Defending the STARTEST
Bryan Greer
I would like to step in here to defend the startest. First some
"theory", and then some talk about practical limitations and common
pitfalls people run into with the startest. I've also responded to
three postings from another thread.
Let us not forget what the "startest" really is--it's just the point
spread function (PSF) of the optical system. Essentially, it is
another representation of the MTF (one Fourier Transform removed, at
least). Thus, the startest contains just as much "information" about
an optical system as the MTF (or, more technically correct, the OTF).
In other words, all of the optical defects are baked in there.
A good example of this principal is the Hubble Space Telescope. One
way the wavefront error of the original defective primary mirror was
verified was by using the startest. And they got more than just a PV
number--defocused stellar images were analyzed, and a very precise
wavefront map (including the secondary mirror obstruction!) was
created from it. This process is described in "Combined approach to
the Hubble Space Telescope wave-front distortion analysis" (C. Roddier
& F. Roddier, "Applied Optics", Vol. 32, No. 16, June 1993, pg. 2992).
There are numerous other examples of wavefront reconstruction by
startest (i.e., PSF) analysis in the professional journals.
I point this out NOT because I think this is a good way for amateurs
to quantify the errors of their telescopes. Rather, it's just to
point out that the startest is not mysterious. Every bit of light
energy in that diffraction pattern is there for an explainable reason,
whether it be optical defects or atmospheric turbulence.
Now, on to the more practical realities of the startest...
Roland Christen wrote:
>
> I have trouble with people who claim that such and such an optic
> tests to 1/8 wave on the star test. This is not realistic. I could
> show you a 1/10 wave system that looks like 1/4 wave on the star
> test, and surprisingly, I have seen a 1/2 wave system that would
> appear to be 1/8 wave on the star test,...<snip>
>
I would not take exception to this statement IF you had qualified it
by saying it is true with SOME refractors, and SOME other catadioptric
designs. I remember well the first time I looked through a
semi-colorfree doublet refractor years ago. After cutting my teeth on
Newtonians, I thought I could root out about any optical problem with
just a turn of the focusing knob. How wrong I was! This particular
refractor had distinctly dissimilar extrafocal patterns, and weird
ones at that, yet performed well. Of course, now I know to use color
filters for such tests, and that telescopes like this have a wide
range of spherical corrections depending on the wavelength. However,
once you understand this, there is no mysterious incongruity between
the startest results (with filters) and the performance on real
celestial objects.
It's also been my experience that true APOs--especially longer focus
ones--do test very much like a Newtonian does. Thomas Back's 180mm
Astrophysics refractor is a good example. I've only had the
opportunity to look through it one time, but the startest behaves very
much like you would expect. Thomas and others have rightfully
"bragged" (I don't mean that in a bad way, Thomas) about this here on
s.a.a. before, and it's well known that telescope is conservatively
better than 1/10 PV.
When it comes to Newtonians, I have yet to run into a paraboloidal
mirror that startests well that doesn't also pass other tests with
flying colors. I guess this is my main point of disagreement with
your statement above. It is a global discrediting of the startest,
and one I strongly disagree with.
However, I will point out some weaknesses of the startest as I see
them:
1) Long Learning Curve. This is easily the biggest problem with
the startest. While anyone with good vision can learn to see basic
things like correction errors in short order, it takes some dedication
to "master" the test. Almost every telescope has multiple sources of
error that need to be carefully teased apart. There are clever
techniques (e.g. masking the aperture to isolate defects), but these
take time to learn, and a methodical mind to implement.
I believe this long learning curve is responsible for a lot of the
criticism. I've run into a few self-proclaimed telescope "experts"
who are, frankly, just too lazy (or disinterested) to learn the
startest properly. Inevitably, they blame the startest as being
inaccurate. (Of course, I'm not putting you in this category,
Roland.)
2) "Seeing" Limited. Under less than good seeing conditions, the
startest is difficult to perform on real stars. While it's still
usually possible to separate the well-, average-, and poorly-corrected
optics, making critical evaluations (see #3) can be tough.
3) Relative Insensitivity to Roughness. Optical roughness
manifests itself in the startest the same as atmospheric seeing, but
without the motion. Thus, while the startest shows optical roughness
just fine, it tends to get lost in the noise from other things that
"roughen" the wavefront. Unlike all the other optical defects,
roughness looks essentially the same on either side of focus. It only
shows up as ragged, or spikey, diffraction rings (or fuzzy, less
distinct, rings with some scales of roughness), but it essentially
looks the same on both sides of focus. This reduces this test to a
qualitative judgement call by the tester, and that requires a lot of
experience, or a telescope of known smoothness sitting next to you for
comparison.
Because of problems #2 and #3 above, I strongly encourage people to
set up a artificial star if they can. Suiter describes how to do this
in his book, and fortunately it takes almost no special equipment.
You will be spoiled by the fantastically stable rings, and this will
finally let you make some accurate judgements on the more subtle
defects like surface roughness (with some experience, of course).
I did a talk once about the startest, and had an artificial star set
up on the rooftop of the observatory I was speaking at. After the
talk, there were about a dozen scopes set up on the front lawn. Even
rank newbies were able to decipher most of the important features of
the startest. I had a woman-friend from California with me who knew
absolutely nothing about telescopes, and after her first 10 second
look through the eyepiece she declared, "It looks overdone!" (she
meant overcorrected, of course), and she was exactly right.
Allan Rahill wrote:
>
> All this story about star testing brought me doing a study one
> year ago. With aperture program creating theorical diffraction
> patterns for all kind of spherical aberration order... I pent 2 weeks
> printing all the results with in and out focus at different
> wavelenght distances. Than I glued all the papers on the wall with
> 1/2 wave at one end to 1/10 wave error on the wavefront at the other
> end of the wall. I really tried to find out any distinct pattern to
> match such and such spherical aberrations with different
> wavelenght errors without any conclusions. In fact I came with
> the conclusion... it is very hard to tell any differences above
> 1/4 wave.
>
This is a bad way to evaluate the startest. As Suiter mentions in the
documentation that comes with the program "Aperture", and in his book,
reproducing accurate looking diffraction patterns in print is
difficult. The printed page has nowhere near the dynamic range that
the real startest has. In fact, one of the important changes in his
book from the First Edition to the Second Edition was tweaking several
of the diffraction pattern images. Due to difficulties in predicting
printing results, some images the first time around looked
unrealistic.
Allan Rahill also wrote:
>
> There is some ATM who claims they can figure a mirror by looking at a
> star test... I have a lot of difficulty to believe such claims...
> because it is pratically impossible to tell where corrections has to be
> applied on the mirror surface to eliminate spherical aberrations.... there
> are too many possibilities.
>
This is spoken like someone who has never made a mirror before. The
startest is a superb way to check an uncoated mirror for correction,
zones, and edge defects, among other things. The Foucault, and other
schlieren-family tests, are better for "in process" verification, but
the startest beats them all in sensitivity. In fact, measuring
spherical aberration and detecting astigmatism (which is VERY tough
with the knife-edge test) are two important strengths of the startest.
Valery Deryuzhin wrote this about the startest:
> It spoiled too many amateurs, lead many of them to a wrong
> conclution, and as rumours tell, a star parties become more
> and more a turnaments - who owned a better scope.
>
There is some truth to this, as I've observed it myself. I constantly
preach to anyone who will listen that the startest is just a tool in
your toolbox. Like any tool, you have to learn to use it correctly.
It's also easy for a beginner to misinterpret chromatic effects (like
I described above) or higher order SA, and wrongly condemn a
telescope. But let's not forget that the startest is used for FAR
more than just checking the quality of the optics. The most common
reasons I pull this "tool" out of the box is for critical collimation,
estimating seeing conditions, and checking for thermal problems.
These reasons alone make it well worth learning the test.
Valery also said:
> The book was, of course, very useful for.... Suiter's pocket.
>
Certainly, you must retract this statement, Valery. It is inaccurate,
way too cynical, and makes you come off as unproffesional. If Mr.
Suiter is reading this, he must be laughing. Have you ever undertaken
the writing of such a book, and added up the hours spent to do it? I
happen to know it took years for Suiter to put his book together. If
you look at the subsequent publishing royalties, he might have done
better financially working the counter at McDonalds. In general, the
ATM authors in this business (R. Berry, H. Suiter, etc.) don't do this
for the money. There just aren't that many of us around to make much
money from.
Sincerely,
Bryan Greer
Columbus, OH
(please remove *takeout* from my address for private replies)
Bryan Greer
I would like to step in here to defend the startest. First some
"theory", and then some talk about practical limitations and common
pitfalls people run into with the startest. I've also responded to
three postings from another thread.
Let us not forget what the "startest" really is…it's just the point
spread function (PSF) of the optical system. Essentially, it is another
representation of the MTF (one Fourier Transform removed, at least).
Thus, the startest contains just as much "information" about an optical
system as the MTF (or, more technically correct, the OTF). In other
words, all of the optical defects are baked in there.
A good example of this principal is the Hubble Space Telescope. One way
the wavefront error of the original defective primary mirror was
verified was by using the startest. And they got more than just a PV
number--defocused stellar images were analyzed, and a very precise
wavefront map (including the secondary mirror obstruction!) was created
from it. This process is described in "Combined approach to the Hubble
Space Telescope wave-front distortion analysis" (C. Roddier & F.
Roddier, "Applied Optics", Vol. 32, No. 16, June 1993, pg. 2992). There
are numerous other examples of wavefront reconstruction by startest
(i.e., PSF) analysis in the professional journals.
I point this out NOT because I think this is a good way for amateurs to
quantify the errors of their telescopes. Rather, it's just to point out
that the startest is not mysterious. Every bit of light energy in that
diffraction pattern is there for an explainable reason, whether it be
optical defects or atmospheric turbulence.
Now, on to the more practical realities of the startest…
Sue_and_Alan_French
Related to the long learning curve is the fact that almost none of us have
access to telescopes that have been adequately quantified. It is one thing
to say that the star test looks like what some text shows for 1/4 wave
undercorrection, and quite another to relate what has been quantified on an
interferometer to what you see in the star test.
Perhaps we can all learn to judge for overcorrection and undercorrection.
Putting some numbers to the correction errors would seem much more difficult
without some "standards" for comparison.
Personally, I am much more comfortable with "in-focus" testing on the
planets. If my telescope reveals a wealth of detail under good seeing
conditions and compares favorably with the best telescopes of a similar
design and aperture that I have encountered, I am quite happy.
Clear skies, Alan
Bryan Greer wrote in message <388788bd...@news.supernews.com>...
[SNIP]
However, I will point out some weaknesses of the startest as I see
them:
1) Long Learning Curve. This is easily the biggest problem with
the startest. While anyone with good vision can learn to see basic
things like correction errors in short order, it takes some dedication
to "master" the test. Almost every telescope has multiple sources of
error that need to be carefully teased apart. There are clever
techniques (e.g. masking the aperture to isolate defects), but these
take time to learn, and a methodical mind to implement.
replies) [SNIP]
Jim Ketchum
forget to see what's in the eyepiece. There is no perfect mirror. Never will
be. It all comes down to what you see.
Jim
Sue_and_Alan_French wrote in message <#ThJys6Y$GA.302@cpmsnbbsa02>...
>Bryan,
>
>Related to the long learning curve is the fact that almost none of us have
>access to telescopes that have been adequately quantified. It is one thing
>to say that the star test looks like what some text shows for 1/4 wave
>undercorrection, and quite another to relate what has been quantified on an
>interferometer to what you see in the star test.
>
>Perhaps we can all learn to judge for overcorrection and undercorrection.
>Putting some numbers to the correction errors would seem much more
difficult
>without some "standards" for comparison.
>
>Personally, I am much more comfortable with "in-focus" testing on the
>planets. If my telescope reveals a wealth of detail under good seeing
>conditions and compares favorably with the best telescopes of a similar
>design and aperture that I have encountered, I am quite happy.
>
>Clear skies, Alan
>
>Bryan Greer wrote in message <388788bd...@news.supernews.com>...
>[SNIP]
>However, I will point out some weaknesses of the startest as I see
>them:
>
> 1) Long Learning Curve. This is easily the biggest problem with
>the startest. While anyone with good vision can learn to see basic
>things like correction errors in short order, it takes some dedication
>to "master" the test. Almost every telescope has multiple sources of
>error that need to be carefully teased apart. There are clever
>techniques (e.g. masking the aperture to isolate defects), but these
>take time to learn, and a methodical mind to implement.
>replies) [SNIP]
>
>
Frank Bov
What you saw is not that uncommon. A bright star on a dark field provides a
very large difference in light over a short distance; it's a high contrast
image. Planetary observation stresses subtle detail from an image that has a
narrow range of brightness compared with a star; it's a low contrast test.
I see the same thing with my 35mm Panoptic; coma at the edges of the field
from star images (high contrast), but absolutely sharp lunar detail as the
image moves out of the field (low contrast.).
A good star test is necessary but not sufficient to a complete evaluation of
an astronomical instrument. Stray light is a good example; just a little
will degrade a planetary image but star tests are fairly imune. However,
star testing is an excellent way to identify a number of optical defects,
and is invaluable when used appropriately.
Hoping your skies are clearer than mine,
Frank
RAnder3127 <rande...@aol.com> wrote in message
news:20000120214429...@ng-cg1.aol.com...
> Something doesn't jive with the
> reality of the star test. My TV85
> had superior star images compared
> to a Meade 102 ED I also had. But,
> the Meade handily beat the TV85
> on planets. The star image in the Meade
> was really bad. So, the star test
> (in focus or out) isn't all it's cracked up
> to be by some modern optic testers.
> -Rich
>
>
> Don't forget to tune into Regis Filbin's
> "Who Wants A Lobotomy?"
WHALEN44
Amen!
Richard Whalen
whal...@aol.com
"Time spent observing the heavens is not deducted from your lifespan"
RAnder3127
Allan Rahill
> Allan Rahill also wrote:
> >
> > There is some ATM who claims they can figure a mirror by looking at a
> > star test... I have a lot of difficulty to believe such claims...
> > because it is pratically impossible to tell where corrections has to be
> > applied on the mirror surface to eliminate spherical aberrations.... there
> > are too many possibilities.
> >
> This is spoken like someone who has never made a mirror before. The
> startest is a superb way to check an uncoated mirror for correction,
> zones, and edge defects, among other things. The Foucault, and other
> schlieren-family tests, are better for "in process" verification, but
> the startest beats them all in sensitivity. In fact, measuring
> spherical aberration and detecting astigmatism (which is VERY tough
> with the knife-edge test) are two important strengths of the startest.
>
Very interesting discussion but you bypass two important words.
If you read carefully... I am speaking about spherical aberrations. I
agree with you that star testing is a good test for astigmatism, turn
down edge, zonal defects... etc. Star test is good also for low order
spherical aberration but gives hard time for higher order one. There are
many telescopes suffering of higher order spherical aberrations.
I do have experience in figuring mirrors.
About the printing of diffraction patterns from aperture... it is true it
doesn't have all the dynamics due to contrast but most of the information
is there. I did testing mirrors and than calculate the diffraction
patterns and finally do a star test... the match is quite close... it
works and it is fun to play with physics phenomenon. It happens to me to
spent hours during an observation session playing around just looking at
different diffraction patterns by changing different mask (different
shapes, different diameters, with two, three, four holes in front of the
telescope... etc. Try it is very instructive.
Sincerly...
~~~~~~
Allan Rahill ___ ( )
/ / ~~/ -- )
/ / (____ CMC )
//@--[]- (_______)
^ /\ ////
/ \ '''
WHALEN44
Did you test them side by side on the same night under the exact same
conditions? Also I would expect a 102mm scope to beat a 85mm scope on the
planets. In this case, aperture wins.
So there is nothing wrong with the star test, just your expectation to see more
in a smaller good scope than a larger average scope. If you were talking about
maybe a 6.8" APO verses a 8" ED it might mean more, as a 6.8" scope has enough
aperture to really start seeing some serious detail on the planets. A 85mm is
just to small.
>Something doesn't jive with the
>reality of the star test. My TV85
>had superior star images compared
>to a Meade 102 ED I also had. But,
>the Meade handily beat the TV85
>on planets. The star image in the Meade
>was really bad. So, the star test
>(in focus or out) isn't all it's cracked up
>to be by some modern optic testers.
>-Rich
Richard Whalen
lude...@my-deja.com
"Sue_and_Alan_French" <Sue_and_A...@email.msn.com> wrote:
> Bryan,
>
> Related to the long learning curve is the fact that almost none of us
have
> access to telescopes that have been adequately quantified.
>
> Putting some numbers to the correction errors would seem much more
difficult
> without some "standards" for comparison.
> >
> Clear skies, Alan
>
Alan,
Brian made a perfect explanation and you answere is the answere, why
Roland know only a very few people who are able to do accuarate
Startesting.
It is correct, that you need a little more than many telescopes and the
book to do accurate startesting.
You need controlled testing conditions, like autocollimation
startesting in stable roomclimate or perfect seeing under sky.
Additional you need some similar reference telescopes to compare ( real
tested) to do comparations and learning.
If you never have had the chance to do your startest in comparation
with real tested scopes of same type and size, than you will be never
able to rate the real quality very close (excapt with many luck).
So the real acurate startest is limited to some people who have an
acces to all the necassary stuff and real tested scopes, becaause only
such people can learn really to see errors, show in testreports, like
Interference lines or much better the topographic map or the starimage
on such testreports, if this testreports are made with good scale and
accurate.
Markus
Sent via Deja.com http://www.deja.com/
Before you buy.
lude...@my-deja.com
> Very interesting discussion but you bypass two important words.
> If you read carefully... I am speaking about spherical aberrations. I
> agree with you that star testing is a good test for astigmatism, turn
> down edge, zonal defects... etc. Star test is good also for low order
> spherical aberration but gives hard time for higher order one. There
are
> many telescopes suffering of higher order spherical aberrations.
> I do have experience in figuring mirrors.
all depense on your experience, if you have enough, you will also
detect the higher order spherical aberation, but here the seeing
conditions must be really perfect, I agree it more difficult to see it,
but still possible.
Markus
>
>
> Sincerly...
> ~~~~~~
> Allan Rahill ___ ( )
Bryan Greer
<Sue_and_A...@email.msn.com> wrote:
> Perhaps we can all learn to judge for overcorrection and
> undercorrection. Putting some numbers to the correction
> errors would seem much more difficult without some
> "standards" for comparison.
>
Yes, Alan, learning to roughly quantify what you see in the startest
is certainly one of the more advanced parts of the test, but it is
possible. I'll also add that for many uses of the startest it's not
particularly important to be able to put a number to it.
Mirror makers have the potential to become good startesters because
they have many comparative "data points" between two very different
tests. I personally don't trust the Foucault test (a.k.a. knife-edge
test) much beyond a 1/10th wave (on the wavefront). However, within
this range, it is a reproducible test, and there are studies showing
it is reasonably accurate for common types of figure errors and
reasonable focal ratios. Thus, the mirror maker who figures by day
using Foucault, and retests at night using the startest, develops a
good ability to judge errors quantitatively.
I was fortunate to learn to make mirrors in a group setting. For
almost ten years, a generous man named Bill Herbert conducted weekend
mirror-making sessions. Several dozen fine paraboloids came out of
that basement. The final Foucault numbers were shared, and some
good-natured bragging took place. My startesting abilities were
greatly enhanced by being able to eventually startest many of those
telescopes.
Additionally, veteran starparty geeks (like me) do run into
interferometrically-tested telescopes on occasion. Again, Thomas
Back's 180 mm refractor is one example. These valuable experiences
provide yet another source for independent corroboration of the
startest.
How accurately do I think I can startest? For a Newtonian under good
seeing, I can estimate errors up to about 1/8th wave (wavefront).
With an artificial star, I can probably estimate a little better. Of
course, I don't claim +/- 2% accuracy. If it looks 1/4 wave to me, I
realize it could be 1/3 to 1/6, but that is sufficient accuracy for my
purposes. Some aberrations, like spherical aberration, are easier to
quantify than others. Astigmatism (pure "potato chip" astigmatism, at
least) is easy to detect, but if it's less than about 1/4th, I can't
honestly pin a number on it.
Again, I've never experienced the odd mismatch between startest and
in-focus image quality that Roland et al talk about--at least with
Newtonians and high-quality APOs. Really, there's no need to debate
this endlessly, as there is a way to come to a conclusion (at least it
would satisfy me). If they could provide me with a specific telescope
design that I could load into OSLO, and that would show the wildly
asymmetrical defocused PSF behavior, well...as Yuri says, I'll eat my
shoes. :-)
Sincerely,
Bryan Greer
Columbus, OH
(please remove takeout from my address for private replies)
Chris1011
in-focus image quality that Roland et al talk about--at least with
Newtonians and high-quality APOs. >>Really, there's no need to debate
this endlessly, as there is a way to come to a conclusion (at least it
would satisfy me). If they could provide me with a specific telescope
design that I could load into OSLO, and that would show the wildly
asymmetrical defocused PSF behavior, well...as Yuri says, I'll eat my
shoes. :-)>>
I should start out saying that I have the highest regard for Suiter and his
book, there are however enough holes in the book vis a vis refractors and
catadioptrics that it cannot be used for all occasions. If you want to see some
wildly or perhaps mildly asymetrical printouts, contact TEC Telescopes. They
can send you some Zemax results of Mak-Cass designs that may want you to start
nibbling.
Now on to my soapbox:
The situation with the star test is more complicated than manufacturers being
afraid to be found out now that amateurs have an easy test to evaluate the
optics. When the optical system gets more complex than a simple parabolic
mirror, then there are inherent aberrations that affect the star test. An
example is the Maksutov Cassegrain. This system can be manufactured many
different ways, but one popular way is to have all surfaces spherical, which
cuts down the need for hand work. Machines exist now that can lay down a 1/20
wave or better spherical surface on a piece of glass without any human
intervention. In this pure form, the Mak-Cass has left over 5th order
aberrations and, depending on design, these can be less than 1/10 wave on the
wavefront. By the way, fast Apo refractors have these same aberrations also.
The RMS value will be better than 1/50 RMS and the Strehl ratio will be
exceedingly high. In other words, the optic will deliver a very high contrast
image, consistent with the high wavefront rating. When tested on the night sky,
the inside and outside diffraction patterns will be quite different. Any
beginner will see it so and may conclude that the optic is of poor quality.
As an example, I have recently finished exhaustive tests of different 10" F14
Mak-Cass systems, some with these inherent aberrations left in, some with them
meticulously removed. All the optics tested between 1/10 and 1/12 wave with
Strehl ratios of 98% or better. The system with the uncorrected or pure
Maksutov curves had the central hole of the donut break out 4 times farther on
one side of focus than the other. It would be judged by the Suiter star test
method as being maybe barely 1/4 wave, if that. The hand aspherized version,
which was no better on the interferometer and had similar Strehl ratio, showed
a donut breakout approximately at the same point on either side of focus - in
other words textbook perfect. At focus, both scopes showed a tight central Airy
disc with a faint first diffraction ring. At focus, it was impossible to tell
them apart. The ratio of brightness of the Airy disc and first diffraction ring
was essentially identical. Both scopes showed the same high contrast on Jupiter
and Saturn. Both scopes could split doubles with equal precision.
The dilemma for manufacturers then is, should we do our best to produce smooth
high contrast optics, or should we please the star test crowd and do some hand
aspherizing to get a more pleasing out-of-focus star image? I can tell you that
it is easy to do some rough compensation with quick local polishing at several
zones to get more equal inside and outside star patterns, but the result will
almost certainly be a loss of contrast. Add to that a nice big central
obstruction to get rid of the offending inner zones, and presto! you have a
nice "fast food" Mak-Cass that doesn't work any better than a typical SCT.
In our case, we will do our utmost to produce the closest faximile to the star
patterns in Suiter's book, but they will never be exactly equal. The overriding
concern will be that the optic has a very smooth and accurate wavefront to
produce the highest contrast possible in the final image, which I assume will
be in focus.
Roland Christen
ASTRO-PHYSICS
Chuck Zdeb
> Again, I've never experienced the odd mismatch between startest and
> in-focus image quality that Roland et al talk about--at least with
> Newtonians and high-quality APOs. Really, there's no need to
> debate
> this endlessly, as there is a way to come to a conclusion (at
> least it
> would satisfy me). If they could provide me with a specific
> telescope
> design that I could load into OSLO, and that would show the wildly
> asymmetrical defocused PSF behavior, well...as Yuri says, I'll eat
> my
> shoes. :-)
> Bryan Greer
> Columbus, OH
Hi Bryan,
The odd mismatch in diffraction patterns (which I had mentioned in
another post in another thread) that I see in my Mak-Cass is very real.
Also, the breakout of the central obstruction shadow is very quick,
which per the book would indicate correction problems. Yet the in-focus
images are nothing short of wonderful, as one might expect from a scope
which has tested at 1/8 wave (632nm) on an interferometer. If you ever
get in around the Minneapolis area, I'd be glad to demonstrate. I can't
claim any great expertise in star testing beyond reading Suiter's book
several times and attempting to wear out the focusers of a number of
scopes, but the behavior is very unusual to say the least (I have never
seen anything similar in a Newt or Apo). I'm not implying the star test
is not a tremendously useful tool, but that there are evidently
designs/implementations that result in behaviors that are at odds with
Suiter's book (not that I'm faulting him for not writing about every
possible case). In the case of my Mak, I gather it's an effect of
low-amplitude higher-order aberration, but the result is what "appears"
to be a poorly corrected scope
Perhaps contact Yuri at TEC and he can explain it in detail. Just in
case, walk around in a pastry bakery first ;-)
Chuck
* Sent from RemarQ http://www.remarq.com The Internet's Discussion Network *
The fastest and easiest way to search and participate in Usenet - Free!
ar...@selena.kherson.ua
lude...@my-deja.com wrote:
> In article <Pine.SGI.4.10.10001210447000.263927929-> > > >
> > Very interesting discussion but you bypass two important words.
> > If you read carefully... I am speaking about spherical aberrations.
I
> > agree with you that star testing is a good test for astigmatism,
turn
> > down edge, zonal defects... etc. Star test is good also for low
order
> > spherical aberration but gives hard time for higher order one. There
> are
> > many telescopes suffering of higher order spherical aberrations.
> > I do have experience in figuring mirrors.
>
> all depense on your experience, if you have enough, you will also
> detect the higher order spherical aberation, but here the seeing
> conditions must be really perfect, I agree it more difficult to see
it,
> but still possible.
>
> Markus
Markus,
I think this is quite different things to SEE a "results"
of a high order aberrations and to DIFFER and PROPERLY
ESTIMATE them. Quite different things!
I bet you will never *estimate* correctly 8 of 10
scopes which does has different numbers of high order
aberrations. Even 5 of 10 you will not properly estimate.
I can simulate in optical design software several systems
wich will has very different wave fronts from very bad to
very good and all them will has almost identical out of
focus images.
For a real example - the 5" F/8 Borg which you astimated as
somewhat around 1/5-1/6 wave front really in Startest show
a pictures which, according to Suiter's pictures, corresponds
to 1/5 wave or so. Now this objective in GOI. It has 1/8.4
wave front quality(in geen). It has residual 5-th order aberration.
Yes, you saw it, but your estimation was not correct - you
think that this was typical 3-rd spherical undercorrection.
Much better for you to buy a simplest interferometrical
equipment and use it. Then you will quickly forget your
Startest sleeping dreams.
Valery.
lude...@my-deja.com
> Markus,> >
> For a real example - the 5" F/8 Borg which you astimated as
> somewhat around 1/5-1/6 wave front really in Startest show
> a pictures which, according to Suiter's pictures, corresponds
> to 1/5 wave or so. Now this objective in GOI. It has 1/8.4
> wave front quality(in geen). It has residual 5-th order aberration.
> Yes, you saw it, but your estimation was not correct - you
> think that this was typical 3-rd spherical undercorrection.
Valery,
a 5th order spherical aberation will show on Zygo a 1/5 wavefront print
out and not a 1/8 wavefront print out. If your interferometer does show
diffrent, buy another interferometer which works more accurate and show
the real world
Markus
ar...@selena.kherson.ua
chri...@aol.com (Chris1011) wrote:
>
> In our case, we will do our utmost to produce the closest faximile to
the star
> patterns in Suiter's book, but they will never be exactly equal. The
overriding
> concern will be that the optic has a very smooth and accurate
wavefront to
> produce the highest contrast possible in the final image, which I
assume will
> be in focus.
>
> Roland Christen
> ASTRO-PHYSICS
I can confirm on 100% all what Roland said. But let me add
some points too. We spent a lot of time e-mailing each other in
discussions - should we follow to a "modern songs" about necessary
identical out of focus images or not. In beginning our points of
view was different - Roland think that this will nice for selling
with no big problems. And I understand him. My point was that this
is nice, of course, - to have an identical diffraction patterns, but
this can't be our goal. In this case the optics must be 100% free of
residual high order aberrations and from low order ones too. In
this case this optics will be close to 1/30 wave front. We can't
produce such optics systematically even for 10x higher price.
Roland reported later, that in-focus images for three different
10" systems was quite close and on a planets, for example, one
can't differ these systems. Out of focus these systems looks different.
Their interferograms are almost identical - we can't make them
easy more identical. This will be too costly labor, and what the
most pity, it will be spent for nothing. No real in-focus improvement.
Finally, as Roland said, we decided that we will try to make a
systems with as close as possible outside of focus images, but only
if this will not impact with a labor cost(increasing).
I agree with Roland's estimation of Suiter book - it is basically
correct with some minor bugs of not principal character. But I still
consider this book as a very harmful for absolut most amateurs.
This book can be carefully used by more or less experienced and
educated in physics amateurs and also with interferograms in hand.
I also still think that with StarTest one can differ very bad scope
from excellent one. All beyond this does not worth even to know about.
No any reliable wave front estimations - unless one knew per advance
what kind of errors he can guess.
Valery Deryuzhin
ARIES INSTRUMENTS Co.
Rockett Crawford
If you say so. It would be nice if we could get the opinions of
experts who were not manufactures of telescopes. Also it would
be nice if Harold was here to defend his book.
take care,
Rockett Crawford
Vahe Sahakian
I have been following this discussion with great interest, it is always
nice to see the experts going at each other with such a passion and
gusto.
As a novice I simply did not learn the art of startesting, as simple as
it is, I can never interpret the results properly and come up with a
definite number for the system wavefront.
Regarding Maksutov Cassegrains in general and the TEC Maks In
particular, prior to getting my 10" f/21 Mak I did a fair amount of
investigation by contacting various TEC Mak owners. Most TEC owners that
I contacted indicated that these Maks simply do not follow the "book"
when it comes to symmetrical/similar patterns inside and outside the
focus. One TEC owner, Eric Roel of Mexico, a pretty experienced
telescope builder, went to great length to technically explain why these
Maks do not behave according to the principals outlined in Suiter's
book.
Aside from the odd mismatch in diffraction patterns, when it comes to
real word in-focus performance the image quality of my Mak is simply
spectacular, and that is good enough for me.
Thanks,
Vahe
PS. Check out Eric Roel's lunar images taken with the same TEC Mak
http://voltaire.csun.edu/roel/roel.html
Chuck Zdeb
wrote:
> Hello Chuck,
> I have been following this discussion with great interest, it is
> always
> nice to see the experts going at each other with such a passion and
> gusto.
It's great, isn't it? Lots to learn from them.
> As a novice I simply did not learn the art of startesting, as
> simple as
> it is, I can never interpret the results properly and come up with
> a
> definite number for the system wavefront.
Same here so far as numbers go, I don't think it's accurate to call
star testing a quantitative method. However, it is very powerful as a
qualitative tool.
> Regarding Maksutov Cassegrains in general and the TEC Maks In
> particular, prior to getting my 10" f/21 Mak I did a fair amount of
> investigation by contacting various TEC Mak owners. Most TEC
> owners that
> I contacted indicated that these Maks simply do not follow the
> "book"
> when it comes to symmetrical/similar patterns inside and outside
> the
> focus. One TEC owner, Eric Roel of Mexico, a pretty experienced
> telescope builder, went to great length to technically explain why
> these
> Maks do not behave according to the principals outlined in Suiter's
> book.
> Aside from the odd mismatch in diffraction patterns, when it comes
> to
> real word in-focus performance the image quality of my Mak is
> simply
> spectacular, and that is good enough for me.
> Thanks,
> Vahe
Exactly, my interest is also in in-focus images and they are a delight
in my Mak as well. Just from a standpoint of curiousity though, it
intrigues me as to why the out-of-focus patterns vary from classical
ideas of symmetry. I don't think Suiter's book is in error, it's
something that he didn't really address. I want to know more about why
the patterns are so much different when higher-order aberrations are
involved, and I'm happy to learn from our experts.
> PS. Check out Eric Roel's lunar images taken with the same TEC Mak
>
> http://voltaire.csun.edu/roel/roel.html
Simply superb-
Chuck Zdeb
Thanks for the info from Mr. Roel. I had spoken with Yuri about it
after I first star tested my TEC scope (I had marvelous images of
Jupiter the first night out, before I star tested, so I knew the optics
surely weren't defective) and had seen the very puzzling out-of-focus
patterns. Yuri sent me some computer-generated diffraction patterns and
some other info that essentially explained things as Mr. Roel has.
Suiter did mention higher-order aberration in passing, more information
would certainly be welcome. What is most curious to me is that these
higher-order aberrations evidently have such a strong effect on the
out-of-focus diffraction patterns, out of all proportion to the actual
effect on the in-focus image. It's as if the star test is much more
sensitive to higher-order aberration than to simple 3rd order. I'm
curious as to why that seems to be, though I may not understand the
answer ;-)
It's interesting that Roland and Valery are considering tweaking optics
to "improve" their star test, even though it doesn't improve the
in-focus images (which is presumably what we do all this for). Are we
amateurs so resistant to learning something new about star testing?
Thanks for your comments-
Chuck
In article <38890660...@flash.net>, Vahe Sahakian
<va...@flash.net> wrote:
> Hello Chuck,
> You are correct, Suiter did not address the unusual star test
> behavior
> of MC's. In all fairness, unlike SCT's which are all essentially
> the
> same from the standpoint of the design, MC's come in variety of
> designs
> all the way from simple/economical spot secondary (Meade) to
> systems
> optimized for high power planetary work (TEC f/20's). There are
> variations in both performance and star test characteristics in
> these
> instruments.
> Here is what Eric Roel had to say about his 10" f/20 TEC MC:
> "Contrary
> to usual belief, the in and out diffraction patters look different
> in
> this type system, because the meniscus produces a balance of the
> third
> and fifth order spherical aberration, and the spherical aberration
> is
> corrected at the wave front of the best focus, so in this type of
> system
> you have at least an 1/8th of correction at the wavefront if not
> up to
> 1/10th, that is if the optics are well made, smooth with no zones".
> Perhaps Yuri can help explain.
> Thanks,
> Vahe
zgse...@netway.com
>> >
>> > There is some ATM who claims they can figure a mirror by looking at a
>> > star test... I have a lot of difficulty to believe such claims...
>> > because it is pratically impossible to tell where corrections has to be
>> > applied on the mirror surface to eliminate spherical aberrations.... there
>> > are too many possibilities.
Well, as difficult as you might find it to believe such claims, most
mirrors (all?) John Dobson made were figured by star testing. A lot
of mirror makers I am acquainted with use the star test for final
figuring -- bench testing can only get you so close. Carefully
performed, the star test can tell you how much under/overcorrection
(spherical aberration) you have and where on the mirror it lives.
What more do you need to know?
Regards,
Gary Seronik
(Remove the "z" for my actual e-mail address.)
Paul Hyndman
<9A833DE2624867DC.35A884A2...@lp.airnews.net>,
Rockett Crawford <Roc...@Audiotel.com> wrote:
> If you say so. It would be nice if we could get the opinions of
> experts who were not manufactures of telescopes. Also it would
> be nice if Harold was here to defend his book.
Valery and Roland have been very generous to me, as with to many
others, in sharing their knowledge and insights. I have implicit faith
in their observations, and believe that their roles as manufacturers
helps to reinforce their credibility (who better to know these facets?).
I have spent much time with Dick Suiter's book (and earlier articles)
and have referred to it often. It is a terrific resource but it is not
without error (I had been in contact with Dick about one minor but
fundamental error, just a few months ago... he said it would be
corrected when a future edition is released).
Please don't mis-construe this post, and you do have a valid concern as
to potential agendas from a manufacturer, but Valery and Roland have
proven their integrity and (oh yeah) they ARE the "experts"!
Clear Skies!
Paul
--
Paul Hyndman pghy...@yahoo.com Madison, CT
Vahe Sahakian
You are correct, Suiter did not address the unusual star test behavior
of MC's. In all fairness, unlike SCT's which are all essentially the
same from the standpoint of the design, MC's come in variety of designs
all the way from simple/economical spot secondary (Meade) to systems
optimized for high power planetary work (TEC f/20's). There are
variations in both performance and star test characteristics in these
instruments.
Here is what Eric Roel had to say about his 10" f/20 TEC MC: "Contrary
to usual belief, the in and out diffraction patters look different in
this type system, because the meniscus produces a balance of the third
and fifth order spherical aberration, and the spherical aberration is
corrected at the wave front of the best focus, so in this type of system
you have at least an 1/8th of correction at the wavefront if not up to
1/10th, that is if the optics are well made, smooth with no zones".
Perhaps Yuri can help explain.
Thanks,
Vahe
ar...@selena.kherson.ua
<9A833DE2624867DC.35A884A2...@lp.airnews.net>,
Rockett Crawford <Roc...@Audiotel.com> wrote:
>
>
> If you say so. It would be nice if we could get the opinions of
> experts who were not manufactures of telescopes. Also it would
> be nice if Harold was here to defend his book.
>
> Rockett Crawford
Rockett,
1. I think this is difficult for this NG - to get an opinion of
independent experts - probably they are simply not interested
in this and in *practical* startest application. Visit our opti-
cal center, please, and ask some scientists about their opinion
about Suiter's book and about ST. In the best case you will see
a smiles. This will be not due their arrogance - this is simple
indicator on a real place of this ST method.
You can belive me 100%, that careful testing of an optics at 1/8
and deeper, especially around 1/12 - 1/15 wave is hell work.
The difference in cost between 1/4 and, say, 1/10 wave optics not
in "polishing" cost - just in frequent check outs of the results.
You simply don't know how difficult is to collimate a systems to
a level which allow to take a worth interferogram.
Don't you think all manufacturers will be so stupid to not use the
ST method instead of costly and time consumption interferometrical
if only ST will be able to give us a bit reliable information?
2. The fact that Harold Suiter is not here (or don't like to drop
here)does not necessary prohibit to express my(our) opinion about
this book and its role in amateur astronomer's brains spoiling.
3. May I ask you, Rockett, to try to add even a bit constructivism
or a bit of something useful to this n.g. instead of your constant
opposition to all what we(others) discussing here and your costant
attempts to spoil other's opinions speaking that SCT is all
what they need?
Take care,
Valery Deryuzhin
ARIES INSTRUMENTS Co.
Sent via Deja.com http://www.deja.com/
Before you buy.
ar...@selena.kherson.ua
lude...@my-deja.com wrote:
> In article <86al69$mus$1...@nnrp1.deja.com>,. There>
> > Markus,> >
> > For a real example - the 5" F/8 Borg which you astimated as
> > somewhat around 1/5-1/6 wave front really in Startest show
> > a pictures which, according to Suiter's pictures, corresponds
> > to 1/5 wave or so. Now this objective in GOI. It has 1/8.4
> > wave front quality(in geen). It has residual 5-th order aberration.
> > Yes, you saw it, but your estimation was not correct - you
> > think that this was typical 3-rd spherical undercorrection.
>
> Valery,
>
>a 5th order spherical aberation will show on Zygo a 1/5 wavefront
>print out and not a 1/8 wavefront print out. If your interferometer
>does show diffrent, buy another interferometer which works more
>accurate and show the real world
>
> Markus
You really think so? According to your above in any regards
brilliant statement a 3-rd order spherical aberration or 3-rd
order astigmatism etc. MUST show on interferogram printouts a
1/3 wave errors only?
If so, then no comments from my side.
Hey, Yuri, Thomas, Brian, Roland, Jim McS., Bill McHale - Help!!!!!
Valery.
ar...@selena.kherson.ua
va...@flash.net wrote:
> same from the standpoint of the design, MC's come in variety of
designs
> all the way from simple/economical spot secondary (Meade) to systems
> optimized for high power planetary work (TEC f/20's).
Vahe,
A fact that a spot is used as a secondary does not necessary
mean that this system is simple and this fact even less mean
that the system is economical. In MC systems it is significantly
harder to reach a good perfomance than in a systems with separate
secondaries. But spot does has its own advantage and this is worth
to spend a bit additional efforts to reach good final wave front.
In another systems a separate secondary does has its own advantages
as well.
Valery Deryuzhin
ARIES INSTRUMENTS Co.
ed_an...@my-deja.com
>
> You are correct, Suiter did not address the unusual star test behavior
> of MC's. In all fairness, unlike SCT's which are all essentially the
> all the way from simple/economical spot secondary (Meade) to systems
> variations in both performance and star test characteristics in these
> instruments.
> Here is what Eric Roel had to say about his 10" f/20 TEC MC: "Contrary
> to usual belief, the in and out diffraction patters look different in
> this type system, because the meniscus produces a balance of the third
> and fifth order spherical aberration, and the spherical aberration is
> corrected at the wave front of the best focus, so in this type of system
> you have at least an 1/8th of correction at the wavefront if not up to
> 1/10th, that is if the optics are well made, smooth with no zones".
> Perhaps Yuri can help explain.
>
> Thanks,
> Vahe
I would better tell a "Star test" story which happened in
1994. At that time I lived in Coshocton, Ohio and owed
one-man company called "Optical Engineering".
When I assembled the fist Mak-Cas. 8" f/20 and tested it
with Thomas Dobbins - we found that telescope performed
very good on planets, but star test in-and-out focus was not
quite identical, so I called to Ed in Kiev (he is here now)
and recieved from him quite good grafical explanation of
the above; addition to that, I called Richard Buchroeder and
he confirmed Ed's opinion, and later send me a letter, that
could be interesting to the readers of this thread:
"Dear Yuri,
Regarding the assimetry of Fresnel diffraction effects on
intra and extra focal star images, the zonal spherical
aberration characteristic of many Maksutov-Cassegrain
telescopes will catch the attention of knowledgable observer.
Unlike a true Cassegrain, or an achromatic refractor, the
meniscus corrector of the Maksutovs produces a balance of
third and fifth order spherical aberration that results in a
final wavefront error of typically less than 1/8 wave, yet it
is still possible to see a difference in Freshnel rings on
opposite sides of focus. One reason for this is that the 'best
focus' does not coincide with the paraxial focus, which
automatically hints at non-identical Fresnel rings.
However, diffraction theory and computer analysis shows
that only the WAVEFRONT at BEST FOCUS determines
the two most important image quality criteria: Strehl ratio
and MTF. This has been understood by microscope
designers for many years, but is at yet not fully understood
by telescope designers.
I would like to assure you that the difference in apperance
of intra- and extra-focal rings does not indicate any flaw in
the telescope, and that if INFOCUS interferogram shows
better than 1/4 wave peak-valley wavefront quality, the
instrument will perform as well as if it had shown virtually
identical out-of-focus rings.
Sincerely, R.A. Buchroeder, PhD.
1.19.95. "
Similar letter was sent to the editor of "S&T", after
publishing "Star test your telescope", March 1995, p.42.
I do not know if that letter was published, but it called
" False Conclusion from Star tests" .
So, I would not sleep well if do not add something myself.
To answer people a question about star test in-and-out
focus we are telling in short, that we do telescopes for
observing INFOCUS. In case of more questions, we can
explain it with Zemax simulated images ( I can e-mail it to
anybody), BTW, this image could be found also on
Valery's table - he was using it to explain all the above to
some die-hard people.
(-:
Ed and Yuri,
TEC
ed_an...@my-deja.com
ar...@selena.kherson.ua wrote:
> In article <38890660...@flash.net>,
> va...@flash.net wrote:
>
> > same from the standpoint of the design, MC's come in variety of
> designs
> > all the way from simple/economical spot secondary (Meade) to systems
> > optimized for high power planetary work (TEC f/20's).
>
>
> A fact that a spot is used as a secondary does not necessary
> mean that this system is simple and this fact even less mean
> that the system is economical. In MC systems it is significantly
> harder to reach a good perfomance than in a systems with separate
> secondaries. But spot does has its own advantage and this is worth
> to spend a bit additional efforts to reach good final wave front.
> In another systems a separate secondary does has its own advantages
> as well.
>
> Valery Deryuzhin
> ARIES INSTRUMENTS Co.
Hi Valery,
the only first five of ours 8" f/20 made in 1994 had the
secondary mirror as a spot, than we found that separate
secondary is more practical and has more advantages in
design, manufaturing and use - just imagine, after 5-10
years your customer will need to recoate secondary spot -
he should use a special mask and send to coating whole
meniscus, instead of only small secondary!
Yuri
William R. Meyers
Hi, Valery,
I have found this thread amusing and also enlightening. I learned a
little bit about optics. But I wonder, if A sends his friend B a test
report giving an error of 1/27 wave or so, and B posts it on s.a.a.,
should B know what kind of skeptical response the post will elicit from
lots of people? If B posts it anyway, is he trolling for responses,
and not doing A any favors by opening him up to criticism?
In addition, I have some questions about language. Is "troll" a
corruption of "trawl," and do only trolls trawl? Just curious.
Thanks,
Bill
ar...@selena.kherson.ua wrote:
> Valery Deryuzhin
> ARIES INSTRUMENTS Co.
>
WHALEN44
You sure stepped in it now :-)
I find this whole discussion quite amusing, with some hypocraisy and
disingenuous behavior on at least one poster's part. You on the other hand,
sure stick to your guns. Your take on the star test has not shifted one mm in
any direction over the last few years. And even though we agree to disagree
somewhat on this subject, I respect you greatly for that.
Being that I am even more stubborn than you, I'm going to let all of this
slowly wear you down, and then strike. I'll change your mind yet :-)
Steady skies my friend,
Richard
WHALEN44
Send me a email (I lost your address)
lude...@my-deja.com
Hi Valery, ED and Yuri,
you can say from manufactor standpoint whatever you want, partly you
are for shure correct, but on the hand I can tell you from now years
experience with Testing in Zygo Mark VI at 632 nm, LOMO Zygo testiong
at 632 nm and 546 nm , from Lytkarino testing in Interferometer at 632
nm and 546 nm from leica douple pass sheme testing and fron extremly
accurate startesting under controlled conditions with same telescopes,
talking about ED Apos, SD Apos, Fluorite Apos , Maksutov Newtonian
Systems, Maksutov cassegrain systems with secondary Spot (Zeiss and
INTES), Maksutov Cassegrain F/10 systems, Maksutov Cassegrain F/15
systems, Schmidt Cassegrain and Classical Cassegrain Systems normal
parabolic mirrors used in Dobs, the accurate made startest showed
always ( with very little excaption , where telescopes have had
stronger local errors)the similar result as all this Interferometerical
tests.
I dont know excactly which interferometer and which software you are
using , but I see that our experience confirm the ability of a accurate
startest to estimate closly the optical quality of a system.
The Startest is of course not real usefull to during prodution and
polishing but very helpfull to estimate a ready to go Telescope.
If you have another opinion, okay we have to excapt it, but I hope you
will be able to explain why over nearly 10 years the Startest was able
to confirm the results of many diffrent Interferometers at diffrent
manufactors. Correspondance to your explanation it should be
impossible, but we have shown that it was very well possible.
Markus
> No any reliable wave front estimations - unless one knew per advance
> what kind of errors he can guess.
lude...@my-deja.com
lude...@my-deja.com
Valery,
one more question.
Could you please explain me following:
In early '90 Carl Zeiss jena invited me for several visit's at here
optical factory to bring many diffrent type of telescopes for testing
to see at which place are the Zeiss amateurtelescopes in the world.
All this tests have been made only, I note only in douple pass sheme
startesting and not in the intetrferometer.
Many times I stayed by a Leica Masteroptician for testing diffrent
telescopes and always he made the test in douple pass sheme startesting.
I asked them why they doing it that way and they answered me, no other
method of testing is able to show total all errors with one method
excapt startesting. You can see there astigmatism, spherical
aberration, highe rorder aberation, coma, decentering, you can even see
if a scope is handfigured or maschine polished.
Are all this people, proffeseionell opticians from famous companys who
having modern Interferometers are so crazy, that they use only simple
startest ? I dont think so
Markus
Derek Wong
> The dilemma for manufacturers then is, should we do our best to produce smooth
> high contrast optics, or should we please the star test crowd and do some hand
> aspherizing to get a more pleasing out-of-focus star image?
...
> In our case, we will do our utmost to produce the closest faximile to the star
> patterns in Suiter's book, but they will never be exactly equal. The overriding
> concern will be that the optic has a very smooth and accurate wavefront to
> produce the highest contrast possible in the final image, which I assume will
> be in focus.
If the uncorrected and hand-aspherized systems have the same Strehl and
the same images in focus, I vote for leaving it uncorrected. Having AP
hand correct a large meniscus is expensive--won't this add a couple
hundred dollars to the cost of the scope?
In separate posts you, Yuri, Valery, and Al Nagler (in an Ask-Al if I
recall) all emphasized the importance of optimizing the in-focus image
rather than optimizing the star test. Why don't you just produce a
brochure illustrating the points in your previous post, in the same
style as Suiter with pictures of diffraction patterns and interferograms
to back them up?
I understand that AP is concerned about damaging it's well deserved
reputation for optical quality. It is just too bad that some energy
will go into making the scope look like pictures in a book which could
be used to make more scopes.
Derek Wong
By the way, this is not a slight on Suiter's book--I enjoy it and learn
a lot from reading it, even though I will never be an expert star
tester.
Enyo
Suiter's software for a 20% obstructed system 1/10 higher order SA and
looked at the inside and outside focus patterns (+and -5 waves). They are
very different and I think we all agree a 1/10 wave is a very good system.
I don't know enough to do analysis of real system with OSLO or other systems
so I will leave it there.
BTW I am on the waiting list for AP and would be more than happy to have a
scope that is uncorrected for a symmetrical start test.
John
If you have
"Derek Wong" <daw...@earthlink.net> wrote in message
news:38897424...@earthlink.net...
Rockett Crawford
Anyway, like I said. It would be nice to get the opinion of
experts who didn't have a vested interest in selling their
products here and I wonder how Harold Suiter's input
would temper the opinions being expressed here about
his book?
take care,
Rockett
ar...@selena.kherson.ua wrote:
> In article
> <9A833DE2624867DC.35A884A2...@lp.airnews.net>,
> Rockett Crawford <Roc...@Audiotel.com> wrote:
> >
> >
> > If you say so. It would be nice if we could get the opinions of
> > experts who were not manufactures of telescopes. Also it would
> > be nice if Harold was here to defend his book.
> >
> > take care,
> > Rockett Crawford
>
> Rockett,
>
<snip>
ed_an...@my-deja.com
005.remarq.com>,
Chuck Zdeb <czdebN...@mho.net.invalid> wrote:
> It's interesting that Roland and Valery are considering tweaking optics
> to "improve" their star test, even though it doesn't improve the
> in-focus images (which is presumably what we do all this for). Are we
> amateurs so resistant to learning something new about star testing?
> Thanks for your comments-
>
> Chuck
I would explain this by their young "Maksutov" age.
I have made my first scope, when AP has not been started,
Valery just started walking and RC was in the third grade.
(-:
Yuri
ar...@selena.kherson.ua
Chuck Zdeb <czdebN...@mho.net.invalid> wrote:
> It's interesting that Roland and Valery are considering tweaking optics
> to "improve" their star test, even though it doesn't improve the
> in-focus images (which is presumably what we do all this for). Are we
> amateurs so resistant to learning something new about star testing?
> Thanks for your comments-
>
> Chuck
Hi Chuck,
The answer is simple as rail. :-)
The reason is the existing of the Suiter's book and his followers. Roland, of
course, know well that the assymmetry of a diffraction patterns does not
necessary mean any seriouse flaws in a telescope perfomance. But he also
know well that most amateurs was so much spoiled by wrong adressed Suiter's
book, by numerouse "star test experts" hypes. His (R.) task is successful
selling of his new scopes and therefore he will feel himself much more
comfortable if his custo- mers will see almost identical diffraction patterns
and will feel theirselves also comfortably. Only due to this he asked us to
try to make the systems with as close as this possible to symmetrical
diffraction patterns. I can say that basing on a Startest estimation, one can
be dissati- sfied even by 1/20 wave front system - as Roland was untill he
test the system in his green interferometer with above result for 3-rd order
spherical aberration correction. Another and hidden side of this is a price
increasing. So, for all such misleading always will pay......a customer,
who read this book or listen too much to "startest experts" . To reach
almost identical patterns will cost a lot of money without any additional
in-focus image improvement. This is a reason why I not tired to repeat that
for typical amateur the Suiter's book is harmful. This book being basically
correct and interesting for specialists was simply wrong addressed and does
not reflect and explain in a simple, understandable for amateurs style, a
lot of questions. This caused a lot of problems.
I can only imagine to what kind of disappointment and confusing the Startest
can lead a less experienced persons. And numerouse "star test expert's "
claims that during SEVERAL(!) years all his estimations were confirmed by a
wide number of very respectable firms, can't be convincing for more or less
knowelegable amateurs. Who can seriously consider such claims? All were
confirmed, all were confir- med etc etc. Whom all this were confirmed? By
similar "star testers"? :-)))))))) And whats about quite RESENT promise to
test visually 12" mirror and then test it interferometrically to see how
close a visual ST results will be to a Zygo one? Whats about a years
confirmation? Why need a new test? Very starnge..... My be the reason is that
according "ST expert's" claim a 5-order aberration MUST show 1/5 wave
error? Very probably, however.
Clear sky,
Valery.
ar...@selena.kherson.ua
ed_an...@my-deja.com wrote:
Hello Yuri!
Thanks for your excellent post. Say my thanks for Dr.Bachroeder
as well. I have the same point of view as he has. Unfortunately,
my english is too weak to write such brief and clear explanations.
I should say thanks also for your print-outs of diffraction patterns.
They help me a lot and lead me to upgrade my Zemax.
I think that if we will continue our efforts to learn amateurs about
ST obviouse holes, may be we will see a days when amateurs will
has a nice scopes and will forgot all nightmares caused by blossoming
of this notorious(!!!!) StarTest . May be at that nice time all "star-
test experts" will retire from this job and amateurs will be focused
on their main goal - sky observing and results and experience
exchange . I truly hope for this and I will continue my modest efforts
to make these days closer.
Best regards, give my best wishes to Ed too, please.
ar...@selena.kherson.ua
Derek Wong <daw...@earthlink.net> wrote:
> In separate posts you, Yuri, Valery, and Al Nagler (in an Ask-Al if I
> recall) all emphasized the importance of optimizing the in-focus image
> rather than optimizing the star test. Why don't you just produce a
> brochure illustrating the points in your previous post, in the same
> style as Suiter with pictures of diffraction patterns and interferograms
> to back them up?
>
> I understand that AP is concerned about damaging it's well deserved
> reputation for optical quality. It is just too bad that some energy
> will go into making the scope look like pictures in a book which could
> be used to make more scopes.
>
> Derek Wong
>
Hi Derek!
Thanks you too for your support. You expressed exacly my points.
See my reply to Chuck's Zdeb post.
I see that some points was clearly explained. About aspherizing.
The aspherizing is required if we would like to have better MC
systems - say, we would like smaller central obstruction, shorter
(and less heavy) OTA, faster F/D with the same central obstruction.
But this necessary aspherizing will not produce a full sysmmetry
of diffraction patterns. Some significant dirrerence will remain.
When Roland said about local aspherizing and central masking,
he did mean an additional local aspherizing for reaching a sysmmetry.
Only THIS additional zonal aspherizing , only in benefit of a star-testers
satisfaction is a waste manufacturer's time and customer's money.
When sysmmetry funs and their teachers will understand this?
Take care,
ar...@selena.kherson.ua
"Enyo" <En...@worldnet.att.net> wrote:
> Not sure where to enter on this thread. I am no expert but I just ran
> Suiter's software for a 20% obstructed system 1/10 higher order SA and
> looked at the inside and outside focus patterns (+and -5 waves). They are
> very different and I think we all agree a 1/10 wave is a very good system.
> I don't know enough to do analysis of real system with OSLO or other systems
> so I will leave it there.
>
> BTW I am on the waiting list for AP and would be more than happy to have a
> scope that is uncorrected for a symmetrical start test.
>
> John
Hi John!
I see one more stands in a line with us. Thanks for the help.
Regards,
ar...@selena.kherson.ua
ed_an...@my-deja.com wrote:
> Hi Valery,
> the only first five of ours 8" f/20 made in 1994 had the
> secondary mirror as a spot, than we found that separate
> secondary is more practical and has more advantages in
> design, manufaturing and use - just imagine, after 5-10
> years your customer will need to recoate secondary spot -
> he should use a special mask and send to coating whole
> meniscus, instead of only small secondary!
>
> Yuri
> TEC
Hello Yuri,
A spot secondary does has it's own advantages and disadvantages.
From my point of view in our design for the AP 10" MC the spot
design does has advantages with weight more than it's disadvantages.
This was Roland's point of view too.
You know, of course, that a separate secondary is not a problem
for us and we simply make our own choise with some ideas in mind.
Valery.
AIC.
valery_...@my-deja.com
ed_an...@my-deja.com wrote:
> In article <0d24a28c.11786a85@usw-ex0101-
> 005.remarq.com>,
> Chuck Zdeb <czdebN...@mho.net.invalid> wrote:
>
> > It's interesting that Roland and Valery are considering tweaking optics
> > to "improve" their star test, even though it doesn't improve the
> > in-focus images (which is presumably what we do all this for). Are we
> > amateurs so resistant to learning something new about star testing?
> > Thanks for your comments-
> >
> > Chuck
>
> I have made my first scope, when AP has not been started,
> Valery just started walking and RC was in the third grade.
> (-:
> Yuri
>
Yuri,
You may be not right. I made my first scope at age 9. This
was simple singlet refractor D=35mm F=1200mm in a cartooon
tube. With deep yellow-green filter it has a nice images - good
enough for me at that moment. :-)
The reason why we try (only try, this is not our real goal) to
make a out of focus images symmetrical was explained in
my reply to Chuck's post. Read it and you will see that no
any connection with Roland's and my "Maksutov" age. :-)
FYI, my first real scope was 3" refractor semi-apo and second
one was 70mm F/10 Maksutov-cassegrain - at that moment
I was 12-13. So, my Maksutov age is a bit older than you may
think. :-))
Valery.
ed_an...@my-deja.com
> Valery,
>
> a 5th order spherical aberation will show on Zygo a 1/5 wavefront print
> out and not a 1/8 wavefront print out.
That is neat!. Markus you are so creative!
If your interferometer does show
> diffrent, buy another interferometer which works more accurate and show
> the real world
>
> Markus
The only brand that can show the real world is a hard-to-
find Zyguto interferometer.
Yuri
Vahe Sahakian
> to "improve" their star test, even though it doesn't improve the
> in-focus images (which is presumably what we do all this for). Are we
> amateurs so resistant to learning something new about star testing?
> Thanks for your comments-
Hi Chuck,
Yes indeed it is interesting, but let us leave Valery out of this, his
ideas are clear by now, he is after all producing the optics for Roland,
per Roland's specifications.
I see a fundamental difference here between TEC and AP as far as
Maksutov designs are concerned. TEC is simply not willing to give in to
the public expectations and produce a system that star tests per "book",
I have a feeling that AP has raised the public expectations on their
upcoming Mak's to a point that, regardless of the cost, they must
produce a product that simply tests "perfect" no matter what type of
test they are subjected to.
So "tweaking" optics to improve the star test is to satisfy the "star
test" experts, and do not underestimate the power of this group, star
testing has become a major pastime in all star parties, many come to
these star parties with the hope of catching one of these experts for an
in-depth evaluation of their telescope.
As a TEC Mak owner, I too am very much interested and can not wait to
look thru one of the AP Maks and to see how it performs on planets, "in
focus".
Thanks
Vahe
Eric
to kill or discredit the startest though. It's still a valuable tool,
especially for the price! Since Suiter is apparently going to revise his book,
maybe Valery, Roland and Yuri could send letters asking that he include this
information in the new version. If ya can't beat 'em, join 'em!
Clear skies to all!
Eric
In article <86cmjn$281$1...@nnrp1.deja.com>, ar...@selena.kherson.ua says...
Chris1011
Anyway, like I said. It would be nice to get the opinion of
experts who didn't have a vested interest in selling their
products here and I wonder how Harold Suiter's input
would temper the opinions being expressed here about
his book?
>>
Since Suiter is not here on SAA, I have sent to Mr Greer an actual
interferogram with resultant star test images of a 1/10 wave P-V, 1/50 wave RMS
optic that shows quite dramatic difference in the intra vs outer star pattern.
It fits Suiter's 1/4 wave criteria, but is indeed 1/10 wave (actually 1/11
wave). It also performs flawlessly on the planets and splits doubles with
exceeding ease even on nights of poor seeing.
I hope he doesn't have to eat his shoes.
Roland Christen
ASTRO-PHYSICS
Chris1011
Could you please explain me following:
In early '90 Carl Zeiss jena invited me for several visit's at here
optical factory to bring many diffrent type of telescopes for testing
to see at which place are the Zeiss amateurtelescopes in the world.
All this tests have been made only, I note only in douple pass sheme
startesting and not in the intetrferometer.
Many times I stayed by a Leica Masteroptician for testing diffrent
telescopes and always he made the test in douple pass sheme startesting.
>.
I can tell you, because I am friends with this master optician. I have talked
to him on many occasions. They did it that way because it was quick and cheap.
Interferometry testing is very time consuming and expensive. The reason Zeiss
optics are good is not because of the double pass test, rather it is because of
the design being conservative and inherently accurate, coupled with good
manufacturing techniques such as slow polishing on pitch.
Roland Christen
ASTRO-PHYSICS
Chris1011
Exactly, my interest is also in in-focus images and they are a delight
in my Mak as well. Just from a standpoint of curiousity though, it
intrigues me as to why the out-of-focus patterns vary from classical
ideas of symmetry. I don't think Suiter's book is in error, it's
something that he didn't really address. I want to know more about why
the patterns are so much different when higher-order aberrations are
involved, and I'm happy to learn from our experts.
>>
This same thing happens in my short focus Apo triplet lenses. They also have
classical 5th order like Mak-Cass systems. I have seen literally thousands of
star patterns on my test setups. I can tell you that the shadow breakout
difference is almost entirely due to slight increase in focal length of the
very inner zones on these optics. That is one half of the 5th order defect.
This causes no problem in definition and contrast for 2 reasons. 1, there is
very little energy as you go toward the middle of the optic, and 2, the depth
of focus approaches infinity at the very center. However, the shadow breakout
is linear, not asymtotic like the encircled energy. This is easily fixed, of
course by commercial makers by simply making a large central obstruction.
The other half of the defect occurs at the outer zone. If this is left
uncorrected, you will see fuzz in the in-focus image which is highly
destructive of contrast. The inside and outside patterns will look the same,
causing you to conclude that the optic is textbook perfect. It is this outer
zone that the competent optician will concentrate on, since this is the place
that contributes most to the definition, resolution and contrast of the optical
system, even though it has little or no effect on the inside/outside star
pattern, particularly where the shadow breakout occurs.
In view of the above, I have seen amateurs at star parties wrongly interpret
the star test and overestimate the quality of one optic that was really not
that good, and underestimate another that was really superb.
Roland Christen
ASTRO-PHYSICS
Doug Looze
> I think that if we will continue our efforts to learn amateurs about
> ST obviouse holes, may be we will see a days when amateurs will
Hi,
I'm really new at this, and certainly not an expert. But, for me, this
statement is the most appropriate of the entire thread. I've read Suiter's
book twice cover to cover since starting in October, and have written Matlab
software to help me visualize and understand much of the material. The star
test serves a number of purposes for me: it helps with colimation, it allows
me to check the seeing, and it provides a quick verification that the scope
has cooled down. The test also provided a confirmation that my mirror (I have
an 8" dob) was reasonable. The latter is not insignificant. I do not have
access to sophisticated testing equipment, and have not yet had a chance to
interact with other local amateurs.
The key point is that the star test is a tool, that when used properly can
provide much information about the status of a scope. As much of this thread
notes, it has limitations. If the manufacturers and experts can help other
amateurs understand the limitations, we all benefit.
As an aside, my scope has softer images inside than out, but the shadow break
out with a 33% mask is almost indistinguishable. I was able to replicate the
patterns I observed in an artificial star test with either a 1/10 wave TDE or
a small (my recollection was 1/10 wave) higher order spherical aberration. I
also recall (I don't have the text handy) that Suiter had several cautions
about too quickly rejecting a mirror for different inside/outside patterns.
Doug Looze
Bryan Greer
> If you want to see some wildly or perhaps mildly asymetrical
> printouts, contact TEC Telescopes. They can send you some
> Zemax results of Mak-Cass designs that may want you to start
> nibbling.
>
Please keep in mind what part of your earlier post I was specifically
referring to. I said in my first post...
"I would not take exception to <Roland's earlier posting> IF
you had qualified it by saying it is true with SOME refractors,
and SOME other catadioptric designs."
I have already acknowledged that some refractors and catadioptrics
will not give the startests shown in most startest "primers"
(including Suiter's book). But Maksutov-Cassegrains, and the other
affected designs you mention, make up a fairly small percentage of the
total population of telescopes out there. Most types of telescopes
can be very adequately evaluated either directly, or with filtering.
Some of the statements made by you and others could come off as a
global discrediting of the startest. It's like telling an aspiring
musician, "Don't waste your time...there's no career in music anyway."
It really sends the wrong message. I think if you refer to my first
post, it should be clear that this was what I was "defending".
Let me also point out that a knowledgeable startester can use his
skill to learn something even about these Maksutov designs. I suggest
we define the "startest" to not just mean we're looking for
symmetrical extrafocal patterns, but simply looking at the patterns
themselves, and comparing them to what they theoretically should look
like (symmetrical in most cases, but some not). There is a LOT more
information in the PSF than just seeing if it's symmetrical on both
sides of focus. (I sometimes wonder if Mother Nature had not given us
the easy ability to see the PSF, if Zygo would have made some $150,000
machine that would. Then we'd be hearing from Valery, "You can have
your silly fringes...I can see the actual point spread function!!"
<g>)
In fact, I think I will generate some computer simulated startest
images for various catadioptric designs, and post them to a webpage.
I suspect it will be educational for more people than myself. If you,
Yuri, or Valery have a generic (non-proprietary) design to share that
you think is representative of this behavior, I'd be glad to include
it (I can import Zemax into OSLO, by the way). I'm very interested to
see how these images compare to subsequent (filtered) startesting.
Roland wrote:
> The dilemma for manufacturers then is, should we do our best to
> produce smooth high contrast optics, or should we please the star
> test crowd and do some hand aspherizing to get a more pleasing
> out-of-focus star image?
>
Believe it or not, I honestly "feel your pain", Roland. Really. In
fact, I empathize not just with you, but even the Newtonian telescope
makers. I am sure any telescope maker has had that occasional
customer who is looking for the "pefect startest", and some of these
customers may not even be qualified to judge what is "good enough".
That must be a "character building" moment for you men, and I guess
that must play some part in your apparent frustration with the
procedure.
Without a doubt, the startest has been misused in all kinds of ways.
This was a major point I tried to make in my talk about it. But IMO
the solution is to write _more_ solid books like Suiter's, write
_more_ webpages with even better graphics, and just keep trying to
educate. The startest is a versatile and powerful tool when used
correctly.
Sincerely,
Bryan Greer
Columbus, OH
(please remove *takeout* from my address for private replies)
Bryan Greer
That must be a "character building" moment for you men.
Bryan Greer
>Could you please explain me following:
>
>In early '90 Carl Zeiss jena invited me for several visit's at here
>optical factory to bring many diffrent type of telescopes for testing
>to see at which place are the Zeiss amateurtelescopes in the world.
>All this tests have been made only, I note only in douple pass sheme
>startesting and not in the intetrferometer.
>Many times I stayed by a Leica Masteroptician for testing diffrent
>telescopes and always he made the test in douple pass sheme startesting.
>
>I asked them why they doing it that way and they answered me, no other
>method of testing is able to show total all errors with one method
>excapt startesting. You can see there astigmatism, spherical
>aberration, highe rorder aberation, coma, decentering, you can even see
>if a scope is handfigured or maschine polished.
>
>Are all this people, proffeseionell opticians from famous companys who
>having modern Interferometers are so crazy, that they use only simple
>startest ? I dont think so
>
Fantastic point, Markus.
I think what is slowly solidifying out of this is:
1) The startest is a reliable--even quantifiable--test of optical
quality for an experienced tester when testing purely reflective
systems.
2) The startest is a reliable test for many refractors if appropriate
color filters are used, and the tester is familiar with how the design
is chromatically optimized. This is especially true for slower f/
designs.
3) The startest, while it may still be informative, is more difficult
to apply with "standard methods" on some complicated (i.e. aspherical)
refractive and catadioptric designs.
At least this is what I'm distilling out of all this. If so, it's
been a very productive discussion.
Bryan Greer
>another post in another thread) that I see in my Mak-Cass is very real.
>
But the statement I made (that you even quoted) said:
"Again, I've never experienced the odd mismatch between
startest and in-focus image quality that Roland et al talk
about--at least with Newtonians and high-quality APOs."
In other words, I made an exception for your Mak-Cass. A Mak-Cass at
Astrofest looked the same way (not one of Roland's, BTW).
Bryan Greer
>The reason is the existing of the Suiter's book and his followers.
>
I was not aware of a "following". How can I join?
> know well that the assymmetry of a diffraction patterns does not
>necessary mean any seriouse flaws in a telescope perfomance.
>
"telescope performance"? ALL telescopes, Valery?
and...
> But he also know well that most amateurs was so much spoiled
> by wrong adressed Suiter's book, by numerouse "star test
> experts" hypes.
>
"hypes"?
Valery, I can see by your purposely vague, imprecise, and provocative
wording that you have no real interest here in furthering this
discussion in meaningful way. Reasonable men almost always find
common ground, but I see that you're only interested in keeping a hot
flame going on s.a.a. Good luck...you're on your own.
Disappointing...
Hasta luego,
Derek Wong
>
> But this necessary aspherizing will not produce a full symmetry
> of diffraction patterns. Some significant difference will remain.
> When Roland said about local aspherizing and central masking,
> Only THIS additional zonal aspherizing , only in benefit of a star-testers
> satisfaction is a waste manufacturer's time and customer's money.
I think most people would agree that additional work only to satisfy the
startest is unnecessary.
However, I agree with Bryan that more information, not less is needed.
If you teach a math class, do you explain everything at the level of the
lowest 1/3 of students or do you give more complex explanations to the
upper 1/3 as well?
The star testing book is out there, and people will use it regardless of
what you do. I think that the Mak and refractor makers should talk to
Dr. Suiter about the next revision of his book. I'm sure he could give
a clear and insightful explanation about why a highly corrected Maksutov
system can have an extremely high Strehl ratio and still have a markedly
asymmetric star test.
If there are doubters (ie. at the major star parties), do some
comparisons between scopes. The best thing would be to bring one scope
with great optics and an asymmetric star test, and another with mediocre
optics and a near symmetric star test...then let people compare the in
focus images of Jupiter or other objects.
If the Maks are as good as advertised, I don't think AP, TEC or anyone
else's sales will suffer.
Derek
Dan McKenna
I have been experimenting with extra focal star tests for automated
alignment of our
1.8 meter F/9. This telescope (VATT) uses a F /1.0 primary and a F/0.8
secondary.
As a result we need to keep our primary to secondary spacing with in 1.5
microns
as well as maintaining less than 10 micron error in tip/tilt and
decenter. To do this we
have a six axis drive on the secondary mirror with sub micron resolution.
As we do not have a telescope operator, the burden of alignment falls on
the observer.
All data, positions, temperatures, etc. are logged so that we can review
setting to keep track of
telescope performance. We find that some observers are not to careful
with alignment especially
when the seeing is not good and to have the telescope performance be as
independent of observer skill
as possible we need to automate alignment.
We are now implementing a star test that will produce two extra focal
images on a ccd image.
this is done by using a simple arrangement of prisms in front of the
guide camera.
The images are reduced with a software package called EF by Laplacian
optics.
The output of this package is error as a function of Zernike , a PSF,
phase map or fringes.
We find that we can resolve better than 1/20 wave if the exposure is
longer than 30 seconds.
I would be willing to run a few images if anyone wants to try it.
I would need two extra focal images between 100 to 200 pixels across.
(one each side of focus.)
The images need to be in FITS format.
E-mail me if you want to test your optics with this method.
It is very interesting to see just how much the PSF changes in a night
due to temperature
and gravity effects.
Dan
Bryan Greer
>to apply with "standard methods" on some complicated (i.e. aspherical)
>refractive and catadioptric designs.
>
Oops...I made a mistake. Take out the "aspherical" part, as it's just
the opposite.
Sincerely,
Mark D'Ambrosio
say that many self-proclaimed star-testing experts know dick.
For whatever reason, I have no idea, but to walk up to a telescope, and
within 5 seconds, say that a scope has such, and such wavefront error,
is IMO full of BS.
With even interferometer testing varying, how can someone walk up, and
make these claims?
As far as identifying a poor optic, yes, I most definitely agree it can
be spotted visually, but to judge one that's exceptional, and put a
number on it, I don't think so! Mark
Mike Spooner
You and Roland and others have described higher order effects on the star
test. I just want to point out that even Suiter addressed the higher order
spherical if folks would have paid enough attention though he does not
specifically point out that it can be a residule when balancing a design
such as a Mak. And the parts where he describes estimating the low order
error by secondary shadow breakout has no doubt contributed to much of the
confusion. For those folks who will continue to star test using this book,
please note the shadows of the high order spherical he shows and compare
them to the low order. That alone should serve as a warning to using this
procedure for the purpose of rating. To be fair to Suiter he mentions in
print about the high order spherical having the caustic pulled back thru
itself. It looks untidy somewhat out of focus but actually cleans up as you
approach focus. Even Newtonians can be affected but with low cost mirrors it
is often a struggle to get basic correction under control and with complex
aberrations, a residule 5th order is mostly a secondary issue anyway. As
Roland pointed out, infocus the energy involved is not much as the area and
true deviation is actually quite small as Valery has pointed out by the
interferometric tests. But because of the effect on the caustic, the
appeareance of the rings is drastically altered from pure, low order error.
One other check you can do, is to defocus Jupiter so there is a 4 or 5
arcsec blur around the planet and see if it is close to the same inside as
out. With a good optic, there will be little or no softness on either side
of focus and of course the most important infocus images will bear this out
in any event with sharpness and contrast. As Roland has mentioned, you will
be amazed how steady an image can be even with obvious seeing effects.
Anyway, thanks for more clearly stating some of the cause and effects of
what is seen in the star test with different scopes and aberrations rather
than just saying that it can not be used. Please note that I agree that it
is more often misused but as has been pointed out, it is one of the only
methods sensitive and available to an amateur wishing to verify his
purchase. I started in an age when 1/25th wave optics were commonplace if
all the advertisements were to be believed. <grin> So it is nice to at least
be able to look for 1/2 wave or better (with a little care).
Cordially,
Mike Spooner
Chuck Zdeb
Thanks, your explanation of the variation of the shadow breakout is
exactly the sort of information I've been looking for. If I understand
correctly, the optic is aspherized slightly to correct the outer zone
defect, so as to achieve a proper in-focus image. I still find it
puzzling that such a small change in 5th order correction causes such a
major change in the diffraction pattern. Roughly what size of
correction is applied (either Apo or Mak-Cass, both are equally
interesting cases)? If you happen to know offhand, could this be
modeled using Suiter's Aperture? Thanks again-
Chuck
In article <20000122142750...@ng-dh1.aol.com>,
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Chuck Zdeb
I'm fully in agreement with the points you've made (I've excerpted
here for emphasis). The star test isn't going to go away, even if that
were a desirable thing, and it would be a great benefit for those of us
who use it if we could learn more about behavior of a variety of
telescope types. I'm learning a lot from this thread!
Chuck
> Let me also point out that a knowledgeable startester can use his
> skill to learn something even about these Maksutov designs. I
> suggest
> we define the "startest" to not just mean we're looking for
> symmetrical extrafocal patterns, but simply looking at the patterns
> themselves, and comparing them to what they theoretically should
> look
> like (symmetrical in most cases, but some not). There is a LOT
> more
> information in the PSF than just seeing if it's symmetrical on both
> sides of focus.
snip
> In fact, I think I will generate some computer simulated startest
> images for various catadioptric designs, and post them to a
> webpage.
> I suspect it will be educational for more people than myself. If
> you,
> Yuri, or Valery have a generic (non-proprietary) design to share
> that
> you think is representative of this behavior, I'd be glad to
> include
> it (I can import Zemax into OSLO, by the way). I'm very
> interested to
> see how these images compare to subsequent (filtered) startesting.
snip
> Without a doubt, the startest has been misused in all kinds of
> ways.
> This was a major point I tried to make in my talk about it. But
> IMO
> the solution is to write _more_ solid books like Suiter's, write
> _more_ webpages with even better graphics, and just keep trying to
> educate. The startest is a versatile and powerful tool when used
> correctly.
> Sincerely,
> Bryan Greer
> Columbus, OH
> (please remove *takeout* from my address for private replies)
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ar...@selena.kherson.ua
Derek Wong <daw...@earthlink.net> wrote:
> The star testing book is out there, and people will use it regardless
of
> what you do. I think that the Mak and refractor makers should talk to
> Dr. Suiter about the next revision of his book. I'm sure he could
give
> a clear and insightful explanation about why a highly corrected
Maksutov
> system can have an extremely high Strehl ratio and still have a
markedly
> asymmetric star test.
Excellent point, Derek. But when the second edition will
be available? I think it will be better if his detailed
explanations will be published in the S&T and then in his
second edition book. I am sure also that it must be much
more clear, than before, mentionad that the star test alone
is very unreliable in the hand of inexperienced persons and
100% reliably about given optical system quality one can
judge only with interferogram in hand.
> If there are doubters (ie. at the major star parties), do some
> comparisons between scopes. The best thing would be to bring one
scope
> with great optics and an asymmetric star test, and another with
mediocre
> optics and a near symmetric star test...then let people compare the in
> focus images of Jupiter or other objects.
This point is also excellent. I think that it is worth to create
specially designed/performed scope with identical diffr. patterns
but with huge amount of wave front distortion to compare it to
full wise-wersa scope. But is this not too high price for a few
mistakes of Mr. Suiter?
Take care,
Valery Deryuzhin
ARIES INSTRUMENTS Co.
Paul Hyndman
ar...@selena.kherson.ua wrote:
> This point is also excellent. I think that it is worth to create
> specially designed/performed scope with identical diffr. patterns
> but with huge amount of wave front distortion to compare it to
> full wise-wersa scope. But is this not too high price for a few
> mistakes of Mr. Suiter?
Hello Valery,
It would be very informative to have something along the lines of Peter
Ceravolo's test.
A quick question if I may... In an earlier posting to this thread, a
fixed spot secondary was mentioned. Does your 10" MCT use this or an
adjustable secondary?
Thanks, and Clear Skies!
Paul
--
Paul Hyndman pghy...@yahoo.com Madison, CT
ar...@selena.kherson.ua
bgreer@*takeout*fpi-protostar.com wrote:
> Valery wrote:
>
> >The reason is the existing of the Suiter's book and his followers.
> >
> I was not aware of a "following". How can I join?
Bryan,
You already in the first row on leader position.
> > know well that the assymmetry of a diffraction patterns does not
> >necessary mean any seriouse flaws in a telescope perfomance.
> >
> "telescope performance"? ALL telescopes, Valery?
If you will be focused on a subject of discussion and will
read my posts being less biased (not trying to hooking me on
a word), you will quickly find some thigs which escaped from
you:
1. I did meant the 10" F/14 Mak-cass .
2. We spoke not about simple parabolic mirrors or apos
where 3-rd order spherical must be corrected,
but about results which high order aberrations
can cause in a startest pictures.
> > But he also know well that most amateurs was so much spoiled
> > by wrong adressed Suiter's book, by numerouse "star test
> > experts" hypes.
> >
> "hypes"?
Correct! Just real hypes!
> Valery, I can see by your purposely vague, imprecise, and provocative
> wording that you have no real interest here in furthering this
> discussion in meaningful way. Reasonable men almost always find
> common ground, but I see that you're only interested in keeping a hot
> flame going on s.a.a. Good luck...you're on your own.
Bryan, I can't argue with you in fair way - your english is your
native, you can express yourself much better and correctly. Inlike
you, english for me is not native, I have a limited active dictionary
and therefore my expressions can looks too sharp - very often I can't
choose a right and appropriate words to be correctly taken. This is
first point you might keep in mind writing me your insulting letter.
You wrote "provacative" . OK, explain in what I was provacative?
From my side I can say - when you will run such kind of business
as I doing now, when the selling and therefore, a salary of many
peoples can be damaged by book's simply not full accomplishing,
by wrong understanding of a simplest things like one of your friend
already demonstrated here in his brilliant post ("5-th order
spherical must show in the interferometer 1/5 wave error"!!!!!)
etc etc, when all this will going to damage all YOUR efforts,
expectations and will going to ruin YOUR (YOUR!) business, then
I like to hear what kind of songs will you sing about ST method and
so called "ST experts".
Just cuiouse, why you didn't reply to above ST-expert nonsense?
Don't you think that a 4-th order spherical should show in any
proper interferometer a 1/4 wave errors or 3-rd order astigmatism
should show 1/3 wave error?
I am 200% sure you know what is wrong. And I am 300% sure you
know well where a real reason of such proclaimed nonsense lies.
But why you didn't correct him. Shame? Simply shame or another
reason? Clarify, please.
You wrote "imprecise".
OK, I see you do have enough time to spend it here for your
own self-confidence increasing among amateur astronomers.
I am free of this. My interest is lies in a plane practical
field - I am intereting that amateurs will be not spoiled
by different unwanted reasons like your songs about excellence
of this ancient as moon method, by wrong addressed book, by
"knoweleges" and "skills" of certain "ST-experts" etc etc.
Clear practical interest which coinside with real interest of
a real amateur sky observes, not "aberration spirit observers".
And my time and quite weak english does not permit me to be
"precise". If you like a precision, you are welcome - let speak
russian and then I will see your precison.
Another point. My todays business is somewhat clear for you.
I am more or less modest person and forgive me, please, my
unmodest to speak about myself - but you, Bryan, is responce to
push me on this, say, clarification. So...
I can assure you that your knoweleges in a wave front
analysis is simple grey dust vs mine. FYI, just yesterday's
morning I wote to Rich Wallen a private letter where I explain
him what I did in my pros astronomy career from 1982 till 1993.
I want that this will be unknown for amateurs here. But OK, I
see I need to let you and many others know: during these years
the astrophysics was only a fourth priority for me. My main task
(I were the principal person here)was the developing of a principally
different wave front analyzers for Soviet "Space Defence Initiative"
if to use Ronald's Reagan words. Second task was developing of
amathematical algorithms for an adaptive mirrors control - for wave
front correction when IR lazer beams pass trough earth atmosphere
to the aims (missiles) . Four differnt types of the wave front
analyzers were created and two different algorithms for adaptive
mirrors was developed as well. In the CAO still in use one of the
very first 48" MMT scope with segmented primary mirror. These segmets
correct a dusturbed (by earth atmosphere)wave front in a real-time
mode.
Also ARIES constantly support an long-time efforts of
Professor Valery Terebizh, who is a world level expert in
the image restoration, wave front modern analysis (for image
restoration tasks) and super-resolution (sub-diffraction)works.
I was very detailed informed about all these works, I was involved
in a numerouse discussions about all these subjects, so my level
of knowelege in a wave front analysis was at least a bit increased
again. For your info. Using 24" Zeiss reflector with wave front
quality RMS=0.038, working at average seeing 2", using also Meade
Pictor416 and the newest algorithms of a wave front analysis, newest
algorithms of image restoration, V.Terebizh team was able to
reach sub-diffraction resolution ( 0.08") in this given case.
I am glad that I was a little involved in this works too - this
increased my baggage of knoweleges a lot.
> Disappointing...
For your own self-confidence.
To all rest s.a.a. amateurs. Don't take me wrong. Honestly say,
I don't like to be considered as expert in any regards. I was
full up to the neck by such definitions during my pros carreer.
I was retired with pleasure and was able to dive back in to amateur
astronomy where I was absent from my first university course.
I like it very much, like in my youth, when my imaginations and
dreams about pros astronomy were clear and sweet. The reality was
almost 100% wise-versa. No any romantic. Sometime this job was
a real kind of a nightmares.
So, I like to be one of you, without any advance position.
This is my truly wish. No any hidden intentions etc.
Another deal is that sometime I loss my patience when I see
clear misleading and wrong understanding of a simplest, from
my point of view, things. It is especially pity that in many
cases my posts with explanations of my own point of view
was (and probably ever will) taken wrong - as attacks to a persons.
And the most pity I consider the fact that some persons clearly
wish to be always and each day considered as "experts" having
absolut no any solid basis for this, except owning some books
and OSLO.....
I know, that may be this letter will not add even one more
supporter to me, but can cause some more haters. May be from
personal iterests it will be easier to keep a silence, but I was
forced to protect myself and I was, at least, sincere in my
position.
And my advice to Bryan G. - take the S. book, OSLO and oculars, then
go to the Itek, the Perkin-Elmer, the Estman Kodak, the Tinsley lab,
the REOSK, the LOMO - where you wish or to any other leading optical
firm and try to establish your name as an optical experts there, not
here. When and if only you will have a success, let us know, please.
Clear sky to a real observers,
Valery Dryuzhin
ARIES INSTRUMENTS Co.
ar...@selena.kherson.ua
zgse...@netway.com wrote:
> Allan Rahill <afs...@ec.gc.ca> wrote:
>
> >> >
> >> > There is some ATM who claims they can figure a mirror by looking
at a
> >> > star test... I have a lot of difficulty to believe such claims...
> >> > because it is pratically impossible to tell where corrections has
to be
> >> > applied on the mirror surface to eliminate spherical
aberrations.... there
> >> > are too many possibilities.
>
> Well, as difficult as you might find it to believe such claims, most
> mirrors (all?) John Dobson made were figured by star testing. A lot
> of mirror makers I am acquainted with use the star test for final
> figuring -- bench testing can only get you so close. Carefully
> performed, the star test can tell you how much under/overcorrection
> (spherical aberration) you have and where on the mirror it lives.
> What more do you need to know?
>
> Regards,
> Gary Seronik
Gary,
I wish to know a real quality of these Dobs mirrors.
Not mirrors which was re-polished and re-figured then,
just mirrors made by ST control.
I am quite curiouse how difficlut poplese can leave
their misleading.
What kind of mirror precision we can expect from ST
if even much more quantitative Fucault one being
claimed 1/27, in reality, as some peoples agreed,
show only 1/5 or so.
Will I be too tolerant if I say that if a mirror will
come out from such figuring (with STest) as 1/4 wave,
then its real quality will be 1 - 1/2 wave?
Valery Deryuzhin
WHALEN44
I think it would be much better to bring two scopes that test around 1/8 wave
on the interferometer, with one that shows equal images on each side, and the
other that shows asymmetric star test.
Then we would truly see the difference if there is any. BUT WAIT, this was done
recently at Astrofest. There was 3 scopes there that tested by interferometer
around 1/8 wave P-V on the wavefront, with two having asymmetrical patterns and
one with close to symmetric patterns. And on both nights, after all scopes had
cooled down late in the evening, the one with the symmetrical star test showed
the best Jupiter and Saturn image. This was confirmed by several experienced
observers, opticians, dealers, and manufacturers. All had the same opinion.
This was after 5 hours of cooldown time.
And the scope that showed the biggest difference in breakout also had the least
desirable image,
though it had the smallest obstruction.
So go figure...........
And I find it interesting that at least one expert made a point about how
important it was to fully correct for 5th order and have symmetrical star test.
Now he seems to be singing a different tune..............
sing to the tune of the Stones " I can't get no satisfaction"
It goes something like:
I can't get no,... symmetric star test....
I can't get no,....symmetric star test...
Though I try, and I try, and I try, and I try....
I can't get no, I can't get no...
When I'm assembling others optics, working on my interferometer, and the man
comes on the phone to say that anything better will cost more dough, I can't
get no,
A no no no....... Hey hey, hey, that's what I say
etc. :-)
Well, time for my medication..........
Richard
Interesting.........
>If there are doubters (ie. at the major star parties), do some
comparisons between scopes. The best thing would be to bring one scope with
Dan Chaffee
wrote:
>Very interesting discussion but you bypass two important words.
>If you read carefully... I am speaking about spherical aberrations. I
>agree with you that star testing is a good test for astigmatism, turn
>down edge, zonal defects... etc. Star test is good also for low order
>spherical aberration but gives hard time for higher order one. There are
>many telescopes suffering of higher order spherical aberrations.
>I do have experience in figuring mirrors.
That the star test is quite sufficient for detecting lower order
spherical aberration is of ENORMOUS consequence to most
amateur mirror makers, since by far the most common scope
that they make, even if that were it's only virtue, which I think
most of us will agree is part of several advantages. Just ask John
Dobson, for whom it is the only test he uses to figure his mirrors. I
don't hear many criticisms of the images his mirrors produce. I'm am
in 100%agreement with Bryan's assessment on the benefits of this test
on the types of systems he cited.
Since I figure my mirrors with a bit more than half of my testing
on a star, the rest with foucault and ronchi readings, I can say
with certainty that my mirrors that test very close to identical
either side of focus perform very well on planets. I can't always say
the same thing about foucault testing, however. My point is that
most amateurs have no other recourse for evaluating the quality
of such scopes as newtonians and most refractors that is as sensitve
as the star test. Personally, I've yet to see a truely perfect star
test on any scope(doesn't mean I think there aren't any), but
obviously the test need not show perfect results on ANY system
to be a fine performer. If there is any "spoilage" that the hype
about the star test is responsible for, it is that people feel ripped
off for aquiring an instrument that doesn't test perfectly and yet,
a careful reading of Suiter's book openly exposes this problem. He
quite plainly states that almost all telescopes will show spherical
aberration(lower order), and that the test is almost too sensitve
to it.
As an experiment for my own purposes, decided to star test my
150mm f/8.4 before parabolizing it ( as a smooth sphere), in which
condition it has effectively slightly less than 1/4 WF
under-correction.The foucault is quite accurate in nulling a sphere,
so there is little to misjudge in this state. This afforded me with
the oppurtunity to look for the known amount of sphereical aberration
in the star test. The patterns on either side of focus were almost
devastingly different, such that even a beginner should have been
capable of readily discerning the difference. I recommend this
to anyone who is working on or posesses a 150mm f/8ish
spherical mirror.
D Chaffee
Kansas City
Dan Chaffee
Hi Valery,
>I wish to know a real quality of these Dobs mirrors.
>Will I be too tolerant if I say that if a mirror will
>come out from such figuring (with STest) as 1/4 wave,
>then its real quality will be 1 - 1/2 wave?
Is this a rhetorical question, or do you really want to know?
If you do, why not arrange to test one of Dobson's mirrors, or
have someone you trust here in the States do so with
a interferometer? Now, I don't know this man, but I suspect he
feels he has nothing to prove, and since the in focus images
are of high quality, would have no interest in what numbers
could be assigned to his handywork.
D. Chaffee
ar...@selena.kherson.ua
Dan McKenna <mck...@as.arizona.edu> wrote:
> We find that we can resolve better than 1/20 wave if the exposure is
> longer than 30 seconds.
Dan,
We reached around the same precision of a wave front restoration
(extraction) in similar system for automatic collmation of 2.6m
reflector. Similar wave front analyzer will be installed on the
1.8m Space Ultraviolet telescope with a active flexible mirror which
will change it figure by special actuators drived by powerful computer.
This also need not only a large expensive CCD, this need a fast right
algorithms of an image processing and a huge computing power.
We should not forget that we speak here about AMATEUR's ability
reliably estimate (extract) a wave front. Their brains can't average
an "images" and do a proper estimations of a brightness in a greed
points over a defocused image, they can't remember this data and then
precess it. If your system is able to reach around 1/20 precision
when all possible factors were taken in to account, with which portion
of scepticism we must consider "ST-experts" claims about their ability
to estimate a wave front errors with 0.1 precision (even in the case
with small amplitude high order aberration).
A certain persons here does has so biased minds, that they simply
unable to listen to others and finally understand that I am not
against of some methods of wave front analysis using ST approach -
how I can criticise the Space Telescope Institute huge work in
Hubble's PSF extracting using only one of two available on orbit
methods (this was similar to Dan's method)? This was necessary to
know (PSF) for Hubble's images restoration. A great number of very
qualified peoples was involved in this works.
But a field and simple "lab" conditions, equipment consisted
from an ocular, S. book in hands (if it there) and "expert's"
mind - all this arms simply does not allow to do more than
differ a very best scope from very bad one. Even in this case
with residual small amplitude high order aberrations one can't
do even such simplest differetiation, not speaking about reliable
wave front quality estimation.
References that this is only a single method to which a simple
amateurs has an access are wrong. This is illusion that they
does NOT has such access. To use a ST approach to evaluate a
wave front with necessary reliability and precision (0.1 wave),
they need to own similar to Dan's equipment and be able to use
it properly. Simple illusion and many (surprize!)would like to
live with such illusion and also some of them (with the most
biased minds and big Ego) would like be "experts" in STesting.
The last fact is especially worth of regret - speculations on
simple peoples unknoweleges are worth to regret in any field, not
only in notorious StarTesting.
Amateur observers, don't allow to some certain persons to spoil
you by a tales about ST method abilities in an amateur conditions!
If you would like to be an owner of a high quality scopes - make
your choise and deal with a reputable manufacturers which care about
their reputation at first of all. This will be the best warranty
what you can only have. Don't allow to such persons to spoil you
so much that you will sell your good scope(s) only because they does
not produce an identical diffraction patterns and/or if they does
has an aberrations which were estimated as unasseptable by cetain
persons - ST experts and folowers. Same with a telescope purchasing.
Do what you want to do - observe! Observe IN FOCUS !
Ocean779
>were a desirable thing, and it would be a great benefit for those of us
>who use it if we could learn more about behavior of a variety of
>telescope types. I'm learning a lot from this thread!
>
>
I agree, Chuck. I'm wondering if the APO makers have shied away from
explaining, if not discrediting, more publically Suiter's book because it might
sound like whining.
A lot of amateur astonomers are slaves of the star test because we've not been
told any different.
sean nolan
Derek Wong
>
> > I'm sure he could give a clear and insightful
> > explanation about why a highly corrected Maksutov
> > system can have an extremely high Strehl ratio and
> > still have a markedly asymmetric star test.
>
> Excellent point, Derek. But when the second edition will
> be available? I think it will be better if his detailed
> explanations will be published in the S&T and then in his
> second edition book.
OK, talk to him about S+T also. Or you, Roland, and others could write
your own article for S+T and perhaps a more technical version for
another magazine (ATMJ?)
> I am sure also that it must be much
> more clear, than before, mentionad that the star test alone
> is very unreliable in the hand of inexperienced persons and
> 100% reliably about given optical system quality one can
> judge only with interferogram in hand.
I have seen a few copies of interferogram reports on Markus' website,
although the ones for the Aries refractors have been removed. Since the
interferogram is done at one wavelength only, how does one use it to
test for chromatic aberration and spherochromatism? I would think you
would have to do multiple wavelength interferograms, and then integrate
the result with the visual response curve to get an overall measure of
visual quality (or same with CCD/film response). I am obviously not an
optician, so forgive me if my understanding is poor.
> But is this not too high price for a few
> mistakes of Mr. Suiter?
I think I will not answer this, as it will lead only to lots of
arguments which people do not want to read. I understand your position,
and I hope you can talk to Dr. Suiter.
Sincerely,
Derek
Derek Wong
> I think it would be much better to bring two scopes that test around 1/8 wave
> on the interferometer, with one that shows equal images on each side, and the
> other that shows asymmetric star test...
> And on both nights, after all scopes had
> cooled down late in the evening, the one with the symmetrical star test showed
> the best Jupiter and Saturn image.
> ...the scope that showed the biggest difference in breakout also had the least
> desirable image, though it had the smallest obstruction.
I think this comparison is very valuable because it shows the inadequacy
of saying "this is a 1/x wave scope". In this case, I assume the scopes
had differing rms values and that the best scope had the best rms due to
smoothness and/or higher order aberrations, right? I hope to see those
scopes at RTMC.
I would still like to see two of Roland's scopes--one with "normal"
aspheric curves and the other with the extra figuring and do a similar
comparison.
> It goes something like:
>
> I can't get no,... symmetric star test....
> I can't get no,....symmetric star test...
You must have cloudy skies like us :-)
Derek
lude...@my-deja.com
chri...@aol.com (Chris1011) wrote:
> >>
> Could you please explain me following:
>
> In early '90 Carl Zeiss jena invited me for several visit's at here
> optical factory to bring many diffrent type of telescopes for testing
> to see at which place are the Zeiss amateurtelescopes in the world.
> All this tests have been made only, I note only in douple pass sheme
> startesting and not in the intetrferometer.
> Many times I stayed by a Leica Masteroptician for testing diffrent
> telescopes and always he made the test in douple pass sheme
startesting.
>
> I can tell you, because I am friends with this master optician. I
have talked
> to him on many occasions. They did it that way because it was quick
and cheap.
both answeres are correct, thank you. But we have to add a third
answere :
Because it show them what they want to know, correct ?
Markus
>
> Roland Christen
> ASTRO-PHYSICS
lude...@my-deja.com
> > BTW I am on the waiting list for AP and would be more than happy to
have a
> > scope that is uncorrected for a symmetrical start test.
> >
> > John
>
> Hi John!
>
> I see one more stands in a line with us. Thanks for the help.
>
> Regards,
>
> Valery.
Hi Valery, Roland, ED and Yuri,
My personal feeling is, that some informations going out totaly wrong
to the public.
All you 4 guys saying that Startest is totaly invalid to estimate a
optical quality. This is wrong.
Correct is, that some systems will show with a 1/10 wavefront more or
less identical images and others show a diffrent image at 1/10
wavefront.
But no system will show a hudge diffrent in starttesting at 1/10
wavefront. People here , who believing your explanations maybe now
thinking, that it is possible, that a 1/10 wavefront system could be
able to show a hugde diffrence. It could and will show a hugde
diffrence in douple pass sheme Startesting , but only a smaller
diffrence under controlled startesting.
A aspherized Mak with interferometrical tested 1/10 wavefront will
never show a bad image, which could be rated by experienced startester
under controlled conditions as a bad 1/2 wavefront, but maybe as a 1/8
wavefront system, since this diffrence is very small small and very
difficult to estimate real accurate.
Please explain this to this customers and don't try , that the startest
is totaly useless, as you do now since a few days.
Markus
Mike Spooner
Derek Wong <daw...@earthlink.net> wrote in message
news:388ACC3D...@earthlink.net...
>
> I would still like to see two of Roland's scopes--one with "normal"
> aspheric curves and the other with the extra figuring and do a similar
> comparison.
Hope he would also have a time involved/price difference also.
>
> > It goes something like:
> >
> > I can't get no,... symmetric star test....
> > I can't get no,....symmetric star test...
>
> You must have cloudy skies like us :-)
>
Actually with the cloudy skies, my 'mirror in a minute' 8" f/1 planetary
newtonian provides a very similiar image to the best 20" Apple chromagnet
refractor. :))))
--Mike Spooner
lude...@my-deja.com
> I agree, Chuck. I'm wondering if the APO makers have shied away from
> explaining, if not discrediting, more publically Suiter's book
because it might
> sound like whining.
>
> A lot of amateur astonomers are slaves of the star test because we've
not been
> told any different.
>
> sean nolan
>
now here:
The first samples of new 4" Apos we got from Russia with
Interferometrical testreport made at green and red laser for same
optics comes out with turned edge and center zones, from bad design.
The testreport showed 1/4 wavefront and 1/25 RMS. The startest showed a
bad image since many light have been pushed into the diffraction rings.
During my visit after receiving this first optics, I gave to the
opticians the Book Startesting. They told me after looking a time to
the images, that they cannot confirm that all in this book is right,
but they agreed to do what I ask.I told them, please no more turned
edge and no more visible zones. The next lenses I received have been
figured and did show not anymore turned edge and no more zones. The
testreport results have been the same as before, which was our first
quality agreement. The Image under sky and in startesting was now much
better but the infocus image also have been improved and the optics
have been much sharper and could handle highe power.
So for this specific optics the Startest is valid , since figuring the
optics to show a better startest resultet in an overal much better
performance.
This show that the startest is valid to evoluate many optics , but not
all accurate.
lude...@my-deja.com
ar...@selena.kherson.ua wrote:
> In article <86bmad$dao$1...@nnrp1.deja.com>,
> ed_an...@my-deja.com wrote:
>
> > Hi Valery,
> > the only first five of ours 8" f/20 made in 1994 had the
> > secondary mirror as a spot, than we found that separate
> > secondary is more practical and has more advantages in
> > design, manufaturing and use - just imagine, after 5-10
> > years your customer will need to recoate secondary spot -
> > he should use a special mask and send to coating whole
> > meniscus, instead of only small secondary!
> >
> > Yuri
> > TEC
Yuri,
I have seen 20 years old and 40 years old Zeiss Mak with secondary
spot, which was never recoated but still looked like new. Maybe this is
a question of coating quality and where and how you store your scope ?
Markus
>
> Hello Yuri,
>
> A spot secondary does has it's own advantages and disadvantages.
> From my point of view in our design for the AP 10" MC the spot
> design does has advantages with weight more than it's disadvantages.
> This was Roland's point of view too.
> You know, of course, that a separate secondary is not a problem
> for us and we simply make our own choise with some ideas in mind.
>
> Valery.
> AIC.
John J. Kasianowicz
differences?
What provides the basis for thinking that a P-V measurement is sufficient to
completely describe an optic's quality?
WHALEN44 wrote:
> I think it would be much better to bring two scopes that test around 1/8
wave
> on the interferometer, with one that shows equal images on each side, and
the
> other that shows asymmetric star test.
>
> Then we would truly see the difference if there is any. BUT WAIT, this was
done
> recently at Astrofest. There was 3 scopes there that tested by
interferometer
> around 1/8 wave P-V on the wavefront, with two having asymmetrical
patterns and
> one with close to symmetric patterns. And on both nights, after all scopes
had
> cooled down late in the evening, the one with the symmetrical star test
showed
> the best Jupiter and Saturn image. This was confirmed by several
experienced
> observers, opticians, dealers, and manufacturers. All had the same
opinion.
[snip]
John J. Kasianowicz
Did he publish his findings anywhere?
Best,
John
Vahe Sahakian wrote:
[snip]
>One TEC owner, Eric Roel of Mexico, a pretty experienced
> telescope builder, went to great length to technically explain why these
> Maks do not behave according to the principals outlined in Suiter's
> book.
[snip]
John J. Kasianowicz
point here is whether a given person (e.g. an amateur astronomer at the EP
of a scope) can correctly interpret ST results.
No flame intended, but arguing that master optician X uses the ST method to
figure optics and master optician Y uses interferometric measurements to do
the same is probably not productive. Rather, it is important to understand
what each measurement process (ST, interferometry, etc.) can reliably tell
us about how well a given optic will perform under the conditions it's used.
Do you believe that there are there factors other than spherical aberration
that affect the quality of an in-focus image?
John J. Kasianowicz
John J. Kasianowicz <sur...@erols.com> wrote in message
news:86f2kc$q3s$1...@bob.news.rcn.net...
> Unless I've misinterpreted Roland's and others' comments in this thread,
the
> point here is whether a given person (e.g. an amateur astronomer at the EP
> of a scope) can correctly interpret ST results.
>
> No flame intended, but arguing that master optician X uses the ST method
to
> figure optics and master optician Y uses interferometric measurements to
do
> the same is probably not productive. Rather, it is important to understand
> what each measurement process (ST, interferometry, etc.) can reliably tell
> us about how well a given optic will perform under the conditions it's
used.
>
> Do you believe that there are there factors other than spherical
aberration
Sue_and_Alan
If I recall correctly, John makes somewhat slower mirrors (around f/6) than
most amateurs seeking large telescopes. It would be nice to have some idea
of how these mirrors actually tested by other methods. I asked a prominent
TN and author once about how John's mirrors tested, and he made a comment
suggesting my question was sacrilegious <G>.
John also benefits, I suspect, from having a lot more experience at this
sort of thing than many mirror makers.
Clear skies, Alan
Dan Chaffee wrote in message <388a8ba5...@news.gvi.net>...
> [SNIP] Just ask John Dobson, for whom it is the only test he uses to
> figure his mirrors. I don't hear many criticisms of the images his
> mirrors produce. [SNIP]
zgse...@netway.com
>In article <3888532c...@news.netway.com>,
> zgse...@netway.com wrote:
>> Allan Rahill <afs...@ec.gc.ca> wrote:
>>
>> >> >
>> >> > There is some ATM who claims they can figure a mirror by looking
>at a
>> >> > star test... I have a lot of difficulty to believe such claims...
>> >> > because it is pratically impossible to tell where corrections has
>to be
>> >> > applied on the mirror surface to eliminate spherical
>aberrations.... there
>> >> > are too many possibilities.
>>
>> Well, as difficult as you might find it to believe such claims, most
>> mirrors (all?) John Dobson made were figured by star testing. A lot
>> of mirror makers I am acquainted with use the star test for final
>> figuring -- bench testing can only get you so close. Carefully
>> performed, the star test can tell you how much under/overcorrection
>> (spherical aberration) you have and where on the mirror it lives.
>> What more do you need to know?
>>
>> Regards,
>> Gary Seronik
>
>Gary,
>
>I wish to know a real quality of these Dobs mirrors.
It will vary from optician to optician.
Regards,
Gary Seronik
(Remove the "z" for my actual e-mail address.)
zgse...@netway.com
My response was to an individual who was disputing that it was
possible to figure a mirror using the star test. It is possible, as
John Dobson (and many others) prove. This would suggest that John
Dobson (and many others) are able to interpret the star test with
sufficient success to figure mirrors of very high quality.
"John J. Kasianowicz" <sur...@erols.com> wrote:
>Unless I've misinterpreted Roland's and others' comments in this thread, the
>point here is whether a given person (e.g. an amateur astronomer at the EP
>of a scope) can correctly interpret ST results.
>
>
><zgse...@netway.com> wrote:
>> Well, as difficult as you might find it to believe such claims, most
>> mirrors (all?) John Dobson made were figured by star testing. A lot
>> of mirror makers I am acquainted with use the star test for final
>> figuring -- bench testing can only get you so close. Carefully
>> performed, the star test can tell you how much under/overcorrection
>> (spherical aberration) you have and where on the mirror it lives.
>> What more do you need to know?
Regards,
Gary Seronik
(Remove the "z" for my actual e-mail address.)
Vahe Sahakian
> Did he publish his findings anywhere?
> >One TEC owner, Eric Roel of Mexico, a pretty experienced
> > telescope builder, went to great length to technically explain why these
> > Maks do not behave according to the principals outlined in Suiter's
> > book.
John,
No, as far as I know he did not publish the Maksutov star test behavior
anywhere.
I was faced with a dilemma, ordering a fairly expensive ota without the
benefit of even one look thru the instrument is always an awfully risky
proposition, so my only option in this case was to contact various
owners of this particular Mak and seek advice. Eric sent me an E-mail
outlining the pros and cons of TEC Maks, and as I said before, he had a
word of warning coupled with detailed explanation as to what to expect
from star test.
The current discussion "Defending the Startest" has been very
educational for me, in fact I am printing some of the posts for
reference.
Thanks,
Vahe
ar...@selena.kherson.ua
Paul Hyndman <pghy...@my-deja.com> wrote:
> Hello Valery,
>
> It would be very informative to have something along the lines of
Peter
> Ceravolo's test.
>
> A quick question if I may... In an earlier posting to this thread, a
> fixed spot secondary was mentioned. Does your 10" MCT use this or an
> adjustable secondary?
>
> Thanks, and Clear Skies!
>
> Paul
Hello Paul,
We discussed this question with Roland Christen before
we started to produce a systems. As you know well both
designs with spot secondary and with separate secondary
does has their own advantages and disadvantages. There are
different points of view on this subject.
The main complain of earlier posts (since several years)
for a spot secondary was misleading that this design is
necessary suffer from coma. In our systems is is not the
case. Our systems are free of coma and does has 50mm circle
of diffraction limited images.
We agreed with Roland that in our pursuing for a highest
perfomance for the high resolution works we must choose
the design with smallest possible c.o. The secondary cell
will always add several % to the c.o. value and therefore
will cause small contrast decreasing. The main advantage
of the secondary mirror is a one more freedom in the design
and possibility to use a secondary for telscope collimation.
But our design does leave nothing to wish better and therefore,
this main advantage of a separate secondary was disappered.
But in trade off we do have a zero down-turned edge on the
secondary (it's precision is probably 1/50 P-V if not better)
and less central obstruction as like total absence of a possible
heat source from separate secondary and it's cell.
Forget all ancient tales about spot significant disadvantages,
at least in our systems.
Clear sky,
Valery Deryuzhin
ARIES INSTRUMENTS Co.
lude...@my-deja.com
coma. In our systems is is not the
> case. Our systems are free of coma and does has 50mm circle
> of diffraction limited images.
Valery ,
could you please explain how big is the circle of unvigneted field and
how you will do a 50 mm photographic circle in the mechanics without
very big primary baffle who will support big straylight ? 50 mm
imagecirle unvigneted and no straylight is even in a 12" very difficult
with this small central obstruction. Thanks for your answeres (it is a
question, not a flame, okay ?)
thanks
Markus
ar...@selena.kherson.ua
zgse...@netway.com wrote:
> >Gary,
> >
> >I wish to know a real quality of these Dobs mirrors.
>
> It will vary from optician to optician.
> Regards,
> Gary Seronik
Gary,
You did not understood me correctly. I wish to know
the real quality of THESE mirrors which you mentioned
- made by J.Dobson and figureg using a StarTest only.
Valery.
ar...@selena.kherson.ua
"John J. Kasianowicz" <sur...@erols.com> wrote:
> Unless I've misinterpreted Roland's and others' comments in this
thread, the
> point here is whether a given person (e.g. an amateur astronomer at
the EP
> of a scope) can correctly interpret ST results.
>
method to
> figure optics and master optician Y uses interferometric measurements
to do
> the same is probably not productive. Rather, it is important to
understand
> what each measurement process (ST, interferometry, etc.) can reliably
tell
> us about how well a given optic will perform under the conditions it's
used.
John,
Don't you think you will be absolut not interested to know
what the manufacturer will told you how good the optics WAS
at final testing/certification in HIS lab conditions using
ST as reliable method :-) and/or Zigutto waves when you see
that your scope does has, say 2x less quality than what you
pay for?
Valery Deryuzhin
ARIES INSTRUMENTS Co.
> Do you believe that there are there factors other than spherical
aberration
> that affect the quality of an in-focus image?
>
> <zgse...@netway.com> wrote:
> > Well, as difficult as you might find it to believe such claims, most
> > mirrors (all?) John Dobson made were figured by star testing. A lot
> > of mirror makers I am acquainted with use the star test for final
> > figuring -- bench testing can only get you so close. Carefully
> > performed, the star test can tell you how much under/overcorrection
> > (spherical aberration) you have and where on the mirror it lives.
> > What more do you need to know?
>
>
John J. Kasianowicz
I don't doubt that a ST can help figure a mirror. What I don't know is
1) whether the opticians use the method quantitatively (i.e. can I as the
consumer obtain a written metric of performance of an optic I want to buy),
2) when and why does it fail,
3) is it superior to other methods (e.g. interferometry) for detecting all
aberrations?
Best,
John
WHALEN44
I don't know about the scope that had the best image, never saw an
interferogram, just basing quality om manufacturers word. All three scopes had
very smooth optics, better than 1/40 wave rms,
the difference was not due to huge differences in smoothness.
The difference in image quality from the best to the least desirable was not
all that close. It seemed to me to correspond quite well to the difference in
breakout.
>did the scopes have other types of interferometrically measurable
>differences?
>
>What provides the basis for thinking that a P-V measurement is sufficient to
>completely describe an optic's quality?
Richard Whalen
whal...@aol.com
"Time spent observing the heavens is not deducted from your lifespan"
ap...@my-deja.com
Can you post how to do the artificial star test with double pass
and Cerovolo system?
BTW, is optical quartz(natural) homogeneous enough to make a reference
element?
Al
> Since Suiter is not here on SAA, I have sent to Mr Greer an actual
> interferogram with resultant star test images of a 1/10 wave P-V, 1/50
wave RMS
> optic that shows quite dramatic difference in the intra vs outer star
pattern.
> It fits Suiter's 1/4 wave criteria, but is indeed 1/10 wave (actually
1/11
> wave). It also performs flawlessly on the planets and splits doubles
with
> exceeding ease even on nights of poor seeing.
>
> I hope he doesn't have to eat his shoes.
>
> Roland Christen
> ASTRO-PHYSICS
ed_an...@my-deja.com
ar...@selena.kherson.ua wrote:
> We discussed this question with Roland Christen before
> we started to produce a systems. As you know well both
> designs with spot secondary and with separate secondary
> does has their own advantages and disadvantages. There are
> different points of view on this subject.
> The main complain of earlier posts (since several years)
> for a spot secondary was misleading that this design is
> necessary suffer from coma. In our systems is is not the
> case. Our systems are free of coma and does has 50mm circle
> of diffraction limited images.
> We agreed with Roland that in our pursuing for a highest
> perfomance for the high resolution works we must choose
> the design with smallest possible c.o. The secondary cell
> will always add several % to the c.o. value and therefore
> will cause small contrast decreasing. The main advantage
> of the secondary mirror is a one more freedom in the design
> and possibility to use a secondary for telscope collimation.
> But our design does leave nothing to wish better and therefore,
> this main advantage of a separate secondary was disappered.
> But in trade off we do have a zero down-turned edge on the
> secondary (it's precision is probably 1/50 P-V if not better)
> and less central obstruction as like total absence of a possible
> heat source from separate secondary and it's cell.
>
> Forget all ancient tales about spot significant disadvantages,
> at least in our systems.
>
> Clear sky,
>
> Valery Deryuzhin
> ARIES INSTRUMENTS Co.
Hi Valery,
If you do not mind, I would add my poit of view.
We do use currently a separate secondary for 8-10" MAKs
for the following reasons:
1. (+) Yes, we have more freedom in design - one 8"
meniscus could be used for a few telescopes with different
f/ratios: f/3.5, f/4.0 (this model was made only as a
prototype and was not practical thanks to it's aspherical
primary), f/11, f/20 and one more f/15.5 was added by
customer request, so we can count here the separate
secondary as a (+)
2. (-) As a (-) it is an additional optical part and few
mechanical parts, assembling time, additional aligning...
3. (+) The other (+) of separate secondary is it's easier
coating/recoating - the meniscus is not involved here, small
secondary is easier to handle.
4. (-) Bigger obstruction (incl. secondary cell) - not
always, the f/20 has less than <22%, f/15.5 - less than 27%
5. (+) But the secondary cell let us to have better baffling -
more efficient cut of unneeded light - to do so in other
design will require bigger baffle on the primary mirror...
6. (-) Turned edge on the separate secondary - not noticable
- the whole system usually 1/8-1/10 or better....
7. (+) Stress from meniscus under operating condition
never affects the separate secondary mirror surface - this
stress could affect the secondary spot surface more then a
turned edge - it is very important (+).
8. (-) Secondary with a cell as a possible heat source (-: -
it is not more heat source, than a removed cenrtal part of
the meniscus with perforation.
Finally, I agree with you - both designs : with separate
secondary or with secondary as a central part of meniscus
have their places and it is a designer choice (not easy one).
Yuri
John J. Kasianowicz
Hi Valery,
Results of star testing and interferometry from a manufacturer aside, the
only criterion I can place on an optic (in an OTA) is its performance under
real observing conditions. If the seeing is excellent and the scope cannot
perform well in focus (i.e. if stars are not points, planets are not
resolved well and lack contrast, the moons of Jupiter are not solid disks,
etc.), I certainly wouldn't buy it. A premium optic must deliver premium
quality images.
That said, anyone who pays a lot of hard-earned money on a premium optic
most likely wants some form of certification of the instrument itself and of
the instrument design (I certainly would). It'd be nice to have quantitative
estimates of how the scope performs under standard test conditions (i.e. how
much does the optic suffer in each of the textbook aberrations: spherical,
coma, astigmatism, curvature, distortion, and chromatic). it would be
comforting to know that the results of the test are traceable to a standard.
Someone here stated that John Dobson used star testing to help him figure
some mirrors he made. I don't know if that's true, but it's certainly
conceivable. It's also conceivable that it is a quick and reasonable method
to detect certain aberrations. Whether the receiver of the optic got a
written certification of the results was not mentioned. I'd guess the answer
is no. That would not make me happy if the mirror was expensive.
As a manufacturer of top-quality optics, what specifications do you think
are the most important for a consumer to look for? What measurements do you
think provide the most reliable estimates for each of the classic
aberrations? What measurements do you perform on your optics and what do
these tests reveal (i.e. what aberrations do they test for)? Finally, why
does the Star Test "fail" when it is used to "evaluate" a premium Mak-Cass?
I felt comfortable buying an fast APO from AP a few years ago because they
published P-V, RMS, Strehl ratio values as well as a color error curve in
their catalog (actually an earlier catalog had curves, a more recent one had
a written specification instead). I assumed, I believe correctly, that the
specs were meant to be representative of all their scopes, not just of
hand-picked ones from the best batch. I honestly don't know if my particular
objective is slightly better or worse than the published specs, but the
scope is essentially color free and produces wonderful images. I'm happy
with the instrument.
I spent some time looking through Roland's 10" Mak-Cass scope at AstroFest
99. It nearly knocked my socks off (ya dumayo schto eta ocheyn krasivie
teleskope!; it's hard to transliterate Russian characters w/an English
keyboard). Frankly, I was shocked at how well a Cassegrain reflector could
form such tack sharp and contrasty images (I kept going back to the line
waiting to use this scope). Although the temperatures did not get low enough
to really test the instrument's thermal equilibration time constant the
scope stabilized quickly and outperformed other reflectors in its aperture
class (I think the temperature dropped into the low 40s Fahrenheit, which
was more than cold enough for me because I forgot my jacket at home in the
Washington, D.C. area!). I certainly want a moderate aperture scope (10"-12"
diameter) that can perform this well.
If you want to send me a top quality scope to visually test for a few years,
I won't complain. ~'8^)
Best,
John Kasianowicz
Valery wrote:
> John,
>
> Don't you think you will be absolut not interested to know
> what the manufacturer will told you how good the optics WAS
> at final testing/certification in HIS lab conditions using
> ST as reliable method :-) and/or Zigutto waves when you see
> that your scope does has, say 2x less quality than what you
> pay for?
JMc
Mel Bartels is famous for doing impromptu star tests, and with his
experience, I'd probably find it hard to call his evaluation of any
telescope "BS".
This thread is very interesting because we apparently see manufacturers
telling us:
1) Interferometry is good, gives quantitative results, and we can do it
indoors all day long. We test each system by interferometer, but don't
reveal the results to our customers because they may not understand them...
2) Customers shouldn't rely on the star test because its just sooo
complicated, and they can't possibly understand the results, especially
for our new line of products no one has really seen yet.
3) H.R. Suiter must die! (OK, just kidding on that one)
It seems they're telling the only thing we're competent at is mailing
them a check!
Jim McSheehy
> Mark D'Ambrosio wrote:
>
> I have no idea, but to walk up to a telescope, and
> within 5 seconds, say that a scope has such, and such wavefront error,
> is IMO full of BS.
> With even interferometer testing varying, how can someone walk up, and
> make these claims?
> As far as identifying a poor optic, yes, I most definitely agree it can
> be spotted visually, but to judge one that's exceptional, and put a
> number on it, I don't think so! Mark
bratis...@my-deja.com
chri...@aol.com (Chris1011) wrote:
> When the optical system gets more complex than a simple parabolic
> mirror, then there are inherent aberrations that affect the star test.
An
> example is the Maksutov Cassegrain.
This is quite true, but even those compound systems follow the same laws
of physics/optics. Fifth order spherical is just as damaging in a
Newtonian as it is in a Mak, and just as detectable. Its contrubution on
a final wavefront will be just the same - regardless of the optical
system.
> As an example, I have recently finished exhaustive tests of different
10" F14
> Mak-Cass systems, some with these inherent aberrations left in, some
with them
> meticulously removed. All the optics tested between 1/10 and 1/12 wave
with
> Strehl ratios of 98% or better. The system with the uncorrected or
pure
> Maksutov curves had the central hole of the donut break out 4 times
farther on
> one side of focus than the other. It would be judged by the Suiter
star test
> method as being maybe barely 1/4 wave, if that.
Let me guess - this was with a small obstruction Mak ? Suiter's rule of
'2X distance for breaking out the central hole' is valid ONLY for
33% obstruction. I was dismayed when I tested my own Mak (spot, so it
did need aspherization) and discovered that doughnut would also break
out more than 4 times the distance in one direction even after
autocollimated null was quite good; but when I installed a proper 33%
mask (instead of a 24% what my secondary spot in reality is), it easily
fell into '2X rule'.
I agree that star testing long focal ratio, low obstruction instruments
differs SIGNIFICANTLY from a typical run of the mill fast Newtonian or
APO. Long f/ratio and small obstruction makes star test EXTREMELY
sensitive - even smallest errors stand out like sore thumb. (that is one
of the reasons for many commercial scopes to have quite large
obstruction - it makes them much easier to 'pass' the star test with an
inexperienced star tester). But it can be done, and it can be learned.
And Suiter's book is still valid.
But to cut to the cheese - we all have copy of Aperture. Why don't give
us an example of a 1/10 wave optics with higher order spherical that
will cause doughnut to break out 4X as far on one side with 33%
obstruction ? That will prove once and for all that Suiter's rule is
invalid. Until then, I remain unconvinced.
Bratislav
Mark D'Ambrosio
Perhaps my first post was a bit harsh.
But I still have to state, that to walk up to a scope, and within 5-6
seconds time on a star test, and then make the claim "Oh, it's 1/6 wave
PV" is very, very hard for me to believe.
I do belive that an experienced star tester will within this time frame
know if it's a peach, or a lemon, but to give reasonably accurate
numbers, no, I don't believe so.
More time will be needed (As Richard has told me) to more accurately
judge a good optic.
Sorry for my spur of the moment earlier post, (With poor English :-))
I should have spent a bit more time writing it. Mark