Schiefspiegler-Project
x
I'm going to construct and build a Kutter-Schiefspiegler plus mounting and
just would like to hear your opinions and comments on such a project.
The instrument will be a Kutter-Schiefspiegler consisting of three optical
elements (primary, secondary and correction lens) with a clear aperture of
250mm and a focal length of 5000mm. Therefore the overall length of the
instrument will be something arround 2300mm. Mirror material will be
zerodur to keep the waiting time before actual observation to a minimum.
The tube, the mirror cells and lens cell will be fabricated out of a
carbonfiber reinforced material to keep the weight down. Metall will only
be used where it is absolutely necessary.
The mounting will be fabricated out of the same material. I'm not really
settled on the type of mounting now. I'm thinking of a german or fork type.
I seldom hear of somebody building his telescope out of this material.
Has anybody comment abouts pitfalls to beware of?
For instance are there any problems to be expected when gluing/laminating
any metal parts to the carbonfiber stucture (thermal expansion
coefficients)?
What about disadvantages of the carbon/epoxy-structure compared to say an
aluminum tube? Perhaps a longer waiting period before thermal equilibrium
is reached?
Another problem I'm interested in, is the mirror cell of the secondary. Is
it sufficient to hold the mirror just with three clamps at the edge like in
conventional mirror cells as this mirror is "hanging" in his cell?
I like to keep such an instrument transportable. Therefore the light-weight
approach. Do you think this is practicable with an instrument 2.3m long?
Thanks for any comments, hits, suggestions, etc.
WG
bratislav
> Thanks for any comments, hits, suggestions, etc.
Join the ATM list. Too many experts on s.a.a. that have never made
a telescope.
Send mail to 'subscribe atm' in the body of the message to
majo...@shore.net
Bratislav
LAnce
scope?? I don't know what kind of scope heard it's using folded optics
technology izzit?
jerry warner
brief.
You have a considerable project there. By all means get atm help inaddition to
any
here. My instinct says you have not done much building before and therefore the
raw
appeal of the Kutter-Scheifspglr,but in fact may be more thanyou bargain for,
and
if results dont please a large dissappointment. Perhaps to start smaller
schief. and work
up and with others who have actual experience with same. I would definately
recommend
starting smaller to begin with. Thermal expansionof the graphite laminates
depands entirely onthe bonds and therein is the whole trick. The bonds mustbe
precise to absorb
stress and survive while also retaining mechanical-optical integrity. This can
be done, but
... you are talking about some pretty costly outlays here in appropriate
graphite materials. I assume you know this. I am sure some atms have
experimented along these lines, but
for this project you need some professional advice also. Consult the aero
designers
nd bicycle builders who work daily in these materials, but be forewarned: this
is no
'bicycle' you are building and bike specs will not satisfy optical demands. You
need
a different type of 'rigid flexibility' in optical mechanics vs bikes or
aeroplanes and
again the 'bonds' are crucial is satisfying 'optical-mechanical' stress
requirements ...
these bonds must survive and perform over time greater than the average bike or
aero.
is designed for. In my experience this is where the weakness of bonds starts to
show
in little more than one year. That does not meet optical/mechanical
requirements.
Again a smaller testproject is well-advised in thse materials applied to
optical work.
The three claw cell you ask about will not suffice in that format (for the size
you have
in mind), in my opinion. But, a graphite housing and retainer rings is an
intriguing
possibility, but ... worth the cost?
I understand and sympathize inyour attempt to meld low thermal expansion, high
strength,
proper flexibility, portability, etal into a neat Kutter-Scheifspglr package
.... at appropriate
cost? I think you need some solid professional advice. Good Luck.
J. Warner
x wrote:
> Thanks for any comments, hits, suggestions, etc.
>
> WG
>
> gr...@biochem.mpg.de
x
<jwa...@idtdna.com> wrote:
> Brat says 'too many experts' here. I wouldnt know about that, but I'll be
> brief.
> You have a considerable project there. By all means get atm help inaddition to
> any
> here.
Thanks! I got this advice the third time now ... :-)
I'm not on the mailing list right now, but I've browsed the ATM-archives.
> My instinct says you have not done much building before
My current experience is limited to building one of those standard 6"
newtonians and several simple german mounts. But I've got some experience
with fiber reinforced material. Therefore I don't consider the problems
related to building the tube assembly major obstacles.
> and therefore the
> raw
> appeal of the Kutter-Scheifspglr,but in fact may be more thanyou bargain for,
> and
> if results dont please a large dissappointment.
I've seen what can be achieved by such a system, therefore I think I will
not be dissappointed. I will not grind the optics by myself, though another
problem I don't have to worry about. The "only" part left to me will be the
tube assembly to hold the optics in place. And this system shouldn't be
much more difficult than that of a Schmidt-Cassegrain.
> Perhaps to start smaller
> schief. and work
> up and with others who have actual experience with same. I would definately
> recommend
> starting smaller to begin with.
> Thermal expansionof the graphite laminates
> depands entirely onthe bonds and therein is the whole trick. The bonds mustbe
> precise to absorb
> stress and survive while also retaining mechanical-optical integrity. This can
> be done, but
> ... you are talking about some pretty costly outlays here in appropriate
> graphite materials.
The overall cost of the carbon fabric (approx. US $ 200,-) + epoxy resin is
minor compared to the cost of the optics (approx. US$ 3000,-).
-snip-
> The three claw cell you ask about will not suffice in that format (for the size
> you have
> in mind), in my opinion.
Are you sure? I asked for the support of the secondary which has a diameter
of approx. 12cm. The primary will of course have a floating 9-point
support.
> But, a graphite housing and retainer rings is an
> intriguing
> possibility, but ... worth the cost?
As I said above: the cost of carbon-fabric is not the problem. Every
ready-made aluminum mirror cell will cost two- or three-times as much.
> I understand and sympathize inyour attempt to meld low thermal expansion, high
> strength,
> proper flexibility, portability, etal into a neat Kutter-Scheifspglr package
> .... at appropriate
> cost?
I think cost is not the point. I see more problems in technical obstacles.
Thank you for your comments/advice!
WG
mes...@my-dejanews.com
Look at
http://www.seds.org/~spider/scopes/schief.html
and links therein - pics and info.
- hartmut
-----------== Posted via Deja News, The Discussion Network ==----------
http://www.dejanews.com/ Search, Read, Discuss, or Start Your Own
Peter Natscher
to believe that a Schief performs as well as a Newtonian 1.8X its aperture size.
I think that's crazy. There are Newtonian's that perform well enought to be
approaching close refractor status, even with 20% secondary obstruction.
Suiter's book on "Star Testing Astronomical Telescopes" says that any telescope
that employs a secondary with 20% primary mirror obstruction has a neglegible
effect on observing. Aperture rules. Two telescopes with equal aperture that are
well made will perform closely—one won't be so much better than the other as to
be in a class with another telescope 1.8X its size. That's a stretch.
It looks as if the Schief design leaves it very vulnerable many undesirable
effects,
namely:
surrounding air temperature/turbulence effects
neighboring person's temperature effects (at star parties)
scattered light
limited small field of view
no photography use
Peter Natscher
Belmont, California
mes...@my-dejanews.com
> I looked at the Schief web site
[http://www.seds.org/~spider/scopes/schief.html]
> and found it to be stretching one's imagination
> to believe that a Schief performs as well as a Newtonian 1.8X its aperture
> size.
> I think that's crazy. There are Newtonian's that perform well enought to be
> approaching close refractor status, even with 20% secondary obstruction.
> Suiter's book on "Star Testing Astronomical Telescopes" says that any
> telescope
> that employs a secondary with 20% primary mirror obstruction has a neglegible
> effect on observing. Aperture rules. Two telescopes with equal aperture that
> are
> well made will perform closely-one won't be so much better than the other as
> to
> be in a class with another telescope 1.8X its size. That's a stretch.
Not so, because of imaging effects. What you say applies (almost) for light
gathering capability (i.e., limiting "faintness" of stars which can be seen).
Well, and of course it is not a matter of belief.
However, it does not apply for what Anton Kutter, inventor of the Schief,
called "image definition", or resolution power, or sharpness. Simply, the
obstruction causes a deformation of the stellar diffraction images by
deforming them from "ideal" small circles with rings, first by pushing light
into the first order Newtonian ring, and second by adding non-rotational
symmetric features. Besides perhaps bringing some few on-the-threshold weak
stars "below" limit, this makes the "point" images larger, so that close
binaries, or close features on planets or moon will no more be separated.
Kutter, at about 1960, estimated this effect to reduce the resolution power
of a Newtonian or (classical) Cassegrain-type scope to that of an
obstruction-free optics of only half aperture - a value which I personally
think is a bit exaggerated, but ..
> It looks as if the Schief design leaves it very vulnerable many undesirable
> effects,
> namely:
> surrounding air temperature/turbulence effects
> neighboring person's temperature effects (at star parties)
> scattered light
> limited small field of view
> no photography use
Hmm, some of the best earth-bound lunar and planetary photos, as well as some
of the best amateur photos of galaxies, have been accomplished with Schiefs.
Look e.g. at Bernd Flach-Wilken's photos exhibited online at
http://www.astro-shop.com/AM13/AM13.html
To say it in short, a (well-aligned) Schief is optically as good as a very
good refractor, for a very much lower price. Only limitation: Large-field
deepsky (same as Cassegrain-type scopes or long-fl refractors). The Schief
owners I know (including myself) have no significant problems handling the
issues you mention.
OTOH, I certainly admit that owners and fans of such odd-type scopes may be
a bit odd-type as well ;-)
x
> I looked at the Schief web site and found it to be stretching one's imagination
> to believe that a Schief performs as well as a Newtonian 1.8X its aperture size.
> I think that's crazy.
True, I wouldn't confirm such statements.
> There are Newtonian's that perform well enought to be
> approaching close refractor status, even with 20% secondary obstruction.
> Suiter's book on "Star Testing Astronomical Telescopes" says that any telescope
> that employs a secondary with 20% primary mirror obstruction has a neglegible
> effect on observing. Aperture rules. Two telescopes with equal aperture that are
> well made will perform closelyone won't be so much better than the other as to
> be in a class with another telescope 1.8X its size. That's a stretch.
You are ok by me on this point. :-)
Just two points to mention:
- it is very difficult to fabricate the mirrors for fast big newtonians
like they are common in current dobson designs. Therefore it is more likely
that you get a non-diffraction-limited system. In Schief-designs only
mirrors with long radii of curvature are needed. These can be produced with
relative ease with the required accuracy.
- if we compare the diffration-limited field of a moderatly fast newtonian
(d=255mm, f=1875mm, f/7) we find it to be a little more than 4mm. Compare
this now to a Tetra-Schief with equal aperture: 40mm! If you would like to
have such a field with the 255mm newtonian your ocular would roughly be 3m
above ground!
> It looks as if the Schief design leaves it very vulnerable many undesirable
> effects,
> namely:
> surrounding air temperature/turbulence effects
> neighboring person's temperature effects (at star parties)
True, but I think open truss tube newtonians have the same problem. BTW
there are also closed tube variants of schiefs.
> scattered light
This is only important for Yolo-Systems! Scattered light is really no
problem for Kutter or Tetra-Schiefs.
> limited small field of view
This applies to Kutter-Systems only.
> no photography use
Sorry but this is simply not true! At least for planets you've got all
advantages you can think of: a minimum of optical surfaces involved, a
(nearly) aberration-free field to the edges, a long focal length (no barlow
etc. needed) and no cromatic aberrations!
If you think of the tilted optical plane of the Kutter-systems: it is no
problem to tilt the photographic elements (film, ccd, etc.) those 5-9
degrees.
Just have a look at some images taken with a good Schief.
WG
Dusty Bleher
your Carbon Fiber usage query. I too had pretty well decided upon a CF
substructure for my next scope (in the absence of other more interesting
alternatives). It seems to me that it might well yield some pleasant
construction characteristics and advantages.
I had planned on making models of my parts out of some easily malleable
plastic material, and then using those to make molds for casting w/CF
composite materials.
TIA,
Dusty
San Jose, CA
x wrote in message ...
Bratislav
> Hmm, some of the best earth-bound lunar and planetary photos, as well as some
> of the best amateur photos of galaxies, have been accomplished with Schiefs.
> Look e.g. at Bernd Flach-Wilken's photos exhibited online at
>
> http://www.astro-shop.com/AM13/AM13.html
Hmm... some of the best earth bound lunar and planetary photos have been done
with OBSTRUCTED telescopes. See Dragesco's book, Viscardi's atlas or ALPO web-site.
Or compare Legault's image of Saturn ( http://perso.club-internet.fr/legault/)
with Bernd-Flash's. Thiery used (HEAVILY) ostructed SCT. To my eyes, his image
is better. What do you say ?
> To say it in short, a (well-aligned) Schief is optically as good as a very
> good refractor, for a very much lower price.
To say in short, I have a friend who is a master optician and Australia's leading
planetary observer, who has made several Schiefspieglers (his planetary scope
is 12" Schief), and in his own words, refractors leave Schiefs "for dead".
Of course, he is exaggerating a bit but I can also confirm that 4" Schiefspiegler
performs significantly worse on planets than a 4" achromat (made by same person, by
the way). A (good) 4" APO would make that difference even more obvious.
Obstruction is only half of the story. There are other aspects of planetary telescopes
that may make certain design a winner or loser. Schiefspieglers are nice planetary
telescopes, but not very cost effective. If you don't believe me, just ask Don Parker
and Isao Miyazaki. And they CERTAINLY don't have any magic properties to bend laws
of physics and optics.
Bratislav
x
> > To say it in short, a (well-aligned) Schief is optically as good as a very
> > good refractor, for a very much lower price.
>
> To say in short, I have a friend who is a master optician and Australia's leading
> planetary observer, who has made several Schiefspieglers (his planetary scope
> is 12" Schief), and in his own words, refractors leave Schiefs "for dead".
> Of course, he is exaggerating a bit but I can also confirm that 4" Schiefspiegler
> performs significantly worse on planets than a 4" achromat (made by same person, by
> the way). A (good) 4" APO would make that difference even more obvious.
>
> Obstruction is only half of the story. There are other aspects of planetary telescopes
> that may make certain design a winner or loser. Schiefspieglers are nice planetary
> telescopes, but not very cost effective.
I can't agree here, compared to achromats and even more to APOs they are
_very_ cost effective!
> If you don't believe me, just ask Don Parker
> and Isao Miyazaki. And they CERTAINLY don't have any magic properties to bend laws
> of physics and optics.
I simply could not see the dissadvantage of the Schief-design.
-They use (in the simplest from) only two spherical mirrors, no chromatic
elements, no aspherical deformed surfaces or any other difficult to
fabricat
optical elements.
-High reflectivity coatings can be applied to all optical surfaces to avoid
light loss (if desired dielectric coatings with _very_ high reflectivities
can
be used).
-They avoid the central obstruction by tilting the primary.
-Both mirrors are used to correct for image aberrations and the residual
errors
are reduces to a dimension smaller than the airy-disk by a long focal
length.
-Closed tube assemblies could be built to avoid air currents.
Therefore I can't see any reason why such a sytem should be inferior to an
achromat or apochromat of the same aperture. I find it even the other way
round: those systems are completely free of any chromatic aberration!
Depending on the coating it is possible to record IR- or UV-spectra. Show
me an APO which is corrected for such a wide spectral range! And with
applied dielectric coatings the light loss may be even less than in a APO!
And on top of this you've got to pay a fraction of an APO!
WG
Ratboy99
>make your Schiefspiegler, then snap some planets thru it. Then I will you
>show
>images done with inferior instruments (SCTs, Newtonians, Cassegrains) of THE
>SAME
>SIZE, and we'll compare them. OK ?
Hey Brat, how come you're making him do all of the work?
rat
~( );>
Witty Anecdote Unrelated To Thread
Peter Natscher
I just can't believe that a Schiefspiegler is all that good. They are hardly
available, anywhere. Who wants to deal with such a instrument that has such a
small bandwidth. They might be very good with only planets if the conditions are
near perfect. If the evening is less than best, you are left with poor
performance. We observers have very few near perfect nights outside. All of us
deal with adverse conditions: average to good seeing in our observing sessions, if
we're lucky. In that case, a good quality f/6-8 Newtonian with 20% obstruction,
Mak-Cass./ or Mak-Newt., or a good apochromatic refractor will work out better
than a Schief on most evenings.
> To say it in short, a (well-aligned) Schief is optically as good as a very
Bratislav
> I can't agree here, compared to achromats and even more to APOs they are
> _very_ cost effective!
But compared to Newtonians and Cassegrains they aren't. It's your money, anyway.
> I simply could not see the dissadvantage of the Schief-design.
> -They use (in the simplest from) only two spherical mirrors, no chromatic
> elements, no aspherical deformed surfaces or any other difficult to
> fabricat
> optical elements.
Shallow spheres are not that easy to make - anyone who has made them will tell you
that.Furthemore, in larger sizes (anything above 6" or so) you need to aspherize them. If
you
haven't tried to make say 60% ellipse on an f/12 mirror, you don't really understand how
tough
that task is. Why do you think 8" Schiefspiegler set costs more than a 20"
(interferometrically
certified) mirror ?
> -High reflectivity coatings can be applied to all optical surfaces to avoid
> light loss (if desired dielectric coatings with _very_ high reflectivities
> can be used).
You still won't avoid scatter.
> -Closed tube assemblies could be built to avoid air currents.
I know of only one Schiefspiegler that has closed tube (by a high quality optical
window); its tube is filled by helium to boot, to speed up thermal equilibration. It
still doesn't allow its owner to make better images of planets than people with old
boring Newtonians (of the SAME size, not 1.8 times bigger). And just for fun, check
how much will certified 1/10 wave optical window in 8 to 10 inch range will cost.
> Therefore I can't see any reason why such a sytem should be inferior to an
> achromat or apochromat of the same aperture. I find it even the other way
> round: those systems are completely free of any chromatic aberration!
> Depending on the coating it is possible to record IR- or UV-spectra. Show
> me an APO which is corrected for such a wide spectral range! And with
> applied dielectric coatings the light loss may be even less than in a APO!
> And on top of this you've got to pay a fraction of an APO!
Fine. We can discuss until we both turn blue in the face. But lets stop theoreticizing.You
make your Schiefspiegler, then snap some planets thru it. Then I will you show
images done with inferior instruments (SCTs, Newtonians, Cassegrains) of THE SAME
SIZE, and we'll compare them. OK ?
Bratislav
x
> > I can't agree here, compared to achromats and even more to APOs they are
> > _very_ cost effective!
>
> But compared to Newtonians and Cassegrains they aren't.
Yes, a little more expensive because they are no mass products like those
mentioned. BTW the price I mentioned was for Zerodur-mirrors not Duran or
Pyrex.
>It's your money, anyway.
Unfortunately true! But I haven't spent it right now. I'll read all the
comments and gather some additional information and decide then.
> > I simply could not see the dissadvantage of the Schief-design.
> > -They use (in the simplest from) only two spherical mirrors, no chromatic
> > elements, no aspherical deformed surfaces or any other difficult to
> > fabricat
> > optical elements.
>
> Shallow spheres are not that easy to make - anyone who has made them will tell you
> that.
We are speaking of radii in the order of 3-7m. I think those mirrors can be
fabricated and tested with relaive ease.
> Furthemore, in larger sizes (anything above 6" or so) you need to aspherize them.
Not necessarily. Tetras don't need aspherization. Kutter-Systems only if
the aperture is larger than aprrox. 22cm.
> If
> you
> haven't tried to make say 60% ellipse on an f/12 mirror, you don't really understand how
> tough
> that task is. Why do you think 8" Schiefspiegler set costs more than a 20"
> (interferometrically
> certified) mirror ?
> > -High reflectivity coatings can be applied to all optical surfaces to avoid
> > light loss (if desired dielectric coatings with _very_ high reflectivities
> > can be used).
>
> You still won't avoid scatter.
Not completely but scatterd light is simply the percentage of not
"correctly" reflected light though not much with those coatings.
> > -Closed tube assemblies could be built to avoid air currents.
>
> I know of only one Schiefspiegler that has closed tube (by a high quality optical
> window); its tube is filled by helium to boot, to speed up thermal equilibration. It
> still doesn't allow its owner to make better images of planets than people with old
> boring Newtonians (of the SAME size, not 1.8 times bigger). And just for fun, check
> how much will certified 1/10 wave optical window in 8 to 10 inch range will cost.
I chose the wrong words. With closed I really ment the normaly very short
tube of the primary is extended to the secondary (Percy Zahl has built such
a type) to prevent air-currents from persons standing around to influence
image quality.
> > Therefore I can't see any reason why such a sytem should be inferior to an
> > achromat or apochromat of the same aperture. I find it even the other way
> > round: those systems are completely free of any chromatic aberration!
> > Depending on the coating it is possible to record IR- or UV-spectra. Show
> > me an APO which is corrected for such a wide spectral range! And with
> > applied dielectric coatings the light loss may be even less than in a APO!
> > And on top of this you've got to pay a fraction of an APO!
>
> Fine. We can discuss until we both turn blue in the face. But lets stop theoreticizing.You
> make your Schiefspiegler, then snap some planets thru it. Then I will you show
> images done with inferior instruments (SCTs, Newtonians, Cassegrains) of THE SAME
> SIZE, and we'll compare them. OK ?
I agree, theoretical design is one thing, the assembled scope another.
And prefect seeing conditions where such a design will show it's qualities
is a totally different matter.
WG
x
> Fine. We can discuss until we both turn blue in the face. But lets stop theoreticizing.You
> make your Schiefspiegler, then snap some planets thru it. Then I will you show
> images done with inferior instruments (SCTs, Newtonians, Cassegrains) of THE SAME
> SIZE, and we'll compare them. OK ?
I forgot to mention a link to images taken with a 300mm Kutter-sytem:
http://www.astro-shop.com/AM13/
Have a look at the moon-images.
WG
Peter Natscher
come apart? A 10" Schief has a long primary to secondary mirror tubing
envelope. Can you make the tubing (housing) splitable--fitting together
accurately enough to keep alignment good?
Have you contacted Lichtenknecker (in Belgium)? I hear they make the best
Schief's. Maybe they can give you a few good points.
Those images by Bernd Flach-Wilken are fantastic. I've never seen such sharp
images by any amateur. They rival observatory photos, with only a 300mm
aperture.
Peter
hfro...@iris.rz.uni-konstanz.de
> mes...@my-dejanews.com wrote:
>
> > Hmm, some of the best earth-bound lunar and planetary photos, as well as
> > some
> > of the best amateur photos of galaxies, have been accomplished with Schiefs.
> > Look e.g. at Bernd Flach-Wilken's photos exhibited online at
> >
> > http://www.astro-shop.com/AM13/AM13.html
>
> Hmm... some of the best earth bound lunar and planetary photos have been done
> with OBSTRUCTED telescopes. See Dragesco's book, Viscardi's atlas or ALPO
web-site.
> Or compare Legault's image of Saturn ( http://perso.club-internet.fr/legault/)
> with Bernd-Flash's. Thiery used (HEAVILY) ostructed SCT. To my eyes, his image
> is better. What do you say ?
Well, these are CCD pics. I've seen better pics from this instrument printed
in various mags (esp. in Germany), but sorry, don't know if he has published
them somewhere on the web.
And yes, there is also no magic which can be done by Bernd or other schief
owners, and I'm the last to deny the beauty, usefulness and value of good
astrophotos obtained with obstructed scopes.
> > To say it in short, a (well-aligned) Schief is optically as good as a very
> > good refractor, for a very much lower price.
>
> To say in short, I have a friend who is a master optician and Australia's
leading
> planetary observer, who has made several Schiefspieglers (his planetary scope
> is 12" Schief), and in his own words, refractors leave Schiefs "for dead".
> Of course, he is exaggerating a bit but I can also confirm that 4"
Schiefspiegler
> performs significantly worse on planets than a 4" achromat (made by same
person, by
> the way). A (good) 4" APO would make that difference even more obvious.
>
> Obstruction is only half of the story. There are other aspects of planetary
telescopes
> that may make certain design a winner or loser. Schiefspieglers are nice
planetary
> telescopes, but not very cost effective. If you don't believe me, just ask Don
Parker
> and Isao Miyazaki. And they CERTAINLY don't have any magic properties to bend
laws
> of physics and optics.
Well, of course there is no magic, again. On optics, however, I can't see the
physical foundation of your arguments, or more acurately, the points you think
are wrong in my previous outline. I still hold that a (well-aligned) Schief is
optically equivalent to a good refractor (APO) of same aperture and focal
length. Price advantage obvious.
OTOH, there is another physical point in favor of unobstructed scopes: You
will admit that unobstructed optics makes sharper images than obstructed,
let's not dispute on the exact value of the factor further. Thus, for a
certain needed "image definition", the aperture will be smaller and less
sensitive against unsteady seeing. Yet another point is that a Schief of a
certain aperture will typically weigh less than a refractor or Newtonian of
same type, thus need a lower-weight lower-cost mount (well SCs will be even
better off here).
x
> In keeping this Schiefspiegler transportable, will you design the housing to
> come apart? A 10" Schief has a long primary to secondary mirror tubing
> envelope.
Actually the housing of the primary is very short (40-50cm). The longest
part will be the tube holding the secondary,the wedge lens and the focuser
at its end. The overall length of this tube will be roughly 2300mm. If the
focuser is set to minimum extension I hope it will be no longer than
2100mm.
> Can you make the tubing (housing) splitable--fitting together
> accurately enough to keep alignment good?
I think it can be made spiltable but you definitely got to recollimate
after every setup. Therefore I will not split the housing and I've got to
live with a 2m beast :-(.
Another possibility is to add a third flat mirror to place the focus to one
side of the instrument and therefore to shorten it considerably.
Despite the length of the tube I hope to keep the weight down. My
estimation is:
primary 4.3kg
secondary 0.4kg
lens 0.2kg
mirror cells 1-2kg
tubes 4-5kg
+ ???
____________________
10-12kg
Perhaps a little to optimistic but I can also live with 15kg compared to
41kg (!) Lichtenknecker specifies for their complete OTA.
For me a package with a weight of 15kg and a length of 2m is quite
transportable but of course not as handy as a SCT.
> Have you contacted Lichtenknecker (in Belgium)? I hear they make the best
> Schief's. Maybe they can give you a few good points.
I know Lichtenknecker Optics for a very long time (since 1980) and I will
definitely order my optics there! All data on the Kutter-Schiefs can be
found in their brochure.
> Those images by Bernd Flach-Wilken are fantastic. I've never seen such sharp
> images by any amateur. They rival observatory photos, with only a 300mm
> aperture.
Yes they are really _very_ good images!
That was one of the reasons why I want to get such a system in my hands!
:-)))
Ciao WG
Peter Natscher
I've heard that they are typically 1/20 lambda RMS and 90% Strehl. Is that right?
I think Lichtenknecker sells their completed 250mm (10") f/20 Schiefspiegler for
about $12,000. I've heard that this size instrument will easily perform as well
as a 10" apo on planets and the moon, but cost half the price. But, this size
instrument is observatory only stuff.
Peter Natscher
Belmont, CA
Scott Rychnovsky
> What quality in the mirrors will Lichtenknecker be giving you in your 10"
optics?
> I've heard that they are typically 1/20 lambda RMS and 90% Strehl. Is
that right?
>
> I think Lichtenknecker sells their completed 250mm (10") f/20
Schiefspiegler for
> about $12,000. I've heard that this size instrument will easily perform
as well
> as a 10" apo on planets and the moon, but cost half the price. But, this size
> instrument is observatory only stuff.
>
Pardon me?
This is a very stange thread. You can get a 10" F7 mirror made to very
high standards for 1/10th the price that that will probably out-perform any
multi-mirror telescope. If you can avoid scattering in a Schiefspiegler
mirror you can do the same thing in a Newtonian mirror.
Scott
--
Scott Rychnovsky
sryc...@uci.edu
Remove the "z" in the e-mail header to respond
bratislav
>
> Well, of course there is no magic, again. On optics, however, I can't see the
> physical foundation of your arguments, or more acurately, the points you think
> are wrong in my previous outline. I still hold that a (well-aligned) Schief is
> optically equivalent to a good refractor (APO) of same aperture and focal
> length. Price advantage obvious.
>
> OTOH, there is another physical point in favor of unobstructed scopes: You
> will admit that unobstructed optics makes sharper images than obstructed,
> let's not dispute on the exact value of the factor further. Thus, for a
> certain needed "image definition", the aperture will be smaller and less
> sensitive against unsteady seeing. Yet another point is that a Schief of a
> certain aperture will typically weigh less than a refractor or Newtonian of
> same type, thus need a lower-weight lower-cost mount (well SCs will be even
> better off here).
I'm not denying advantages of unobstructed telescopes - an off axis Maksutov
Newtonian is still on my drawing boards. But in this thread several inaccuracies were
spelled out : that Schiefspiegleer will perform as well as Newtonian of 1.5 (or
even 1.8!) times the aperture (WRONG!), and that Schiefspiegler will be equal to
a same aperture APO (also wrong). I can only suggest you get a good 4" APO and try
it yourself (your Schief is 4" ?). Reasons are not so easy to explain in one sentence,
but scatter is one, and image motion from which Schief will suffer far, far more than
a refractor (its light path zigzags SEVERAL times thru an open air) is second.
The 'equivalent aperture' you are talking about is much closer to the Schief's raw
aperture than 'unobstructed buffs' will admit. If we factor in Schief's extra
sensitivity to seeing (CERTAINLY more than in a Newtonian), the difference all but
vanishes. I may allow few percents difference to say 4" class scopes, but 4" Schief
will be absolutely demolished by a well made 6" Newtonian. At 10", nice, well made
f/7 or so Newtonian will show you EVERYTHING a 10" Schiefspiegler ever could. Of
course everyone is entitled to his own opinion, and I'm no different from you or
everyone else in that respect. But I HAVE done the direct comparison, and have heard
experiences from a person who has observed planets for YEARS with both 4" Schiespiegler
and 4" (NON APO) refractor. And trust me, he KNOWS what he's talking about, both
telescope making wise (he's been making telescopes for at least 30 years) and
observing planets (even longer). His experiences match mine exactly : a 4" refractor
is a superior planetary instrument to a 4" Schiespiegler. In vacuum it might be
different story, though.
By all means, Schiefspieglers are great telescopes, and when well made, mighty fine
performers on any object, especially planets. But they are not best value for money,
and they can also be bitch to accomodate and collimate. 12" Newtonian will cost less,
will take about the same space as 8" Schiefspiegler, and be much lower to the ground.
And will run circles around the Schief planetary performance wise. But each of us
makes his/her own choices.
Bratislav
mes...@my-dejanews.com
well, Bratislaw, I wonder why I don't know people who, from personal
experience with these telescopes, share the views you expressed on
uncomfortable design. My own, as can be seen somewhere in my webpages
(http://www.seds.org/~spider/scopes/schief.html) is indeed a 4.3-inch (11 cm)
f/24.odd which has served me well with visual observations of all kinds of
objects accessible to scopes of these optical parameters. I found sensitivity
for air turbulence far less problmatic than for any scope in a dome.
My personal first dislike was more the sensitivity against dew if air was not
dry enough. Also, I never found adjusting or handling the Schief difficult.
Well, there are of course objects (M33) which are not for such long-fl
scopes. OTOH, planets were great, and not better in the 4-inch refractors I
had occasion to use, definitely better than in 4-inch Newton's I used, and
basically the same as in the 6-inch Newton and the C-8 I had access to.
Better, of course, in 5-inch and 6-inch refractors, and 6-inch Yolo. For
really BIG scopes (12-inch+ Dobson, 20-inch classical Cassegrain), you need
much steadier air to have any advantage, which might be enourmous. In view of
this, one should also keep in mind that the "scope-made" (aperture- and
obstruction-caused) "unsharpness" will fall below that generated by the
atmosphere for apertures larger than, or about, 10-inch. This is the reason
why optical design is of minor priority for the large scopes.
Well, I want to stress here again that Schiefs are not only good for planets
(and double stars), but also for every kind of objects (esp. galaxies and
other deepsky objects, and comets) except large-field ones.
> By all means, Schiefspieglers are great telescopes, and when well made, mighty
> fine
> performers on any object, especially planets. But they are not best value for
> money,
> and they can also be bitch to accomodate and collimate. 12" Newtonian will
> cost less,
> will take about the same space as 8" Schiefspiegler, and be much lower to the
> ground.
Still, it will weigh about double or more (thus need a much more expensive
mount), unless you have an open construction which is at least as sensitive
for (local) turbulences as the Schief of same size.
> And will run circles around the Schief planetary performance wise. But each of
> us
> makes his/her own choices.
That is how it should be, but for my taste, the knowledge of less usual
scopes is not widespread enough - and you can't choose for a Schief unless
you know it (not to speak of availability from commercial sources).
Peter Natscher
as highly as an unobstructed instument with equal mirror quality. And then, there's
apo refractors that don't have to be as high a quality in their lenses as mirrors
do to perform as well with the light wave (lens systems being better performers
than mirror systems). Light wavefronts deteriorate less passing through lenses than
bouncing off mirrors, so long as the lens systems are top notch.
Peter Natscher
Belmont, CA
Del Johnson
argument. And what if the mirror is also top-notch?
It is true that a lens surface creates half the error than a mirror surface
of the same quality, but there is always the second side of the lens so the
end result is the same.
Del Johnson
Peter Natscher wrote in message <3676FD61...@natscher.com>...
Del Johnson
refracting surfaces? You need at least four or six for a color correcting
designs. Add them up and your apo math breaks down.
Your calculations would conclude that the Mak-Newtonian cannot perform very
well, as it to contend with the sum of both surface type errors.
Do you know how many reflective surfaces there are in the corrected HST?
Funny how it all still works.
Del Johnson
TMBack wrote in message <19981215235548...@ng35.aol.com>...
>Del wrote:
>
>
>>It is true that a lens surface creates half the error than
>>a mirror surface of the same quality, but there is always
>>the second side of the lens so the end result is the same.
>>
>>Del Johnson
>
>Del,
>
> Your volumes of misinformation has finally got to me.
>You really don't know what you are talking about. I see
>I'm forced to do a re-post.
>
> The effect of surface roughness can be estimated by
>considering the Strehl ratio. Strehl ratio = 1 - (RMS/
>lambda) 2 where lambda is the wavelength of light and
>RMS is the root mean square surface roughness. The
>Strehl ratio gives the ratio of the Airy disk peak to the
>peak of the unaberrated Airy disk. For a given surface
>roughness, a refractive surface introduces about half
>wavefront roughness as a mirror. This comes about
>because upon refraction or reflection the wavefront is
>deformed by the factor (n' - n) where n and n' are the
>indexes of refraction of the media before and after
>refraction or reflection. For light refraction the factor is
>1/2, for reflection in air it is 2.
>
> Since surface roughness enters as the square in the
>Strehl ratio formula, light is scatter by a refractive surface,
>and a reflective surface in air in the proportions of 1 and 16.
>
> The ratio of error in optical path between mirror and
>lens surface for the same mathematical departure from
>figure is 2/(n - 1) or about 4. The error in optical path
>affects the contrast rendition in the diffraction pattern
>as the square. The two factors combined lead to a 16 to
>1 ratio.
>
> If anyone wonders why a well made refractor gives
>such high contrast/low scatter images for its aperture,
>the above is the reason why. Not to mention scatter
>from mirror coatings. In the optical industry, a very
>high quality aluminized coating is considered such,
>when it only adds 2x scatter. Most coated amateur
>mirrors are worse than this figure.
>
>
>Thomas Back
x
> What quality in the mirrors will Lichtenknecker be giving you in your 10" optics?
> I've heard that they are typically 1/20 lambda RMS and 90% Strehl. Is that right?
I don't have the brochure at hand but if I remember correct it was 35nm RMS
at a wavelength of approx. 630(?) => 1/20 lambda RMS is correct. BUT it is
the maximum overall deviation for the SYSTEM! Therefore the single mirrors
have a better surface quality. BTW, you can get the interferogramm to your
optics!
> I think Lichtenknecker sells their completed 250mm (10") f/20 Schiefspiegler for
> about $12,000.
The Duran-version sells for about DM 14000,- and the Zerodur-version for
about DM 19000,-. The OTA has a weight of 41 kgs! The Zerodur-optics alone
are around DM 5000,-.
> I've heard that this size instrument will easily perform as well
> as a 10" apo on planets and the moon, but cost half the price. But, this size
> instrument is observatory only stuff.
As I mentioned in my posts before: if you buy the OTA from Lichtenknecker
with its 41 kgs you definitely got to mount it permanently. My idea was to
pack the optic (total weight around 5kg) in a neat light-weight carbon-tube
with total weight of under approx. 15kg and therfore keeping it
transportable.
Ciao WG
x
> Pardon me?
>
> This is a very stange thread. You can get a 10" F7 mirror made to very
> high standards for 1/10th the price that that will probably out-perform any
> multi-mirror telescope. If you can avoid scattering in a Schiefspiegler
> mirror you can do the same thing in a Newtonian mirror.
Those prices are for OTA and not single mirrors!
And those systems have are very high garanteed surface accuracy which also
adds to the price.
Scattering is not the only thing to avoid in a mirror system.
There are more reasons for the one or the other mirror system but I really
don't want to repeat those useless discussions once again. If you want more
information get the older messages or have a look at some Schief-WWW-pages.
Ciao WG
bratislav
> well, Bratislaw, I wonder why I don't know people who, from personal
> experience with these telescopes, share the views you expressed on
> uncomfortable design.
Inexperience ? Personal bias ? I don't know. Why do you think no professional
planetary observatory EVER used a Schiefspiegler ? After all, all these obstructed
Cassegrains and horrible (non APO) refractors could have been replaced with instruments
1.8 times smaller ? Why do you think Dragesco, of all 32 telescopes he owned praised
most the old boring Newtonian ? Or are people like Parker, Miyazaki, Viscardi, Arsidi,
Therin just fooled by a mainstream conspiracy from 'obstructed' buffs ?
I have no problem with Schiefspieglers or their owners. What I have problem with is
when people start stories like these (about 'inherent superiority' of a particular
design, claiming all sorts of baloony performance gains). Same as APO ? 1.8 times
Newtonian ? Somehow, I don't think so.
But as usual, everybody has the right to an opinion.
> That is how it should be, but for my taste, the knowledge of less usual
> scopes is not widespread enough - and you can't choose for a Schief unless
> you know it (not to speak of availability from commercial sources).
Yes, I agree - they should be popularized. People should make them, they are
wonderful optical and mechanical challenges; one can learn a lot from building one.
But they should be not advertized by claiming impossible performance gains. Laws
of optics and physics clearly define shortcomings from obstruction. From Hubble
study to books like Texereau and Suiter - effect is analyzed and rather well known.
To claim that Schiefspiegler will outperform a Newtonian 1.5, or even 1.8 (!) size
just because of the obstruction (as Schiefspiegler does not enjoy any of the other
advantages APO does) is simply a fallacy.
Bratislav
TMBack
lude...@my-dejanews.com
bratislav <epa...@asac.ericsson.se> wrote:
> mes...@my-dejanews.com wrote:
>
> > well, Bratislaw, I wonder why I don't know people who, from personal
> > experience with these telescopes, share the views you expressed on
> > uncomfortable design.
>
> Inexperience ? Personal bias ? I don't know. Why do you think no professional
> planetary observatory EVER used a Schiefspiegler ? After all, all these
obstructed
> Cassegrains and horrible (non APO) refractors could have been replaced with
instruments
> 1.8 times smaller ? Why do you think Dragesco, of all 32 telescopes he owned
praised
> most the old boring Newtonian ? Or are people like Parker, Miyazaki,
Viscardi, Arsidi,
> Therin just fooled by a mainstream conspiracy from 'obstructed' buffs ?
Hi Bratislav,
may I can answere your above question. You say : no proffesional planetary
observer ever used a Schiefspiegler . This is wrong information. The best
Planetary images ever shooted by amateurs are made with Schiefspieglers, see
images from Bernd Flach Wilken and some others guys from germany and Austrai.
Note, the images made in germany, where the seeing is much worser than in
Florida. What do you think, how this images would look like , if they where
been made in Florida ? Be shure , close to Huble images. Why owned Dragesco so
many telescopes ? Because he made commercial advertisements for the
telescopedealers who gave him that telescopes with big discount or free of
charge.
Why Schiefspieglers not used in the USA? I saw a few self made Kutter and Tri
Schiefspieglers. They have been so badly made, that an perfect alignment is
impossible and therefore the images looks terrible of course.
In Europe Schiefspieglers are since very long time known as the biggest
competitor of Apos, because the commercial made Schiefspieglers by
Lichtenknecker Optics and AOK/Swiss are perfectly maschined and therefore
working perfectly.
Even in japan, where all amateurs eating for breakfast an small Takahashi ,
Kasai Trading could introduce the AOK commercial made Schiefspieglers , why ?
Because he showed at japanese Starpartys the power of this instruments. Of
course you need an very good seeing, but an real critical Planetary observer
know that he need with any telescope time to see the faintest details.
Conclusion: Schiefspieglers are welknown and famous in europe and japan,
because they are commercial made and perfect working. They have no reputation
in USA and they are not famous in USA, because until today no company offered
high quality mechanics for the optical sets.
Can we live with this conclusion ?
Markus
>
> Bratislav
TMBack
>The single lens surface is attractive, but how many
>refractors have single refracting surfaces?
The popular triplet apochromat has two air-to-glass
surfaces, the air-spaced doublet achromats and apos
have 4.
>You need at least four or six for a color correcting
>designs. Add them up and your apo math breaks down.
Would you please explain how my "apo math" (that's
a good one) breaks down? Nothing wrong with the
math, just the guy that doesn't understand it.
>Your calculations would conclude that the Mak-
>Newtonian cannot perform very well, as it to
>contend with the sum of both surface type errors.
Let's get something straight, I never said a mirror
or mirror/lens system cannot perform well. It is just
a matter of tighter surface and smoothness tolerances,
that are needed in these systems.
>Do you know how many reflective surfaces there are
>in the corrected HST? Funny how it all still works.
Sure. And do you know smooth these Tinsley
mirrors are? Phenomenally smooth (~1A RMS).
Thomas Back
Del Johnson
refractor than a reflector. The costs of refractors suggests otherwise.
Del Johnson
TMBack
>What is your point?
Quality. Sheesh, how much clearer can I make it?
>You seem to be suggesting...
Wrong.
Thomas Back
Del Johnson
"Best Planetary Telescopes So Far". Somehow, those pesky Newtonians are not
obeying your lens equations.
Del Johnson
TMBack wrote in message <19981216132845...@ng27.aol.com>...
MitchAlsup
>>refractors have single refracting surfaces?
>
> The popular triplet apochromat has two air-to-glass
>surfaces, the air-spaced doublet achromats and apos
>have 4.
And the 3 lens APOs also have 4 surfaces which interface
between oil and glass. It has been suggesteed in the past
that these surfaces don't have to be as accurately polished
as air-glass interfaces because the refractive index of the
oil is similar to that of the glass. I take this to mean that the
oil prevents reflections, and also minimizes the sensitivity
of the actual surface between the oil-glass interface.
But doesn't the roughness of 'not fully polished' glass hurt
the final image by some fraction (like 1/8th) of an air glass
interface which has been fully polished?
Mitch
Mitch Alsup
Mitch...@aol.com
TMBack
>But doesn't the roughness of 'not fully polished'
>glass hurt the final image by some fraction (like
>1/8th) of an air glass interface which has been
>fully polished?
Who says that they are not fully polished? Astro-
Physics fully polishes all 6 surfaces -- I know, as
I have seen many of them in the assembly stage.
Ask Roland if you don't believe it.
Thomas Back
vale...@my-dejanews.com
lude...@my-dejanews.com wrote:
> In article <3676DC34...@asac.ericsson.se>,
> bratislav <epa...@asac.ericsson.se> wrote:
> > mes...@my-dejanews.com wrote:
> > planetary observatory EVER used a Schiefspiegler ? After all, all these
> obstructed
> > Cassegrains and horrible (non APO) refractors could have been replaced with
> instruments
> > 1.8 times smaller ? Why do you think Dragesco, of all 32 telescopes he owned
> praised
> > most the old boring Newtonian ? Or are people like Parker, Miyazaki,
> Viscardi, Arsidi,
> > Therin just fooled by a mainstream conspiracy from 'obstructed' buffs ?
>
> Hi Bratislav,
>
> may I can answere your above question. You say : no proffesional planetary
> observer ever used a Schiefspiegler . This is wrong information. The best
> Planetary images ever shooted by amateurs are made with Schiefspieglers, see
> images from Bernd Flach Wilken and some others guys from germany and Austrai.
> Note, the images made in germany, where the seeing is much worser than in
> Florida. What do you think, how this images would look like , if they where
> been made in Florida ? Be shure , close to Huble images. Why owned Dragesco so
> many telescopes ? Because he made commercial advertisements for the
> telescopedealers who gave him that telescopes with big discount or free of
> charge.
Good seeing is a good seeing - no difference where, in Germany or in
Florida. The difference is only haw often you can have such images.
So, the images made with S---ler will be same in both locations.
Note, please, that D.P.'s images is looks like cose to diffration limit.
"Hubble-like images" this is only ads claims. :-)
> Why Schiefspieglers not used in the USA? I saw a few self made Kutter and Tri
> Schiefspieglers. They have been so badly made, that an perfect alignment is
> impossible and therefore the images looks terrible of course.
> In Europe Schiefspieglers are since very long time known as the biggest
> competitor of Apos, because the commercial made Schiefspieglers by
> Lichtenknecker Optics and AOK/Swiss are perfectly maschined and therefore
> working perfectly.
>
> Even in japan, where all amateurs eating for breakfast an small Takahashi ,
> Kasai Trading could introduce the AOK commercial made Schiefspieglers , why ?
<snip>
> Conclusion: Schiefspieglers are welknown and famous in europe and japan,
> because they are commercial made and perfect working. They have no reputation
> in USA and they are not famous in USA, because until today no company offered
> high quality mechanics for the optical sets.
I am seriously doubt that this S----ler design is well known in the rest
Europe (exept Austria, Germany Switzerland). You can ask some peoples in
Russia, Spain, Portugal, N.Ireland etc. What do you think will be the an-
swers? :-))
I clearly remember the comparition of two scopes 8" S--ler and 10" newto-
nian with small c.o. ( I forgot the F/D). The 10" newt. was owned by Markus's
friend Mathias - very experienced "star-tester".
The 8" S-ler was hads down beaten compare to this 10" newt. So, any
speculations about 1.5 or 1.8 rate for the S-ler win is a tales for
childrens.
If someones here directly compare 8" F/6 mak-newt with 22% c.o. to 8" apos,
why they can't accept that S-lers and Newts with the same apertures are
the same in perfomance. This is really a great miracle, at least for me. :-)
Clear sky,
Valery.
bratislav
> Hi Bratislav,
>
> may I can answere your above question. You say : no proffesional planetary
> observer ever used a Schiefspiegler . This is wrong information. The best
> Planetary images ever shooted by amateurs are made with Schiefspieglers, see
> images from Bernd Flach Wilken and some others guys from germany and Austrai.
Sigh. Can you actually read what have you written above ? Let's go sentence by
sentence.
"You say : no proffesional planetary observer ever used a Schiefspiegler"
Yes, that is what I said.
"This is wrong information"
OK, prove me wrong. Give us names of professional observatories with Schiefspiegler
installations, or publications where professional stated (names again please) that
he/she used a Schiefspiegler for gathering the data for the study.
"The best Planetary images ever shooted by amateurs are made with Schiefspieglers"
Nope. Unless they are kept in their owners' bedrooms ! Where are they published ?
I couldn't see them in mainstream magazines, there is no book dedicated to Schiefs
(Dragesco did have few images in his book, but they are CLEARLY not 'best'), what
I could see on the outlined web pages is also clearly NOT better than the work of
people using obstructed telescopes (SCTs and Newtonians).
> Note, the images made in germany, where the seeing is much worser than in
> Florida. What do you think, how this images would look like , if they where
> been made in Florida ? Be shure , close to Huble images.
Gimme a break. Ever heard of old Latin saying 'Hic Rhodos, hic salta' ? This
is a most pathetic excuse.
> Why owned Dragesco so
> many telescopes ? Because he made commercial advertisements for the
> telescopedealers who gave him that telescopes with big discount or free of
> charge.
W-R-O-N-G. Question reads : WHY, of all telescopes Dragesco owned, he praised
MOST the old boring Newtinian ? Read what I say, do not construct questions yoursef
and give meaningless answers.
> Why Schiefspieglers not used in the USA? I saw a few self made Kutter and Tri
> Schiefspieglers. They have been so badly made, that an perfect alignment is
> impossible and therefore the images looks terrible of course.
Can't comment. Schiefspieglers I saw are in Australia. And they are well made,
thanks for asking.
> In Europe Schiefspieglers are since very long time known as the biggest
> competitor of Apos, because the commercial made Schiefspieglers by
> Lichtenknecker Optics and AOK/Swiss are perfectly maschined and therefore
> working perfectly.
They can only work as well as thir aperture and design allows. Contrary to what
you try to insinuate, they DO follow laws of optics and physics. No surprises here.
> Even in japan, where all amateurs eating for breakfast an small Takahashi ,
> Kasai Trading could introduce the AOK commercial made Schiefspieglers , why ?
> Because he showed at japanese Starpartys the power of this instruments. Of
> course you need an very good seeing, but an real critical Planetary observer
> know that he need with any telescope time to see the faintest details.
Fine. There is market for Schiefspieglers in Japan. So what ? The world's most
improbable optical design (SCT) is selling in numbers larger than all other
telescopes together. Does that mean that SCT is a 'magical telescope'. Nope.
There is a market for off axis paraboloids, Schupmanns, Houghtons, Jones-Bird
and whole load of other, more obscure designs. And that is good, nothing wrong
with that. Fact that Schiefspieglers will sell in Japan doesn't prove their
'magical prowess'; sorry.
> Conclusion: Schiefspieglers are welknown and famous in europe and japan,
> because they are commercial made and perfect working. They have no reputation
> in USA and they are not famous in USA, because until today no company offered
> high quality mechanics for the optical sets.
Again, don't know anbout USA, I live in Australia. But you again missed my point
by a LOOOONG shot. Claims that I disputed are that Schiefspieglers will work
equally well as the same aperture APO, and 1.8 times Newtonian. I say
"this is bullshit". Do you dispute _this_ ?
> Can we live with this conclusion ?
I'm not quite sure, but if your conclusion was that 'Schiefspieglers are welknown
and famous in europe and japan', I guess I can live with that.
Cheers,
Bratislav
PS may I ask you why your 'dream telesocpes' (your words, on yor web page) include
only APOs, windowed Newtonian and Maksutovs ? No Schiefs ?
lude...@my-dejanews.com
vale...@my-dejanews.com wrote:
> In article <758fei$en6$1...@nnrp1.dejanews.com>,
>
> Good seeing is a good seeing - no difference where, in Germany or in
> Florida. The difference is only haw often you can have such images.
> So, the images made with S---ler will be same in both locations.
The general limit in germany for seeing is around 0.5 arcsec, in Florida it
goes down to 0.2~0.3 arcsec, so we can never reach in germany the resolution
as in Florida. You understand , Valery ? > >> > I am seriously doubt that
this S----ler design is well known in the rest
> Europe (exept Austria, Germany Switzerland). You can ask some peoples in
> Russia, Spain, Portugal, N.Ireland etc. What do you think will be the an-
> swers? :-))
In Spain they will know it, in Portugal are only a handfull amateurs. Russia
is east europe, about which i not talked, most People from North Irland know
better the guns than telescopes.
>
> The 8" S-ler was hads down beaten compare to this 10" newt. So, any
> speculations about 1.5 or 1.8 rate for the S-ler win is a tales for
> childrens.
wrong. You stayed onnly short time there and not waited the good seeing for
the Schiefspiegler, so you are an not pashioned observer.
> If someones here directly compare 8" F/6 mak-newt with 22% c.o. to 8" apos,
> why they can't accept that S-lers and Newts with the same apertures are
> the same in perfomance. This is really a great miracle, at least for me. :-)
Newtonian have central obstruction , Schiefspiegler not, Newtonian have
spiders schiefspiegler not. About comparation 8" Apo with 8" Mak-newt. we
must clarify, that Apo was 1/5 wavefront and Mak-newt. 1/10 wavefront. This
discussion is only fair and correct if we talk about same quality of each
scope. Apples and Oranges are still 2 diffrent fruits.
>
> Clear sky,
thanks i have it since 2 days and its black in the night.
paka
Markus
lude...@my-dejanews.com
bratislav <epa...@asac.ericsson.se> wrote:
> lude...@my-dejanews.com wrote:
> >
> "This is wrong information"
> OK, prove me wrong. Give us names of professional observatories with
Schiefspiegler
> installations, or publications where professional stated (names again please)
that
> he/she used a Schiefspiegler for gathering the data for the study.
Mr. Bern Flach Wilken,
Website: http://www.astro-shop.com/AM13/AM13.html
and http://www.aokswiss.ch
>
> "The best Planetary images ever shooted by amateurs are made with
Schiefspieglers"
> Nope. Unless they are kept in their owners' bedrooms ! Where are they
published ?
In the german Magazin, called " Sterne und Weltraum" , the magazin from
austria "STAROBSERVER", in the frnace magazin " Ciel Espace" , as you see in
europe. In our German magazin , ther has been publish as much pictures of
Schiefspieglers as in Sky and telescope have been published pictures from
Astrophysics Apos.
> > > > > > > > > > Cheers,
> Bratislav
>
> PS may I ask you why your 'dream telesocpes' (your words, on yor web page)
include
> only APOs, windowed Newtonian and Maksutovs ? No Schiefs ?
Because I like closed systems, which dont need any service or removing optical
parts for cleaning.
clear sky
Markus
lude...@my-dejanews.com
mitch...@aol.com (MitchAlsup) wrote:>
> But doesn't the roughness of 'not fully polished' glass hurt
> the final image by some fraction (like 1/8th) of an air glass
> interface which has been fully polished?
It does slightly hurt, thats the reasons why Zeiss Jena fully polished even
the inside surfaces before oiling.This is printed in the Zeiss magazin.
Markus
>
> Mitch
> Mitch Alsup
> Mitch...@aol.com
vale...@my-dejanews.com
> I'm not quite sure, but if your conclusion was that 'Schiefspieglers are
welknown
> and famous in europe and japan', I guess I can live with that.
>
> Cheers,
> Bratislav
>
> PS may I ask you why your 'dream telesocpes' (your words, on yor web page)
include
> only APOs, windowed Newtonian and Maksutovs ? No Schiefs ?
------------------------------------------- ============== !!!!!!
Bravo Bratislav! In this tread the main, but deeply hidden idear was:
do what I say, not what I do!
Markus, you are quite wrong that I do not familiar with S---ler (can't
spell it, sorry) . One of the largest S-ler D=1.2m in the world is
in our observatory. This is a solar vertical telescope. I were involved
in this project many years ago. All advantages and disadvantages of this
very special scope is well known for me.
Concerning my impatience in observing. Did you ever awaited the sky during
one week of whole sleepless nights as I did this a lot of time during my
pros job? I not even not say about good seeing moments which we always
awaited with a patience like leo on the hunting.
As for the Austria Star Party case with S-ler vs Newt comparition, the
images were good enough and during some moments they were exellent. This
was enough to compare. Newt was a clear winner.
If anyone bring me S-er for a present, I will not take it. Only in a
terrible nightmare I will be able to see myself as S-ler happy owner.
Best regards,
Valery.
Peter Natscher
medium-transmitted wave (through lenses) does. This being so even if both
systems are top notch.
Peter Natscher
Belmont, CA
Del Johnson wrote:
> Sounds like the old "a top-notch lens will outperform a mediocre mirror"
> argument. And what if the mirror is also top-notch?
>
Peter Natscher
best ever done.
Peter Natscher
Belmont, CA
TMBack wrote:
Peter Natscher
meters. That's an awfully long piece of mirror housing to transport, isn't it?
Peter Natscher
Belmont, CA
x
> In article <19981216150339...@ng145.aol.com>,
> mitch...@aol.com (MitchAlsup) wrote:>
> > But doesn't the roughness of 'not fully polished' glass hurt
> > the final image by some fraction (like 1/8th) of an air glass
> > interface which has been fully polished?
>
> It does slightly hurt, thats the reasons why Zeiss Jena fully polished even
> the inside surfaces before oiling.This is printed in the Zeiss magazin.
There was another reason why Zeiss used oil in their APQ-systems.
The CaF_2-crystal in between the two other lenses is not as hard as glass
and also is somewhat brittle because of its crystal structure. Therefore it
was nearly impossible (and very costly) to give the CaF_2 a perfect polish.
The oil was used as an "interface medium" to lessen the effect of a not
perfectly polished surface.
Ciao WG
bratislav
>
> In article <367838B8...@asac.ericsson.se>,
> > OK, prove me wrong. Give us names of professional observatories with
> Schiefspiegler
> > installations, or publications where professional stated (names again please)
> that
> > he/she used a Schiefspiegler for gathering the data for the study.
>
> Mr. Bern Flach Wilken,
> Website: http://www.astro-shop.com/AM13/AM13.html
> and http://www.aokswiss.ch
Sorry, but I could only see commercial pages (advertisements for two companies, one
making Schiefspieglers, one selling them). I couldn't find any indication that Mr.
Wilken is a professional planetary observer, nor that his observatory installation
is "professional", nor any indication that his images are used for any scientific
study. Maybe we should talk about wording 'professional' ?
> > "The best Planetary images ever shooted by amateurs are made with
> Schiefspieglers"
> > Nope. Unless they are kept in their owners' bedrooms ! Where are they
> published ?
>
> In the german Magazin, called " Sterne und Weltraum" , the magazin from
> austria "STAROBSERVER", in the frnace magazin " Ciel Espace" , as you see in
> europe. In our German magazin , ther has been publish as much pictures of
> Schiefspieglers as in Sky and telescope have been published pictures from
> Astrophysics Apos.
[ Please note that high resolution photos in S&T seldom come from A-P APOs.
Most of them are done with Newtonians, Cassegrains or SCTs ]
So those images are better than Parker, Miyazaki, Viscardi, Therin etc. ? What
is the smallest detail they could do on the Moon ? Funny that only few made
it to Dragesco's book. Another conspiracy against Schiefspiegelers ? :-)
Cheers,
Bratislav
lude...@my-dejanews.com
it is not any more worth for me to spend more time in discussion with you
about Shiefspieglers. Valery talk about an 1.2 m Schiefspiegler, where any
Newtonian also never will have stable pictures and about an 30 minutes
try-to-observe at Austria. Of course you can easy say I am wrong , because
most german observers who can confirm that you not really observed there are
not in the internet . Bratislav, I am shure you are perfect in theoretical
knowledge and you know very well about scopes which you used, but you have no
real Idea about power of an very well made Shiefspiegler. Maybe both of you
will come next year august to the Solareclipse in germany , than I will
arrange for you both an observing night with Shiefspieglers 6" and 12" and
from that night on you will shut forever your mouth to say only 1 bad word
about this system.
Now answere what you want, i will not answere to this thread anymore.
You are welcome to the Shiefspiegler night in Germany .
best wishes from the Schiefspiegler Country Germany
Markus
vale...@my-dejanews.com
lude...@my-dejanews.com wrote:
> Valery and Bratislav,
>
> it is not any more worth for me to spend more time in discussion with you
> about Shiefspieglers. Valery talk about an 1.2 m Schiefspiegler, where any
> Newtonian also never will have stable pictures and about an 30 minutes
> try-to-observe at Austria. Of course you can easy say I am wrong , because
> most german observers who can confirm that you not really observed there are
> not in the internet .
Markus,
As Bratislav (or may be Thomas Back) said, you are too sensitive and
we can see your unbased offence. It is not worth be be offenced due to
discussion about some designs.
Concerning my Austria Star Party activity. I REALLY compared the 8"
S-ler (made by two guys, who are fun of ZERO obstructed scopes, as they
personally told me) vs Mathias's 10" Newt. Mathias can clearly confirm
you that I spend as minimum 2.5 hours jumping between three scopes:
8" S-ler, 10" Newt and 6" APQ. We observed Saturn and lately Mars.
I don't remember about Mars, but Saturn was very detailed in all instru-
ments, but 10" Newt. was the best.
I did not said that this is due to S-ler disadvantage - the 10" is simply
larger and so, has better resolution. This fact is simply disillused your
overoptimistic claims that S-ler has 1.5 - 1.8(!) aparture gap against
Newt.
What Bratislav said? In one of his post he said that it is more easy to
make and handle the 12" Newt(or 10" ? ) than 8" S-ler and that in the
case of good optics this Newt will show much more than 8" S-ler.
This all. Nobody said here that unobstructed scope will work poorer
than same size obstructed one in the case of similar optics quality.
We only said that this design is not usable compare to some much more
compact ones and that a S-ler can't has 1.5 times aperture gap.
Or you still disagree with this?
Valery.
mes...@my-dejanews.com
> > experience with these telescopes, share the views you expressed on
> > uncomfortable design.
>
> Inexperience ? Personal bias ? I don't know. Why do you think no professional
> obstructed
> Cassegrains and horrible (non APO) refractors could have been replaced with
> instruments
Well, I gave you an outline of this physics underlying Kutter´s statement
that the "image definition" of an unobstructed scope is a factor better than
that of an obstructed scope (Kutter said "factor 2", I gave you my estimate
of "factor 1.8"). Up to now, neither you nor any other poster has attacked
this statement with physical arguments, not to speak of calculations or
measurements. So I don´t see any reason to go one inch behind this statement;
deliver physics, and if this disproves Kutter, I will hurry to change my
statement in these pages. The physics is more outlined in Kutter´s book and
some easier available, but still old articles in various mags; sorry, didn´t
yet find time to make a webpage from this.
For a fair comparison of these scopes "in-action", you should keep in mind
that there are of course many objects looking better with low magnifications,
esp. extended DSOs. So it is required that you use same magnification for
each if compared visually. And then still you will have all your likes and
"this-cannot-be" view with you .. please account for this.
Moreover, for an "image definition" test, close double stars are the best
testers: Look if this or that pair of stars is separated or not. This _will_
show you the difference, definitely: Schief separates a lot of more pairs than
an "obstructed" of same aperture.
Another point: While I´m not really surprized, I find it remarkable to see
such an anti-Schief "gang", or how should I call it, forming. Bratislav has
at least given some arguments for his opinion, but others just claimed boldly
without anything behind. Is it really necessary, or what is the fun, to argue
this way ? Let´s return to a more experience and science oriented way of
discussion, please. At last, I will continue to offer (and hopefully extend,
slowly) these Schief infos on the web, regardless what someone argues around
this, and while open for reasonable discussion and physical arguments, I will
not allow any influence of bold claims of people who have not even tried to
understand the physics behind it.
In this sense, oblique and Schief views of a very clear skies,
hartmut
http://www.seds.org/~spider/
ar...@mercury.kherson.ua
mes...@my-dejanews.com wrote:
> Well, I gave you an outline of this physics underlying Kutter´s statement
> that the "image definition" of an unobstructed scope is a factor better than
> that of an obstructed scope (Kutter said "factor 2", I gave you my estimate
> of "factor 1.8"). Up to now, neither you nor any other poster has attacked
> this statement with physical arguments, not to speak of calculations or
> measurements. So I don´t see any reason to go one inch behind this statement;
> deliver physics, and if this disproves Kutter, I will hurry to change my
> statement in these pages. The physics is more outlined in Kutter´s book and
> some easier available, but still old articles in various mags; sorry, didn´t
> yet find time to make a webpage from this.
You need to change your statement! Simple example. If the S-ler has 2 times
smaller aperture than, say, Newton, this mean S-ler's Airy disk will has
same diameter as first diffraction ring in Newt. I think that your knowelege
is enough to understand that Newt will deliver much sharper images.
So, Kutter was a lier or poor physicist. You can choose one of them with which
you feel more comfortable.
Sincerely,
Valery.
P.S. 10" Newt with 23% central obstruction (typical value for F/6 scopes)
has better MTF in low and high frequencies and the same in middle
ones. So, the 1.25 larger Newt. will be winner in the most cases
and in the poorest case will be same good as S-ler.
I am physicist and I know about what I am speaking.
Thierry Legault
lude...@my-dejanews.com wrote:
> may I can answere your above question. You say : no proffesional planetary
> observer ever used a Schiefspiegler . This is wrong information. The best
> Planetary images ever shooted by amateurs are made with Schiefspieglers, see
> images from Bernd Flach Wilken and some others guys from germany and Austrai.
> Note, the images made in germany, where the seeing is much worser than in
> Florida. What do you think, how this images would look like , if they where
> been made in Florida ? Be shure , close to Huble images. Why owned Dragesco so
> many telescopes ? Because he made commercial advertisements for the
> telescopedealers who gave him that telescopes with big discount or free of
> charge.
After comparing my recent image of Saturn, taken from France
with a 33 % obstructed 12" Schmidt-Cassegrain telescope
(http://www.club-internet.fr/perso/legault/sat98.gif)
with the one of Mr Flach Wilken
(http://www.astro-shop.com/AM13/SA011197.jpg)
taken with his 12" Schiefspiegler, should I conclude that an
obstructed telescope is better than an unobstructed one ?
No, of course ! So please, do not jump to conclusions about
Schiefspiegler if you see images of a lower quality than
Mr Flach ones. Besides theoretical factors like obstruction,
there are many practical factors that are generally predominant
(even if manufacturers, sellers or books rarely talk about
them) : thermal equilibrium, collimation, focusing and sampling
in CCD, etc.
I think that Jean Dragesco had many instruments not because of
advertisement (I never saw him involved in ads), but because he tried
to find the 'very best' telescope for high resolution. But he never
really found it for a simple reason : it does not exist. As long
as the instrument is of a good mechanical and optical quality,
the most important thing is not a bit less obstruction here or
a bit more lambdas there. It is the element we never talk about :
the user. The amateurs mentioned in Dragesco's book
have proven that any kind of telescope can give very good results
in high resolution...but only if it is properly mastered.
You can be refractor-aficionado, Schiefspiegler-aficionado,
Maksutov-aficionado or anything you want, you can take the best
high tech camera, if you do not have the knowledge, the
perseverance and the meticulousness necessary to master the system,
it's just hot air. No doubt that Mr Flach Wilken is a skilled
amateur, and that he would have obtained close results with for
example a good Newtonian or Cassegrainian of the same aperture.
I have used different kind of telescopes (Schmidt-Cassegrain,
Maksutov-Cassegrain, Cassegrain, Newtonian) of similar aperture
(between 8" and 10") and I have noticed that their results in high
resolution CCD imaging were remarkably close. As close as can be
the resolutions of the lunar images that Mr Flach and I obtained
with our respective 12" telescopes, despite the profound difference
of design of the instruments.
About the seeing, average values can give an idea of the
probability of good conditions in a given place over a large
amount of nights, but it is meaningless for a specific night.
When a night is very good in Germany or in France, it is very
good, period. No reason why it would be inferior to a very good
night in Florida, even if working in our countries indeed
requires a higher perseverance because of the lower quantity
of good (or simply clear !) nights and because of the lower
position of the planets above the horizon.
Regards
Thierry Legault
http://www.club-internet.fr/perso/legault/
jjgoss
well of Shiefspieglers (we in US need to Anglicize this into
'unobstructed off-axis mirror telescope' - at least we can spell
that....).
I will be pursuing high-res. astrophotos next year, and will try
it with a variety of scope designs. I doubt I would spend the
money on a Shief, though, just too awkward a scope.
J.Goss
Thierry Legault wrote in message <367C2A...@club-internet.fr>...
brat...@my-dejanews.com
> Well, I gave you an outline of this physics underlying Kutter´s statement
> that the "image definition" of an unobstructed scope is a factor better than
> that of an obstructed scope (Kutter said "factor 2", I gave you my estimate
> of "factor 1.8"). Up to now, neither you nor any other poster has attacked
> this statement with physical arguments, not to speak of calculations or
> measurements. So I don´t see any reason to go one inch behind this statement;
> deliver physics, and if this disproves Kutter, I will hurry to change my
> statement in these pages. The physics is more outlined in Kutter´s book and
> some easier available, but still old articles in various mags; sorry, didn´t
> yet find time to make a webpage from this.
I think your definition of 'physics' needs some serious rethinking. Kutter
talks a lot about vague and dubious terms like 'definition'. I suggest you
read a REAL physics book. Start with wavefront analysis, MTF and then see
about the obstruction and degradations from it. If Mahajan and Born & Wolf
are too difficult, try Schroeder, Suiter, Rutten & van Venrooij and Zmek
(in reverese order re difficulty of reading/following/understanding).
> For a fair comparison of these scopes "in-action", you should keep in mind
> that there are of course many objects looking better with low magnifications,
> esp. extended DSOs. So it is required that you use same magnification for
> each if compared visually. And then still you will have all your likes and
> "this-cannot-be" view with you .. please account for this.
At no stage I was talking about low magnifications, DSOs or other crap. I was
always very specific in discussing PLANETARY performance. And if you still
think that your 4.5" would have any chance against a good 8" Newtonian, well
you are welcome to try.
> Moreover, for an "image definition" test, close double stars are the best
> testers: Look if this or that pair of stars is separated or not. This _will_
> show you the difference, definitely: Schief separates a lot of more pairs than
> an "obstructed" of same aperture.
Uhh, I think that your understanding (?) of obstruction shows. Obstructed
telescopes have SMALLER Airy disc, hence will split closer (equal) doubles
than Schief will. Anyway, your claim was 1.8 or 2X the size, remember ?
My 8" Newtoninan does a clean split of 14 Ori and 32 Ori (listed as 0.8
and 0.7 arc sec). What your Schiefspiegler does with these, it should be
at least as good, if not better ??? :-)
> Another point: While I´m not really surprized, I find it remarkable to see
> such an anti-Schief "gang", or how should I call it, forming. Bratislav has
> at least given some arguments for his opinion, but others just claimed boldly
> without anything behind. Is it really necessary, or what is the fun, to argue
> this way ?
There is no 'gang'. Only (understandable) reaction to some outrageous claims.
Or did you expect people to sit back and watch Schiefspiegler 'gang' claiming
things like 'equal to Newtonian 1.8 (and now even 2X!!!) the size' ??? I said
before, and I'll say it again : I have nothing against Schiefspieglers, their
owners or admirers. What I have problem with is claims that they are so
'magical' (whatever term you choose - 'definition', 'beautiness',
'unobstructioness', you name it) to equal other instruments (Newtonian)
twice the size. That a call CRAP. No gang, no masses. Just me.
> Let´s return to a more experience and science oriented way of
> discussion, please. At last, I will continue to offer (and hopefully extend,
> slowly) these Schief infos on the web, regardless what someone argues around
> this, and while open for reasonable discussion and physical arguments, I will
> not allow any influence of bold claims of people who have not even tried to
> understand the physics behind it.
You're blaming a professional telescope maker, physicist by training
(Valery) and experienced Telescope Nut (me) of 'not understanding the physics
behind' Schiefspiegler ? Well, I can only say, read the books outlined above.
If you doubt those people as well, good luck in Kutterland.
Bratislav
Bratislav
e
ley.edu!nntp2.dejanews.com!nnrp1.dejanews.com!not-for-mail
mes...@my-dejanews.com
> > Well, I gave you an outline of this physics underlying Kutteræ„€ statement
> > that the "image definition" of an unobstructed scope is a factor better than
> > that of an obstructed scope (Kutter said "factor 2", I gave you my estimate
> > of "factor 1.8"). Up to now, neither you nor any other poster has attacked
> > this statement with physical arguments, not to speak of calculations or
> >statement;
> > deliver physics, and if this disproves Kutter, I will hurry to change my
> > some easier available, but still old articles in various mags; sorry, didnæ„’
> > yet find time to make a webpage from this.
>
> I think your definition of 'physics' needs some serious rethinking. Kutter
> talks a lot about vague and dubious terms like 'definition'. I suggest you
> read a REAL physics book. Start with wavefront analysis, MTF and then see
> about the obstruction and degradations from it. If Mahajan and Born & Wolf
> are too difficult, try Schroeder, Suiter, Rutten & van Venrooij and Zmek
> (in reverese order re difficulty of reading/following/understanding).
Not too difficult. Studied physics long enough (was my learned profession),
and still..
Yet I did not, and could not, due to lack of time, give you here an outline of
the relevant physics I had in mind. As I have currently no time to write a
deeper one, I'll give you here a four-line summary only - sorry but ..:
Basically Kutter said that for obstructed scopes (of his days, around 1960)
a significant portion of the light will go from the central Airy disk into
the Newtonian rings, making the first Newtonian ring much brighter and more
or less merging with it: Larger image (first-ring diameter).
Why Airy disks of same-aperture obstructed scope should be smaller remains
obstructed to me at this time. Also, if your argument was right, why is it
tried to minimize secondary diameters, and why does anyone use these expensive
unobstructed APOs, and not to use very large, rather thin ring mirrors for
getting "sharper images" (well, I know some do with these interferometers).
Btw, please remember that I did not claim (nor anyone else) that
(a) the "obstruction problem" remains for larger apertures
(because atmospheric turbulences will give the limits there)
(b) obstructed scopes don't have their useful application
(c) that Newtons (esp. Dobsons) may not be cheaper than Schiefs for
same "separation power"; you may well find a Dobson for less than
a half-sized Schief, and will certainly find a good equatorially-mounted
Newton for far less than a half-sized APO.
(d) that Schiefs are anyway optically better than other unobstructed scopes;
all arguments are simply against obstruction for scopes smaller than
about 10-inch aperture.
(e) there are more important factors determining if you are successful
as observational (amateur) astronomer: Dark skies, steady air, etc.
Btw, if you or anyone else convinces me that this physics is wrong I'll
mention it in the webpages (maybe this will take some time but..). But only
in this case.
Anyway, this discussion seems to turn more and more useless, as IMHO all
relevant things concerning obstruction have been said (and should have been;
this is why I posted anything at all). At last, if you can't convince me,
I'll don't change or add to my webpages, and I have no hope, no intention,
and little ambition to convince you. If you think Newtons are best value,
this is absolutely no problem for me, personally. Simply, my potentially
biased opinion, observation and experience is that unobstructed scopes make
sharper images of some objects; this will be my last remark on this thread
(unless someone brings in really new stuff).
- hartmut
jjgoss
of you would be perfect fits for the US Congress.
J.Goss
bratislav wrote in message <36817467...@asac.ericsson.se>...
>mes...@my-dejanews.com wrote:
>
>> > If Mahajan and Born & Wolf
>> > are too difficult, try Schroeder, Suiter, Rutten & van Venrooij and
Zmek
>> > (in reverese order re difficulty of reading/following/understanding).
>>
>> Not too difficult. Studied physics long enough (was my learned
profession),
>> and still..
>>
>> Yet I did not, and could not, due to lack of time, give you here an
outline of
>> the relevant physics I had in mind. As I have currently no time to write
a
>> deeper one, I'll give you here a four-line summary only - sorry but ..:
>>
>> Basically Kutter said that for obstructed scopes (of his days, around
1960)
>> a significant portion of the light will go from the central Airy disk
into
>> the Newtonian rings, making the first Newtonian ring much brighter and
more
>> or less merging with it: Larger image (first-ring diameter).
>>
>> Why Airy disks of same-aperture obstructed scope should be smaller
remains
>> obstructed to me at this time.
>
>I'm afraid it will remain obstructed for quite a long time, as your
knowledge of
>subject (or lack of it) is glaringly obvious. Despite your claim that
Mahajan is
>"not too difficult" and that you have "studied physics long enough" it is
quite
>clear to me that you have never read one word from the authors I've
mentioned,
>and that your knowledge of optics is rather severely limited.
>Kutter's design is revolutionary; no doubt about it. But some of the
'physiscs' he's
>talking about just doesn't hold any water at all. And your interpretation
of it,
>with "Newtonian rings" is, well, almost amusingly simplistic.
>
>Bratislav
bratislav
TMBack
>well. Either of you would be perfect fits for the US
>Congress.
>
>J.Goss
I thank you.
Thomas Back
Martin Tom Brown
mes...@my-dejanews.com wrote:
> > > Well, I gave you an outline of this physics underlying Kutteræ„€ statement
> > > that the "image definition" of an unobstructed scope is a factor better than
> > > that of an obstructed scope (Kutter said "factor 2", I gave you my estimate
> > > of "factor 1.8"). Up to now, neither you nor any other poster has attacked
> > > this statement with physical arguments, not to speak of calculations
You need to be very careful using vague terms like "image definition".
What precisely are you saying is a factor of two better ?
The unobstructed aperture will have a slight advantage in contrast
over an obstructed aperture but for sensible Newtonians optimised
for planetary viewing though it is a close call.
> > > deliver physics, and if this disproves Kutter, I will hurry to change my
> > > statement in these pages. The physics is more outlined in Kutteræ„€ book and
> > > some easier available, but still old articles in various mags; sorry, didnæ„’
> > > yet find time to make a webpage from this.
> >
> > I think your definition of 'physics' needs some serious rethinking. Kutter
> > talks a lot about vague and dubious terms like 'definition'. I suggest you
> > read a REAL physics book. Start with wavefront analysis, MTF and then see
> > about the obstruction and degradations from it. If Mahajan and Born & Wolf
> > are too difficult, try Schroeder, Suiter, Rutten & van Venrooij and Zmek
> > (in reverese order re difficulty of reading/following/understanding).
> [..]
>
> Not too difficult. Studied physics long enough (was my learned profession),
> and still..
Oh dear.
> Basically Kutter said that for obstructed scopes (of his days, around 1960)
> a significant portion of the light will go from the central Airy disk into
> the Newtonian rings, making the first Newtonian ring much brighter and more
> or less merging with it: Larger image (first-ring diameter).
>
> Why Airy disks of same-aperture obstructed scope should be smaller remains
> obstructed to me at this time.
He is right, but it is a relatively small effect- the main lobe of
the point spread function becomes narrower and loses some intensity.
Meanwhile some of the Airy rings gain power (bad for contrast).
It is the power going into the Airy rings which decreases contrast.
But for small enough secondaries the effect is very small.
> Also, if your argument was right, why is it
> tried to minimize secondary diameters, and why does anyone use these expensive
> unobstructed APOs,
APO's have a lot to do with religious fervour.
Minimising the secondary diameter much below 1/10 of the aperture
diameter will not make very much difference.
> Btw, if you or anyone else convinces me that this physics is wrong I'll
> mention it in the webpages (maybe this will take some time but..). But only
> in this case.
I suggest you compute the diffraction pattern to be expected from
a range of perfect obstructed apertures before investing too much
time on your search for the Holy Grail.
I am surprised a set of these are not already on the Web somewhere.
Schiefspieglers are interesting as they allow a long focus instrument
to be made with no chromatic abberations. However, it is a lot more
work and you might well find a long focus Newtonian almost as good.
Regards,
--
Martin Brown <mar...@nezumi.demon.co.uk> __ CIS: 71651,470
Scientific Software Consultancy /^,,)__/
mes...@my-dejanews.com
before posting a lot of more crap like you did, let me make clear some facts:
a) because of my German nationality, I indeed read other authors' books on
the subject, in German language, of course. Therfore, some of the terminology
is more familar to me in German than in English, so maybe I understand some
English terms (Airy disk, e.g.) a little poorly. However, be rather sure that
I indeed know this stuff.
b) You are wrong with one small fact, and thus I don't believe anything else
you claim (note I won't claim this demonstrates any limits of your optical
knowledge, let the readers judge, but please never again post your misguided
views on what you think is my knowledge):
The central circular image of a point source is not smaller for an obstructed
scope. Visualize it for one dimension less: The unobstructed and the
obstructed slit, which can be briefly outlined here. The image of a parallel
bundle of light rays, in this case, has an angular width which can be
calculated quite easily. Consider the following situations:
1. ______ ______ unobstructed slit of width L
<--- L --->
2. ______ ___ ______ obstructed slit of width L and central
obstruction l
> l <
-----------+-----------> X
Then light of wavelength Lambda, going out at (small) angle Alpha wrt the
orthogonal to the slit, has, if starting at coordinate X in the slit's plane,
a phase shift Phi against the central ray's phase, given by
Phi = (2 pi/Lambda) * X * sin(Alpha)
Thus the amplitude is given by
A(X) = A_c * exp( i * Phi(X) )
where A_c is the central amplitude, and the intensity of the whole ray bundle
is given by
L/2
I_u = INT A(X)^2 dX
- L/2
in the unobstructed case and
- l/2 L/2
I_o = ( INT + INT ) A(X)^2 dX
- L/2 l/2
in the obstructed case. Inserting the values given, the integral evaluates to
INT A(X)^2 dX = (A_c)^2 * (Lambda / (2 pi sin(Alpha))) *
* (1/2i) exp(i (2 pi/Lambda) * X * sin(Alpha))
Inserting the limits and summarizing factors to I0, this yields
I_u = I0 * sin (pi (L/Lambda) sin(Alpha))
I_o = I0 * [sin (pi (L/Lambda) sin(Alpha)) - sin (pi (l/Lambda)
sin(Alpha))]
which has zeros for
pi (L/Lambda) sin(Alpha_u) = n * pi
pi (L/Lambda) sin(Alpha_o) = pi (l/Lambda) sin(Alpha_o) + n * pi
(integer n giving the order of the minimum) or
sin(Alpha_u) = n * Lambda / L
sin(Alpha_o) = n * Lambda / (L-l) > sin(Alpha_u)
(!; qed)
Thus, obstructed slits' images are indeed greater (wider) than those from
unobstructed slits. Same but longer and less simple to demonstrate for
circular disk images. BTW: Note that with smaller "l", the difference gets
smaller (as expected).
Now, that's all from me here on this obstructed thread ;) at this time.
If ever time permits, I'll add some more on this to the Schief webpage at
http://www.seds.org/~spider/scopes/schief.html
Cheers and a happy New Year, and once more, oblique views of a very clear
skies,
- hartmut
Derek Wong
>
> <snip>
>
> b) You are wrong with one small fact, and thus I don't believe anything else
> you claim (note I won't claim this demonstrates any limits of your optical
> knowledge, let the readers judge, but please never again post your misguided
> views on what you think is my knowledge):
Sorry for butting in, but I'm a reader who is just trying to learn
optics.
I don't want to insult anyone or any design, but I want to follow what
is
going on.
> Then light of wavelength Lambda, going out at (small) angle Alpha wrt the
> orthogonal to the slit, has, if starting at coordinate X in the slit's plane,
> ...
> Inserting the values given, the integral evaluates to
>
> INT A(X)^2 dX = (A_c)^2 * (Lambda / (2 pi sin(Alpha))) *
> * (1/2i) exp(i (2 pi/Lambda) * X * sin(Alpha))
OK, here there is a (2 pi sin(Alpha)) term in the denominator
> Inserting the limits and summarizing factors to I0, this yields
>
> I_u = I0 * sin (pi (L/Lambda) sin(Alpha))
>
> I_o = I0 * [sin (pi (L/Lambda) sin(Alpha)) - sin (pi (l/Lambda)
> sin(Alpha))]
Either I am not understanding this correctly, or something went wrong
with your computations. For Alpha = 0, shouldn't you have maxmimum
intensity? Your I_u and I_o both are 0 at Alpha = 0. Also, I_u and I_o
should not be negative for any alpha, correct?--there is probably a
square you are missing somewhere.
I think the problem here is that you omitted the denominator term (2 pi
sin(Alpha)), since you probably want I0 is a constant.
Also, in the second expression (for I_o), the peak intensity should be
different for the larger and smaller obstructions, right? In other
words, the expression should be
I_o = I0_L *... - I0_l * ...
where I0_L does not equal I0_l
Even if the rest of the analysis is correct this may affect your minima.
Please feel free to correct anything I have said which is wrong.
> Thus, obstructed slits' images are indeed greater (wider) than those from
> unobstructed slits. Same but longer and less simple to demonstrate for
> circular disk images. BTW: Note that with smaller "l", the difference gets
> smaller (as expected).
The books I have prove that a circular aperture with a central
obstruction has a slightly narrower Airy disk width than that of an
unobstructed scope.
Nevertheless, I would love to see your Schiefspiegler if you ever make
it to the U.S.
Derek Wong
daw...@earthlink.net
mes...@my-dejanews.com
these formulae in (marked "F>" below), sorry everybody (but I never said I
can type in equations ;) ):
In article <767hjg$3ki$1...@nnrp1.dejanews.com>, I wrote:
> The central circular image of a point source is not smaller for an obstructed
> scope. Visualize it for one dimension less: The unobstructed and the
> obstructed slit, which can be briefly outlined here. The image of a parallel
> bundle of light rays, in this case, has an angular width which can be
> calculated quite easily. Consider the following situations:
>
> 1. ______ ______ unobstructed slit of width L
>
> <--- L --->
>
> 2. ______ ___ ______ obstructed slit of width L and central
> obstruction l
> > l <
>
> -----------+-----------> X
>
> Then light of wavelength Lambda, going out at (small) angle Alpha wrt the
> orthogonal to the slit, has, if starting at coordinate X in the slit's plane,
> a phase shift Phi against the central ray's phase, given by
>
> Phi = (2 pi/Lambda) * X * sin(Alpha)
>
> Thus the amplitude is given by
>
> A(X) = A_c * exp( i * Phi(X) )
>
> where A_c is the central amplitude, and the intensity of the whole ray bundle
> is given by
>
F> L/2
F> I_u = INT A(X)^2 dX
F> - L/2
L/2
Correctly: I_u = [ INT A(X) dX ] ^2
- L/2
as amplitudes and not intensities are superimposed, as most of us know.
> in the unobstructed case and
>
F> - l/2 L/2
F> I_o = ( INT + INT ) A(X)^2 dX
F> - L/2 l/2
- l/2 L/2
Correctly: I_o = [( INT + INT ) A(X) dX ]^2
- L/2 l/2
> in the obstructed case. Inserting the values given, the integral evaluates to
>
F> INT A(X)^2 dX = (A_c)^2 * (Lambda / (2 pi sin(Alpha))) *
F> * (1/2i) exp(i (2 pi/Lambda) * X * sin(Alpha))
Irrelevant, moreover the relevant integral is
INT A(X) dX = A_c * (Lambda / (pi sin(Alpha))) *
* (1/2i) exp(i (pi/Lambda) * X * sin(Alpha))
and its square, taken at the limits, is the intensity.
> Inserting the limits and summarizing factors to I0, this yields
>
> I_u = I0 * sin (pi (L/Lambda) sin(Alpha))
Correctly: I_u = I0 * [sin (pi (L/Lambda) sin(Alpha))]^2
> I_o = I0 * [sin (pi (L/Lambda) sin(Alpha)) - sin (pi (l/Lambda)
> sin(Alpha))]
Correctly: I_o = I0 * [sin (pi (L/Lambda) sin(Alpha)) - sin (pi (l/Lambda)
sin(Alpha))] ^2
Well, of course this "I0" here is not constant, but has the value
I0 = (A_c)^2 * (Lambda / (2 pi sin(Alpha))) ^2
with a sin(Alpha) in the denominator. Obviously, this denominator has
a zero at Alpha = 0. As the numerator has one at the same value of X,
you may evaluate the function's value for Alpha=0 e.g. by de l'Hospital's
rule and find that for this value, the intensity has a maximum, as it
should be. It should perhaps be noted that A_c will increase linearly with
"free slit width", L or (L-l), respectively.
The rest of the outline, however, remains unchanged, as the squaring
operation doesn't change zeros of a function.
> which has zeros for
>
> pi (L/Lambda) sin(Alpha_u) = n * pi
>
> pi (L/Lambda) sin(Alpha_o) = pi (l/Lambda) sin(Alpha_o) + n * pi
>
> (integer n giving the order of the minimum) or
>
> sin(Alpha_u) = n * Lambda / L
>
> sin(Alpha_o) = n * Lambda / (L-l) > sin(Alpha_u)
> (!; qed)
>
> Thus, obstructed slits' images are indeed greater (wider) than those from
> unobstructed slits. Same but longer and less simple to demonstrate for
> circular disk images. BTW: Note that with smaller "l", the difference gets
> smaller (as expected).
Now, correctly presented, hopefully.
- hartmut
http://www.seds.org/~spider/
Zane
(mathematical derivation snipped)
Hello Hartmut
I had some trouble following your derivation, but you are trying to
disprove a well established fact. It's fairly easy to calculate the point
spread function for an obstructed system using equations formulated using
Bessel functions. (It's not very hard even by hand.) I'm not familiar
with German optical books, but this equation should be pretty easy to find.
You will find that the width of the central peak in the point image, as
measured at the point where its amplitude is 1/2 the peak, is indeed
narrower for an obstructed system.
Also, if you look at the modulation transfer function for different amounts
of central obstruction, you will see a peak at the high frequencies which
gets higher with more obstruction. This is because the central peak in the
line spread function (and therefore also the point spread function) is
getting narrower.
Zane
Derek Wong
complete bull*#$!, so I need HELP from someone who has actually had a
course in optics after high school :-)
>It should perhaps be noted that A_c will increase linearly with
>"free slit width", L or (L-l), respectively.
>The rest of the outline, however, remains unchanged, as the squaring
>operation doesn't change zeros of a function.
I know the mimima will not change despite the extra term in the
denominator. However, you did not account for the other portion of my
post.
> Thus the amplitude is given by
>
> A(X) = A_c * exp( i * Phi(X) )
I think this is key--the amplitude varies with the slit width, as you
said above.
> I0 = (A_c)^2 * (Lambda / (2 pi sin(Alpha))) ^2
***I0 also varies with slit diameter, proportional to L^2 or l^2
> L/2
> I_u = [ INT A(X) dX ] ^2
> - L/2
> in the unobstructed case and
OK, this seems to be true.
> - l/2 L/2
> I_o = [( INT + INT ) A(X) dX ]^2
> - L/2 l/2
>
> in the obstructed case.
> Inserting the values given, the integral evaluates to
> INT A(X) dX = A_c * (Lambda / (pi sin(Alpha))) *
> * (1/2i) exp(i (pi/Lambda) * X * sin(Alpha))
> Inserting the limits and summarizing factors to I0, this yields
> I_u = I0 * [sin (pi (L/Lambda) sin(Alpha))]^2
> I_o = I0 * [sin (pi (L/Lambda) sin(Alpha)) - sin (pi (l/Lambda)
> sin(Alpha))] ^2
Here's where my question lies: I don't think you can evaluate this
obstructed integral as simply as you did, because even though A_c is not
a function of X, it does change with different slits diameters. I think
that you need to think of this as the difference of two unobstructed
integrals:
For an unobstructed slit of width L:
INT A(X,L) dX = A_c(L) * (Lambda / (pi sin(Alpha))) *
* (1/2i) exp(i (pi/Lambda) * X * sin(Alpha))
For an unobstructed slit of width l:
INT A(X,l) dX = A_c(l) * (Lambda / (pi sin(Alpha))) *
* (1/2i) exp(i (pi/Lambda) * X * sin(Alpha))
For the obstructed system:
L/2 l/2
I_o = [( INT A(X,L) DX - INT A(X,l) dX ]^2
- L/2 -l/2
= K1 * [A_c(L)*sin (pi (L/Lambda) sin(Alpha))
- A_c(l)*sin(pi (l/Lambda) sin(Alpha))]^2
= K2 * [sin((pi*L/Lambda)sin(Alpha))
- R*sin((pi*l/Lambda)sin(Alpha))]^2
where K1,K2 = constants and
R = l/L = "Central obstruction ratio" (little L over big L)
since A_c(L) is proportional to L
This I_o would not have the same minima you stated in your other post,
although to complete the proof one would need to go through more
calculations.
Anyway, I may not be completely accurate, but the concept will still
hold. In _Optics_ and other texts there are pictures and calculations
of circular apertures using Bessel functions which show that the
diameter of the Airy disk with a 50% obstructed aperture, for example,
has a width approximately 20% less than the Airy disk of an equivalent
unobstructed aperture.
Derek Wong
daw...@earthlink.net
mes...@my-dejanews.com
> Here is my stab at breaking "Hartmut's Paradox". This post may be
> complete bull*#$!, so I need HELP from someone who has actually had a
> course in optics after high school :-)
Hmm, you named it .. actually I don't see why "it" should be a "paradox",
just a phenomenon. And I won't classify your post.
> >It should perhaps be noted that A_c will increase linearly with
> >"free slit width", L or (L-l), respectively.
Hmm, don't take this "linearly" too serious; actually, it will not vary
necessarily vary this way. See below.
> >The rest of the outline, however, remains unchanged, as the squaring
> >operation doesn't change zeros of a function.
>
> I know the mimima will not change despite the extra term in the
> denominator. However, you did not account for the other portion of my
> post.
As only the minima (defining dimensions of images) were subject of my original
post, and actually I did not yet reply to your post, just corrected mine.
> > Thus the amplitude is given by
> >
> > A(X) = A_c * exp( i * Phi(X) )
>
> I think this is key--the amplitude varies with the slit width, as you
> said above.
Hmm, yes, but keep in mind that for one slit under one illumination, A_c
is a constant. Same light source, it varies with "free" slit width, L-l.
> > I0 = (A_c)^2 * (Lambda / (2 pi sin(Alpha))) ^2
>
> ***I0 also varies with slit diameter, proportional to L^2 or l^2
Where do you see an "L" or "l" here ? It is still hidden in the A_c.
> > L/2
> > I_u = [ INT A(X) dX ] ^2
> > - L/2
> > in the unobstructed case and
>
> OK, this seems to be true.
>
> > - l/2 L/2
> > I_o = [( INT + INT ) A(X) dX ]^2
> > - L/2 l/2
> >
> > in the obstructed case.
>
> > Inserting the values given, the integral evaluates to
>
> > INT A(X) dX = A_c * (Lambda / (pi sin(Alpha))) *
> > * (1/2i) exp(i (pi/Lambda) * X * sin(Alpha))
>
> > Inserting the limits and summarizing factors to I0, this yields
>
> > I_u = I0 * [sin (pi (L/Lambda) sin(Alpha))]^2
>
> > I_o = I0 * [sin (pi (L/Lambda) sin(Alpha)) - sin (pi (l/Lambda)
> > sin(Alpha))] ^2
>
> Here's where my question lies: I don't think you can evaluate this
> obstructed integral as simply as you did, because even though A_c is not
> a function of X, it does change with different slits diameters. I think
> that you need to think of this as the difference of two unobstructed
> integrals:
>
> For an unobstructed slit of width L:
> INT A(X,L) dX = A_c(L) * (Lambda / (pi sin(Alpha))) *
> * (1/2i) exp(i (pi/Lambda) * X * sin(Alpha))
Yes, my formula above.
> For an unobstructed slit of width l:
> INT A(X,l) dX = A_c(l) * (Lambda / (pi sin(Alpha))) *
> * (1/2i) exp(i (pi/Lambda) * X * sin(Alpha))
>
> For the obstructed system:
>
> L/2 l/2
> I_o = [( INT A(X,L) DX - INT A(X,l) dX ]^2
> - L/2 -l/2
I fear this may be wrong, as A_c will have one value also for this
configuration. Actually, a parallel light bundle will fall in with some
intensity (energy per second and square inch, or in one dimension less,
energy per second and [width] inch) which is independent of slit
configuration and dimensions. Energy will certainly be conserved, and
this will define A_c = A_c(L) or A_c(L,l) [of course independent of X].
> = K1 * [A_c(L)*sin (pi (L/Lambda) sin(Alpha))
> - A_c(l)*sin(pi (l/Lambda) sin(Alpha))]^2
>
> = K2 * [sin((pi*L/Lambda)sin(Alpha))
> - R*sin((pi*l/Lambda)sin(Alpha))]^2
>
> where K1,K2 = constants and
> R = l/L = "Central obstruction ratio" (little L over big L)
> since A_c(L) is proportional to L
>
> This I_o would not have the same minima you stated in your other post,
> although to complete the proof one would need to go through more
> calculations.
>
> Anyway, I may not be completely accurate, but the concept will still
> hold. In _Optics_ and other texts there are pictures and calculations
> of circular apertures using Bessel functions which show that the
> diameter of the Airy disk with a 50% obstructed aperture, for example,
> has a width approximately 20% less than the Airy disk of an equivalent
> unobstructed aperture.
Well, after all, I will probably have some days in January left for doing
some studies of optics texts, and perhaps take another attack on the
obstruction prob. If anything notable should come out there I'll perhaps
put it on web and announce.
- hartmut
Martin Tom Brown
mes...@my-dejanews.com wrote:
> Bratislav,
>
> before posting a lot of more crap like you did, let me make clear some facts:
>
> a) because of my German nationality, I indeed read other authors' books on
> the subject, in German language, of course.
I doubt you will find any German optics texts which make such
terrible mistakes in deriving simple 1-D diffraction patterns.
> b) You are wrong with one small fact, and thus I don't believe anything else
> you claim (note I won't claim this demonstrates any limits of your optical
> knowledge, let the readers judge, but please never again post your misguided
> views on what you think is my knowledge):
>
> The central circular image of a point source is not smaller for an obstructed
> scope.
It is and you are apparently unaware of this relatively well known result.
Had you done the 1-D case properly you would have been able to prove it!
You owe Bratislav an apology...
> Visualize it for one dimension less: The unobstructed and the
> obstructed slit, which can be briefly outlined here.
[complete garbage snipped]
Since we are only concerned here with the shape and intensity
of the psf of two different apertures in focus it is possible to
short circuit a lot of tedious mathematics by using the principle
of superposition to generate the required obstructed aperture.
A unit width unobstructed aperture gives amplitude sin(x)/x and
then for another width of aperture b the amplitude response is
Width PSF
1 sin(x)/x
b sin(bx)/x
The obstructed aperture can be made up of a positive contribution
over a unit aperture summed with a negative contribution over the
smaller region b and the diffraction pattern of the obstructed aperture
can be computed as the difference of these two diffraction patterns.
This gives a diffraction pattern for the 1-D unit aperture
with a symmetric central obstruction 0<b<1 which looks like:
sin(x)/x - sin(bx)/x
And since we are sensitive to power the diffraction pattern
or PSF looks like the square of the above function.
D(x) = [ sin(x)/x - sin(bx)/x ] ^ 2
For a unit aperture with central obstruction width b the expression
sin(x)/x - sin(bx)/x
can also be rewritten
(2/x).sin((1-b)x/2).cos((1+b)x/2)
which is very convenient for our purposes here which is to
show that the distance to the first zero of the function
decreases as the the obstruction b increases.
The zeroes are when cos((1+b)x/2)=0 or sin((1-b)x/2)=0
hence (1+b)x/2 = (2n+1).pi/2 (any n) or (1-b)x/2 = n.pi (n>0)
which gives a first zero at
x = pi/(1+b)
So as the size of the obstruction b increases from 0 to 1
the position of the first minimum in the diffraction pattern
moves closer into the main lobe. The main lobe gets narrower. QED
You don't have to trust my algebra (typos are inevitable)
it is trivial to program sin(x)/x and sin(bx)/x in a
spreadsheet and plot the results for yourself for 0<b<1.
This derivation also works for centrally obstructed circular
apertures too, but you then have to include the area contribution
of the aperture and get Bessel functions instead of sin(x)/x
The 2-D area contributions make the effects less pronounced and
there is no cute factorisation trick for circular apertures.
(someone posted the circular aperture results fairly recently!)
John Ongtooguk
: I suggest you compute the diffraction pattern to be expected from
: a range of perfect obstructed apertures before investing too much
: time on your search for the Holy Grail.
An easier way to do such things is with Suiter's program that is
available as mentioned in his book on star testing. It seems to be
fairly complete for calculating the impact of circularly symmetric
changes to an aperture, such as zones, obstruction, apodizing screens,
spherical aberration and such, with the output available in
different formats. Following is some work that I did using his
program, rescaling the OTF to 'equivalent ideal aperture':
From: jo...@vcd.hp.com (John Ongtooguk)
Newsgroups: sci.astro.amateur
Subject: Displaying the OTF
Date: 22 Apr 1996 08:15:44 GMT
Part 1 of 3
Following is a method that I developed in an attempt to make
optical transfer function (OTF) values easier to understand.
I haven't seen OTF values displayed this way before but
I'm just a working stiff and not an optical engineer or
physicist [the method seems to be fairly good at estimating
the performance of decent scopes, where there aren't a
multitude of problems....John O. 25Jul96].
OTF values are used to indicate optical performance, usually
in the following manner:
Fraction
of Max Perfect
Freq OTF OTF Ratio
0.05 0.865 0.936 0.923
0.10 0.723 0.873 0.828
etc.
0.90 0.038 0.037 1.003
0.99 0.001 0.001 1.061
While an OTF of 0.723 at a frequency of 0.1 is lower than
the perfect OTF of 0.873, and the ratio is 0.83, it's not
intuitive as to what it really means. The frequency can
expressed in cycles per arc second given an aperture and
a wavelength, per the following example (I used arc seconds
per cycle):
4in 8in 16in
FREQ arcsec arcsec arcsec
0.05 22.33 11.17 5.58
0.10 11.17 5.58 2.79
etc.
0.80 1.40 0.70 0.35
0.90 1.24 0.62 0.31
0.99 1.13 0.56 0.28
It's seen that a frequency of 0.8 is slightly more than
the familiar Rayleigh resolution criteria. One can even
calculate a series of such values and plot them versus
the perfect OTF, plot an aberrated OTF on top of the
perfect values, and see what the 'equivalent perfect
aperture' is, a method that I mentioned in an earlier
thread. An easier method is to use a ratio of frequencies
in the following manner:
Perfect 8in Perfect
FREQ OTF OTF arcsec Aperture
0.06 0.835 0.924 9.30
0.13 0.835 4.29 3.74
1. The OTF under consideration is 0.835 at a frequency
of 0.06, which is less than the perfect OTF of 0.924.
2. What perfect aperture has an OTF of 0.835 at 4.29
arc seconds ? A 3.74in aperture has such a value.
[edited out some text per a good suggestion from
J. Freeman.........John O. 25Jul96]
8in with 1/2 wavelength of spherical aberration
8in 25% Obstruction 33% Obstruction
FREQ arcsec %aper Aperture %aper Aperture
0.05 11.17 0.294 2.35 0.286 2.29
0.10 5.58 0.273 2.19 0.273 2.18
0.20 2.79 0.332 2.66 0.348 2.78
0.30 1.86 0.453 3.62 0.471 3.77
0.40 1.40 0.555 4.44 0.578 4.62
0.50 1.12 0.663 5.31 0.685 5.48
0.60 0.93 0.843 6.74 0.833 6.67
0.70 0.80 0.925 7.40 0.951 7.61
0.80 0.70 0.892 7.14 0.909 7.27
0.90 0.62 0.951 7.61 0.957 7.66
0.99 0.56 0.999 7.99 0.999 7.99
The reason for doing 1/2 wave scopes first, with typical
obstructions for Newtonians and SCTs, is that these are
evidently common values. In the S&T article on the scopes
made by Ceravolo to known, different quality levels the
1/2 wave scope seemed to typify an average quality level.
In another S&T article on 'beginner's' scopes an 80mm
refractor did better on the planets than a 10in reflector,
indicating that the reflector was exhibiting performance
like that indicated above. One can use the equivalent
perfect scope values in an attempt to ballpark correction
errors by comparing views of one of known quality.
8in with 1/4 wavelength of spherical aberration
8in 20% obstruction 33% obstruction
FREQ arcsec %aper aperture %aper aperture
0.05 11.17 0.538 4.31 0.496 3.96
0.10 5.58 0.508 4.06 0.477 3.81
0.20 2.79 0.571 4.57 0.532 4.26
0.30 1.86 0.680 5.44 0.608 4.86
0.40 1.40 0.754 6.04 0.697 5.58
0.50 1.12 0.856 6.85 0.821 6.57
0.60 0.93 0.965 7.72 0.953 7.62
0.70 0.80 0.965 7.72 1.011 8.09
0.80 0.70 0.956 7.65 0.987 7.89
0.90 0.62 0.983 7.86 0.993 7.95
0.99 0.56 0.999 7.99 0.999 7.99
The above shows the impact of obstruction on 1/4 wave
scopes, where it's apparent that there isn't much to
choose between the two. The spherical aberration is
obscuring the effects of obstruction. Before worrying
too much about obstruction make sure that your scope
can take advantage of any possible improvements.
8in with 1/8 wavelength of spherical aberration
8in 33% Obstruction 0% Obstruction
FREQ arc sec %aper aperture %aper aperture
0.05 11.17 0.624 4.99 0.873 6.98
0.10 5.58 0.609 4.87 0.846 6.77
0.20 2.79 0.632 5.05 0.861 6.89
0.30 1.86 0.664 5.31 0.891 7.13
0.40 1.40 0.743 5.94 0.929 7.43
0.50 1.12 0.873 6.98 0.967 7.73
0.60 0.93 0.993 7.95 0.985 7.88
0.70 0.80 1.030 8.24 0.988 7.90
0.80 0.70 1.011 8.09 0.987 7.89
0.90 0.62 1.006 8.05 0.995 7.96
0.99 0.56 0.999 7.99 0.999 7.99
But, obviously obstruction can't be ignored as the
table above indicates.
Both have 1/8 wavelength of spherical aberration
4in apo refractor 8in reflector, 15% obs.
FREQ arcsec aperture FREQ arcsec aperture
0.05 22.33 3.49
0.10 11.17 3.38 0.05 11.17 6.05
0.20 5.58 3.44 0.10 5.58 5.94
0.30 3.72 3.56 0.20 2.79 6.34
0.40 2.79 3.71
0.50 2.23 3.87
0.60 1.86 3.94 0.30 1.86 6.76
0.70 1.60 3.95
0.80 1.40 3.95 0.40 1.40 7.07
0.90 1.24 3.98
0.99 1.13 4.00 0.50 1.12 7.56
0.60 0.93 7.97
0.70 0.80 7.95
0.80 0.70 7.92
0.90 0.62 7.98
0.99 0.56 7.99
The above shows that aperture can be hard to beat given
equal quality, even including some obstruction.
Part 2 will be examples of 8in scopes and Part 3 will be
general tables of aperture reduction, allowing examples
to be generated for any aperture with a minimum amount
of arithmetic.
John Ongtooguk (jo...@vcd.hp.com)
Paulo Almeida
>In Spain they will know it, in Portugal are only a handfull amateurs. Russia
>is east europe, about which i not talked, most People from North Irland know
>better the guns than telescopes.
Your reputation just went down the drain with this statement. I live
in Portugal and my local astro-club counts more then 600 menders. Not
all of then have scops , that's true, never the less they count as
amateur astronomers.
Paulo Almeida
C11 @ 39,52N-8,56W
lude...@my-dejanews.com
My apology to you. i didn't know that. I have an customer in Portugal since
many years and he told me many times, that nearly no amateurastronomers living
in Portugal. please accept my apology.
best wishes
Markus