8" F-7.5 Zambuto Primaries For Sale
Ed Erbeck Jr.
Selling some Primaries that the Blank supplier sent with the wrong curve
pregenerated on them.
So what I'm Selling are 8" F-7.5 Zambuto Primaries with Standard
Coatings and 1.3" Diagional Mirrors Manufactured by AXE with 96% Enhanced
Coatings with Carls Documentation for his Mirror and Documentation for the
Diagional.
Price is $950 which includes Shipping in the U.S.A.
Ed Erbeck Jr. (Owner Crazy Ed Optical)
Online Catalog:
http://www.crazyedoptical.com
Snail Mail:
Crazy Ed Optical
P.O. Box 446
Pearce, AZ 85625-0446
Phone / Fax 520-826-1484
ste...@binghamton.edu
<e...@crazyedoptical.com> wrote:
Curious as to how much the primary weighs?
Also, how many do you have?
Larry Stedman
Vestal, NY
Ed Erbeck Jr.
> Larry Stedman asked:
> Curious as to how much the primary weighs?
~5lb
>
> Also, how many do you have?
Larry and everyone else. This has been the most asked question and I'm
not trying to be coy or evasive, but I'm not going to give that information
out (and to end the second most asked question, Nor I will not sell the
"Lot" to a single buyer-which would be impossible now anyhow).
The reasoning behind this (and Carl agrees) is we both want to see these
find their way into the hands of people that will build Scopes out of them
and enjoy them, not someone who's only interest may be to own X of Y run of
Mirrors. These were (as are all of Carls Mirrors) made to collect Photons
not Dust. So knowing how many there are really shouldn't matter.
I just hope you and the others understand.
jerry warner
J Warner
Ed Erbeck Jr.
> Jerry Warner asked:
>
> Why so pricey, if I may be so naive as to ask?
I guess "Pricey" is a relitive term. Yes compaired to mass produced
undocumented Optics these may seem "pricey". But these are neither.
For an Example of Cost Deltas, say your Car needs a new Engine, you've
got 2 Options, 1. Head to the nearest garage and have them drop in a rebuilt
outta a Crate and you're on your merry way with a running Car but at an
unknown quality, or 2. You go to a "Perfomance Shop" at several times Option
1's coat and have the Engine Blue Printed and Dyno Tested so now you know
the Engine will do all it is suppost to and have documention to prove it.
So these are for (and this is about as close to "Marketing Hype" as I
ever care to get) Performance users that want to know "What's Under the
Hood" and understand the Price of knowing. To them these are not "Pricey"
I hope this helps in your understanding and thanks for asking.
Sue_and_Alan_French
Actually, I had the same thought. A price of $950 for even a custom made 8"
mirror seems a bit steep (even with a diagonal). There are certainly other
folks quite capable of making a fine 8" mirror and, I believe, at lower
prices. Mirrors have traditionally been somewhat underpriced, but if this
is the start of a trend for custom mirrors we may well see more folks trying
their hand at mirror making.
Personally, I think I'd be much happier with an interferogram than knife
edge readings as documentation, especially when paying such a premium for
the mirror.
Clear skies, Alan
"Ed Erbeck Jr." <e...@crazyedoptical.com> wrote in message
news:B7E364F9.2F747%e...@crazyedoptical.com...
>
>
> > Jerry Warner asked:
> >
> > Why so pricey, if I may be so naive as to ask?
>
> I guess "Pricey" is a relitive term. Yes compaired to mass produced
> undocumented Optics these may seem "pricey". But these are neither.
> [SNIP]
SAS
& polished an 8" f/8 mirror for the first time. It takes some time & work,
but I found the project to be extremely rewarding and I've learned a lot.
The 8" f/8 is also one of the easier mirrrors to make since the parabola
doesn't deviate that much from a sphere. In fact, it only took one rotation
around the mirror to get it properly figured. And the cost was pretty
cheap.
However, if I were to place a price on my work. I'd probably set it at
around the $1000 mark too. Admittedly you can buy a 8" f/6 for a fraction
of the price from some company like Orion. But it just wouldn't be worth it
to me to sell a mirror for a bare fraction over the cost to buy the kit and
coat the mirror.
-SAS
Simple199986
Take a Corvette and a Chevette. Boths cars will get you from point A to point
B.
However the Corvette will out perform the Chevette in very aspect, except gas
mileage. It's just better but for a price.
Plus alot of commerical mirror's out there are hit and miss. Sometimes they
ship out really good mirrors for $300 or $400, and sometimes they ship out
really bad mirror's for the same $300 or $400.
Carl Zambuto is know for making some of the best, if not the best, mirror's
in the world. You know when you buy a "Z" mirror that you will get a "Good"
one.
Stedystate
It is not about a "good" mirror vs a "bad" one. That's the wrong ballpark
altogether. It is about unexcelled performance. These mirrors are not an option
for those who are considering or comparing the general market offerings. These
mirrors will in reality go to 100 power per inch. They have no limitations
outside of their physical aperture. They are incredibly smooth as the
long-focus aspect allowed them to be figured with a 69% diameter lap. Because
of this they are also incredibly accurate, moreso than a common shop
interferometer (with a 1/10 to 1/20 wavefront internal error calibration) is
capable of determining. They have a single aluminum layer with an ion-deposited
quartz overcoat for very high contrast.
They did not parabolize in "one turn around the barrel". After an elongated
polish for an essentially perfect scratch/dig surface rating they figured
during many hours of smoothing, adjusting and precise measurement over a period
of days.
An 8" Newtonian optical set of this caliber (Goff/Zambuto combination) will
outperform any 6" refractor of any quality for resolution and detail when
assembled in a premium telescope with all the appropriate parameters addressed.
Is a top notch 6" APO considered "pricey"? That perspective is also relative.
These mirrors are for the advanced amateur who knows how to build a high
performance telescope and who knows what they are after. If someone is
fortunate enough to acquire one of these sets it is something that should be
kept for a lifetime, as such optics sets are rare.
Another thing, these are available for immediate shipment. There is no 7-9
month waiting period. However as I understand it half of them are already gone,
and this is a one-time situation. If someone is serious about performance from
an 8" system with a 60 inch focal length, they have an opportunity.
Carl Zambuto
SAS
elongated
> polish for an essentially perfect scratch/dig surface rating they figured
> during many hours of smoothing, adjusting and precise measurement over a
period
> of days.
Elongated polishing makes sense of course. But I'm not sure I understand
the long
figuring time for an 8" f/8. More than a few minutes and it seems like you
would
risk missing the figure you were after. Please educate an amateur mirror
maker
if you don't mind!
Stedystate
From: "SAS" sasch...@earthlink.net
------------------------------------
Dear SAS,
In order to properly answer your query I have to impart more information than
just a note on what it takes to fully correct a mirror. It is not so simple.
we’re working from different places and different perspectives. And
realistically, our goals are probably different as well. Because you have
asked as a mirror maker, I will take the time and answer this, once. I’m
happy to do this in part because it does give me an opportunity to “blow our
horn”. I am going to address you as a mirror maker.
I was an amateur mirror maker for eight years before I ever threw the switch on
my first machine. When I did that, my new education began. I had to relearn
many aspects of mirror making. I will not go into all of it because it is a
re-think of many processes. However I will address specifically what applies.
But first, I have to backtrack.
Contrast is everything. Period. The items involved are the following:
1. A thorough grind with appropriate durations at each stage that first
eliminate not only the generator marks left by the curve generation process
(diamond generated curve) but also the sub-surface fractures that exist
because of them. Each ensuing grit has a minimum operating time designed to
eliminate not only the pits and surface, but also the sub-surface of the
previous grit.
2. The same goes for polish. There are things we have learned about polish and
glass from some approx 1,000 mirrors to date that is not talked about in
amateur forums. We polish at high speed, that is, a minimum of 60 RPM with an
80% diameter lap on top (mirror face up) for no less than 12 hours total time
for small mirrors for rough and finish polish. Large mirrors have a minimum
rough-polish time of the same duration, then we add a finish polish beyond that
with our final compound for another 6 hours minimum. This might be equivalent
to walking around the barrel for a week non-stop on a small mirror. -Maybe. the
reason we do this is there are things that happen beyond that first
“optical” appearance of a full polish. We are actually removing glass
stock. For an idea of how much, we can polish out a deep scratch in two days of
work-time, or 24 hours by machine. A note here, We see commercial mirrors that
were only polished for a very short duration as in a couple or three hours, as
we have seen haze around edges, and see surfaces that have a gray finish, and
so on. This stock removal I’m speaking of removes the fractures that exist
under the surface left by the grind. Even though a polish appears to be
complete optically we can “feel” the surface change in smoothness as the
polish progresses. We have a technique with an acetone wipe that tells us about
this. This accomplishes three things- one, the glass becomes a stronger piece
as the surface is made more contiguously smooth right into its substructure.
Two, as the sub-surface fractures are eliminated we “strangely” note that
sleeks that exist early in the polish, disappear. Even though the mirror
already looked done optically, the surface characteristics changed, in that it
becomes so smooth that it does not grab the larger cerium particles and
sleek. What is happening is all aberrations in the glass introduced by the
curve generation and grind, both on and below the surface, are removed. What is
left after our finish polish (that is, with our finishing compound) is a very
strong piece of glass that has a low surface angstrom roughness interrupted by
nothing, right down into the glass. Visually, under high-intensity beam one can
see nothing. The glass has no surface whatsoever. (It is a beautiful thing for
the mirror-maker to observe.)
Why do I say all this? Because this is what we start with before the mirror
ever goes to figuring.
3. Now lets talk about the figure. We now have a rough sphere, either slightly
oblate or prolate with a decent edge and a very low surface angstrom roughness.
It has been checked for figure of revolution (astigmatism) and is ready for the
figuring machine.
At a focal ratio of F/7.5 it is imperative to get a very good sphere, first.
This is typically done with a 60% diameter tool used on top (TOT). We are able
to continuously adjust the stroke length and offsets, never staying long in any
one position. In the case of F/7.5 I was able to use a 69% diameter lap
(larger than normal) which allows extreme control of the macro-surface with
this relatively slight deviation from the parabola. When using a sub-diameter
tool on top, going through the center deepens the center, and going to the 70%
zone flattens it back out to the sphere. The edge is corrected by a
full-length stroke combined with offset (See Mel Bartels instruction on
figuring large mirrors at his website for further information) Somewhere
in-between we are able to precisely control the overall shape, using stroke
length, offset and durations. So first we get a magnificently smooth sphere
with a perfectly balanced edge, then we begin the correction. All of this is
“figuring time”.
Now, about the typical MOT and the barrel-method. With the mirror on top of a
soft lap using a “W” stroke can one dig out F/7.5 correction as measured
from center to edge in one trip around the barrel? Of course. That is easy to
do. But what is the amplitude of your primary ripple, and how accurate to the
parabola is each place on the mirror? Can you finish with less than lambda/100
microripple when you have removed 12 microinches of glass in a couple of
minutes? Can you control with your “W” stroke (MOT) in one trip around the
barrel *every* point on the mirror by radius to better than 1/20 wavefront? I
dare say not in either case.
I used to do small, high-quality mirrors by hand, and developed a technique to
get a smooth figure with narrow channels, extremely thin, decanted cerium and
soft pitch producing a hard vacuum between the mirror and pitch and doing a
straight-over long stroke for one minute, then pressing for 15 minutes. It
took more time than it does now, but I got drop-dead gorgeous smooth pieces.
(there is a lot of similarity in concept between that and what we do now).
So my point is to you, how smooth is that mirror? We cannot discuss that
directly because we do not have the same reference, that is, you are not here
in this shop comparing pieces directly. Inotherwords,”how smooth is
smooth”, we cannot say, between us. What I can say is I can pretty well bet
that one trip around that barrel does not produce the mirror we are speaking of
here.
Now for comparison, the best I can do is give you a visual example of "smooth".
Go to the Mag 1 optics page:
http://www.mag1instruments.com/optics.html
Click on the focogram images, and be *certain* you view the 182K tiff images,
as those have the resolution. (You will probably need Quicktime to decode
them.) Being as you are a mirror-maker you probably (hopefully) have
Texereau’s book. Find figure 37-D of his finished mirror imaged with a
focogram on the bench. Note his estimated lambda/50 microripple and hold the
image up to the computer screen and compare. You will see that the mirror on
your computer screen is visibly smoother, with microripple that is a fraction
(if you can see it at all) of the amplitude of Texereau’s mirror. (the
vertical lines are knifedge diffraction) Furthermore, note the shadows of the
Mag-1 mirror and how they “roll”. You see no zones whatsoever. you are
looking at a zone-free approx 1/20 wavefront mirror as measured by a zonal
Foucault test. That is the kind of figure we are talking about here. This is
what is required for the kind of contrast we enjoy.
So now to your question, in essence, how do we do it? We use a very slow
stroke with only ounces on the quill and over a period of hours of machine time
we shape first the sphere, then slowly remove the 12 microinches of glass
from edge to center in the smoothest possible fashion, by balancing the
center against the edge and everything in-between with precise adjustments
down to as little as 1/8” changes in stroke length and offset. Now
understand that we removed enough glass to figure the mirror probably fifty
times. But therein lies one of our secrets. Our figuring techniques are working
the whole mirror *simultaneously* where we are able to control the shape
precisely and predictably by favoring one area over another. Another way to
express it is we smooth the entire “canvas” of the mirror continually,
making very small and precise adjustments to tweak the figure through its
progression. How long would it take in actual machine time to go striaght to
the parabola from a perfect sphere in this size and F-ratio using this method?
About 2-1/2 hours ideally for the 8” f/7.5 mirror, although it rarely works
that way. So the mirrors in question spent several hours of machine time during
figuring, over a course of days employing measurements and adjustment. (Note
that we do not do one piece at a time, we will typically have several in play
equilibrating and being figured in rotation.)
The final result is mirrors which show surfaces on the bench that will “take
your breath away” for smoothness, with a measured figure in a 4-zone knifedge
test where the worst one in this particular run returned lambda/33 at the
wavefront.
All the above factors results in the contrast we enjoy in our reputation. In
other words, it does not come easy, and it does not come cheap. Based on what
you have just read, Mssrs SAS, you might agree with me by now that many owners
of these optics do not know what they have in their hands. And it is typically
different than what the general amateur mirror maker will produce. Now, can the
advanced, seasoned amateur produce such a mirror? Certainly. But it will likely
take weeks from start to finish. I know it always did for me.
I’m not certain I have answered your question in the way you intended, but my
aim was to give you some perspective as a mirror maker. I hope I have
accomplished that.
thanks for making your query.
Carl Zambuto
PS Being as this was a Saturday AM I was willing take some time during my
relaxation and reading to write this. However I’m not going to make a habit
of it, if you understand my meaning :)
Enyo
"Stedystate" <stedy...@aol.com> wrote in message
news:20011013141700...@mb-fh.aol.com...
>
>Big SNIP>
> "optical" appearance of a full polish. We are actually
removing glass
> stock. For an idea of how much, we can polish out a deep
scratch in two days of
> work-time,
I recall a conversation I had with some Perkin Elmer guys 15
years ago and they said to get a good final surface for
every grinding and polishing step you had to take off at
least 3x the depth removed from the previous one. This is
required to remove sub surface damage.
Ted Kord
Thanks for a very informative explanation. I've never tried my hand at
mirror
making, but it sounds interesting.
Just let me say that I once had an opportunity to look through a
Starmaster
with one of your mirrors in it. Now I'm not an optical expert, and
know nothing of focault tests, interferometers, etc... but the view
brought tears to my eyes. Not only for the clarity/contrast that I
saw, but because I knew my old dob would never satisfy me again.
If I had the money to spare, I'd snap one of these up in a
heartbeat...
Ed Erbeck Jr.
> If I had the money to spare, I'd snap one of these up in a
> heartbeat...
Sorry but too many Heart Beats have passed as there are no more in
Stock.
ValeryD
Tell us, please, what is an internal error in your own interferometer and
what an average quality of these mirrors? Or were they tested by knife
method with somewhat colose to 1/63 wave P-V ??
If your interferometer has smaller than 1/20 wave internal error calibration,
which one did you used for calibration of your own?
Valery Deryuzhin.
Stedystate
I apologize for causing you tears with respect to your "loving old telescope".
I agree that once such a view is had it is difficult to go back. Perhaps
Starmaster needs to put a warning label on their scopes...
I don't have any advice at this point, except that when I was a beginning
mirror-maker I had little money for the hobby and more time. I used to pride
myself on how little my complete scopes cost. Of course i had weeks worth of
labor into any one of them. Perhaps mirror-making/refiguring would be a hobby
of interest to you, in conjunction with a local mirror-making group or the ATM
list. That is a way to learn a new thing and gain great satisfaction in the
result.
Best regards,
Carl
---------------------
jerry warner
should be framed and hung at the Smithsonian. Now you know why
Zambuto mirrors are what they are and why Mr. Zambuto is a very
special premier optical craftsman and a true artisan as well as a man
in science. What a joy!
J Warner
Ted Kord
disappointed.
Now, I've been thinking a trying an off-axis dob experiment. If Mr. Zambuto were
to cookie-cut a smaller mirror from one of his larger ones, I might have to
forego the mortgage for a month for that...
"Ed Erbeck Jr." <e...@crazyedoptical.com> wrote in message news:<B7EE8DDE.30ED9%e...@crazyedoptical.com>...
Howard Lester
> Perhaps mirror-making/refiguring would be a hobby
> of interest to you, in conjunction with a local mirror-making group or
the ATM
> list. That is a way to learn a new thing and gain great satisfaction in
the
> result.
Carl, very well put! 22 years ago I debated long and hard with a friend,
who, in my eyes, happened to be a master telescope maker, as to where I
could buy a good 6" mirror. One day I said to him, "You know, if I'm going
to learn anything about this, I might as well make my own." His reply:
"That's what I've been trying to tell you all along!"
Several months of my weekends at the barrel, under his tutelage, gave me a
superb mirror. I still use it with pride.
The amazing thing to us both is that, on the final day/night of polishing, I
actually bypassed the sphere stage and went right into the paraboloid,
touched it up twice, and stopped. SOMEthing was working just right that
night.
Howard Lester
Tucson, AZ
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Ed Erbeck Jr.
> Hi Ed, Just read your post on s.a.a. saying that the mirror's are all gone.
> So now that they are gone can you tell me, how many mirror's were availible
>to begin with? I guess I want to feel special or one of the lucky ones:)
> Thanks
> Dave Scott
As too being Special or Lucky, well frankly you are in both cases. But
to them all being sold, What I said on s.a.a. was there are no more "In
Stock".....
da Crazy Man
Ted Kord
Thanks for the reply. I'd actually like to try my hand at mirror making, but
sadly I have very little free time even for viewing, let alone anything else.
But, once the new baby is born, and I get through the first couple of years
expenses, I'll be able to put aside the cash for a premium scope. It's just
a matter of time, and maybe there will be some advances between now and then.
Patience is a virtue, especially in this hobby...
stedy...@aol.com (Stedystate) wrote in message news:<20011014165952...@mb-fw.aol.com>...
> Mr Kord,
>
> I apologize for causing you tears with respect to your "loving old telescope".
> I agree that once such a view is had it is difficult to go back. Perhaps
> Starmaster needs to put a warning label on their scopes...
>
> I don't have any advice at this point, except that when I was a beginning
> mirror-maker I had little money for the hobby and more time. I used to pride
> myself on how little my complete scopes cost. Of course i had weeks worth of
> labor into any one of them. Perhaps mirror-making/refiguring would be a hobby
> of interest to you, in conjunction with a local mirror-making group or the ATM
> list. That is a way to learn a new thing and gain great satisfaction in the
> result.
>
> Best regards,
>
> Carl
>
<snip my own particular brand of wisdom(?)>
Scott Milner
for, and at least of same quality.
"Ted Kord" <Ted...@excite.com> wrote in message
news:ae625994.01101...@posting.google.com...
Stedystate
what an average quality of these mirrors? Or were they tested by knife
method with somewhat colose to 1/63 wave P-V ??
If your interferometer has smaller than 1/20 wave internal error calibration,
which one did you used for calibration of your own?
Valery Deryuzhin.
>>
Mr Deryuzhin,
Our measurements tell us the average quality of the 8” mirrors in this
particular production run (F/7.5) are very close (plus and minus) in accuracy
to the 8” that Markus tested on his bench using his collimated star and the
complete optical system in his Portaball. At this longer focal ratio it is more
doable to get the entire run to that accuracy rather than one or two as is
typically the case in a typical F/6 run. If you recall, one of Markus’
reference was a Zygo-certified Leica-made 8” mirror at approx lambda/27 at
the wavefront. He noted that the extra-focal patterns for his Leica mirror in
his bench star test showed the very slightest of difference, I believe
essentially the least he had seen to date. With the Portaball mirror under test
at 160 power-per-inch (1,280x) he indicated he could find no difference in the
extra-focal pattterns, even using a measuring eyepiece on the fresnel rings.
That partcular (Portaball) mirror as measured in a 5-zone knifedge test in this
shop returned a value of approx lambda/40 at the wavefront. So we did measure
error in that mirror, however the accuracy of the optic is essentially to the
point where we believe the ability of the knifedge test begins to break down
for an 8” piece.
We were pleased to have this corroboration, as our observations (Markus and
ours) using two complelety different methods essentially agreed to a very high
degree of accuracy, as one can note the difference between lambda /27 and our
measured lambda /44 is .014, which is on the order of 1/70 at the wavefront.
This is of course not to say we measured to that accuracy, but rather that we
did indeed detect differences and were in agreement. (If only Perkin-Elmer had
been so careful...)
I would say that Markus’ observations are a testament to his astuteness as a
bench tester, and to the accuracy of his bench system. I was pleased to have
that opportunity. So to answer your question with respect to calibration, we
have in this particular instance (8” size) calibrated our knifedge results
against the interferometer that tested the Leica mirror.
We do not use interferometry in our shop, however as far as further and more
complete calibration goes, a project is currently under way where we are doing
more extensive documentation on this very subject. It is a long-term project
involving many individuals. When it is completed next year the results will be
made public. I cannot comment further on it at this time, so we will have to
wait for those results.
Carl Zambuto
ValeryD
> Mr Deryuzhin,
Mr. Zambuto,
With all respect to you, your work level and to Markus, let me still
say, that it is impossible to speak seriously about optical quality
"documented" by eye. Really, all this is far from serious.
I don't remember even one case, when Markus, ordering any mirror optics
from us was agree with such sertificate of quality like you do mean.
An interferogram (better several ones) with full processing data was
always required.
We all know very well, that even 1/8 wave spherically corrected mirror
(with smooth surface) can deliver an exceptional image quality. No any
question about this. Another side is that we don't know how typical
8" mirror (which Markus tested) is for your porduction. It is quite
obvious, that if a master _really_ can achieve such quality which was
constantly claimed, he will be VERY MUCH INTERESTED in its confirmation
for EACH his optical set. In this case any doubts will leave us and
any another persons who know optics.
So far, I never saw ANY optical laboratory where interferometer was
forgotten and a very old and inaccurate knife method was in use as a
main test and certification method.
So, all what were said here is far form serious. May be for amateurs?
I personally doubt even in this. And the only way to go is to use an
interferometer. It is not too expensive if make it and use a professionally
made reference sphere and large flat mirror.
Valery Deryuzhin.
ARIES.
jerry warner
this not critical at all but constructively but I can well understand
the 'commercal' need for verification-quantifiable proof.
jerry warner
Del Johnson
his mirrors. Give it a rest.
Del Johnson
"ValeryD" <ar...@mercury.kherson.ua> wrote in message
news:5c4a4ee7.01101...@posting.google.com...
> stedy...@aol.com (Stedystate) wrote in message
>
>
> With all respect to you, your work level and to Markus, let me still
> say, that it is impossible to speak seriously about optical quality
> "documented" by eye. Really, all this is far from serious.
>
>
> Valery Deryuzhin.
> ARIES.
Chuck Olson
I know I don't need to remind you how effective modern electronics is in analyzing video
pictures. Use of a quality video camera in conjunction with a Vickers "Intensity Profile
Display", for instance, makes the analysis of knife-edge shadows a simple procedure, and much
more accurate than the old "intensity comparison by eye" method. It doesn't seem unreasonable
that this method might be employed for precise enough measurement to justify the performance
figures that a skilled optician can achieve.
Chuck
"ValeryD" <ar...@mercury.kherson.ua> wrote in message
news:5c4a4ee7.01101...@posting.google.com...
Mike Spooner
news:5c4a4ee7.01101...@posting.google.com...
--snip--
>
> So, all what were said here is far form serious. May be for amateurs?
> I personally doubt even in this. And the only way to go is to use an
> interferometer. It is not too expensive if make it and use a professionally
> made reference sphere and large flat mirror.
>
>
> Valery Deryuzhin.
> ARIES.
Hi Valery,
I talked with a professional optician this weekend. He stated that the only way
to truely document would be to use a phase shifting interferometer. I found
that to be an interesting statement.
--Mike Spooner
--
Posted from pg1-line19.az.net [209.145.196.26]
via Mailgate.ORG Server - http://www.Mailgate.ORG
ValeryD
> Carl Z. has his methods and his methods work. Nobody is complaining about
> his mirrors. Give it a rest.
>
> Del Johnson
Del,
I think you missed the point. Carl himslef touched a theme of
interferometers and their internal calibration errors. This
looks fun, especially because he do not use interferometry in his
process, but questioned interferometers.
Also, I can easily believe, that his mirrors are good enough for
successfull observing, but they are a bit far from claimed by Carl
precisions like 1/40 wave front of so. If only he forgot these small
parts of wave lengths and will speak about his own units of mesurements
like 1/70 cz , then, OK, OK, we all (who use interferometers) will
never comment any Carl's claims.
BTW. I a past I made myself a lot of mirrors, include Dall-Kirkhams and
Cassegrains without any interferometers and they worked just fine - no
complains from me and their owners. But, of course, that were far from
1/40 wave fronts. I can say this straight and honestly.
When I hear these claims about Carl Zambuto mirrors, they remind me
similar claims about lack of false colors in fast "fake" ED achromats.:-)
Valery Deryuzhin.
ValeryD
> "ValeryD" <ar...@mercury.kherson.ua> wrote in message
> news:5c4a4ee7.01101...@posting.google.com...
>
> --snip--
> >
> > So, all what were said here is far form serious. May be for amateurs?
> > I personally doubt even in this. And the only way to go is to use an
> > interferometer. It is not too expensive if make it and use a professionally
> > made reference sphere and large flat mirror.
> >
> >
> > Valery Deryuzhin.
> > ARIES.
>
> Hi Valery,
>
> I talked with a professional optician this weekend. He stated that the only way
> to truely document would be to use a phase shifting interferometer. I found
> that to be an interesting statement.
> --Mike Spooner
Mike,
I do understand. Tell this to Carl. :-) May be YOU can convince
him?
Valery Deryuzhin.
CHASLX200
>From: ar...@mercury.kherson.ua (ValeryD)
>Date: 10/16/01 4:19 PM Eastern Daylight Time
>Message-id: <5c4a4ee7.01101...@posting.google.com>
>
>Del,
>
>I think you missed the point. Carl himslef touched a theme of
>interferometers and their internal calibration errors. This
>looks fun, especially because he do not use interferometry in his
>process, but questioned interferometers.
>
>Also, I can easily believe, that his mirrors are good enough for
>successfull observing, but they are a bit far from claimed by Carl
>precisions like 1/40 wave front of so. If only he forgot these small
>parts of wave lengths and will speak about his own units of mesurements
>like 1/70 cz , then, OK, OK, we all (who use interferometers) will
>never comment any Carl's claims.
>
>BTW. I a past I made myself a lot of mirrors, include Dall-Kirkhams and
>Cassegrains without any interferometers and they worked just fine - no
>complains from me and their owners. But, of course, that were far from
>1/40 wave fronts. I can say this straight and honestly.
>
>When I hear these claims about Carl Zambuto mirrors, they remind me
>similar claims about lack of false colors in fast "fake" ED achromats.:-)
>
>
>Valery Deryuzhin.
>***************************************
I have had 3 Zambuto Mirrors, and i can tell ya they are the best i've ever
used, hands down!!!
My dream Zambuto mirror would be a 12.5" F/8, and i would pay top price for
one...
Chas P.
Michael Lott
sucker and its ladder around? ;)
Chris1011
used, hands down!!!>>
So why did you get rid of them? Sorry, I couldn't help asking.
>>My dream Zambuto mirror would be a 12.5" F/8, and i would pay top price for
one...>>
I made a 12" F8 Newt once, first big scope I ever built. I had to stand on the
roof of my car to see stuff with it. An aquaintance had one that was mounted on
a large German equatorial. He actually stood on the dec axis to observe
straight up. That was before the Dob revolution.
Roland Christen
CHASLX200
>Date: 10/16/01 5:48 PM Eastern Daylight Time
>Message-id: <mlott-16100...@user-2inimf6.dialup.mindspring.com>
>
>
>Chas- I assume you'd also pay top price for the 18-wheeler to carry the
>sucker and its ladder around? ;)
>
*******************************
I'm used to big long scopes, i've had three 12.5" F/8's, a 14.5" F/7 and a 10"
F/10!
Chas P.
CHASLX200
>Date: 10/16/01 5:52 PM Eastern Daylight Time
>Message-id: <20011016175219...@mb-fo.aol.com> Roalnd wrote. Snip.
>
>So why did you get rid of them? Sorry, I couldn't help asking.
>
***********************************
Well ya know me, i'm a scope nut!
I love to play with as many scopes as i can, guess if i had unlimited money, i
would have a 100 scopes...
Chas P.
Chuck Gulker
You got that right Del. Everyone is raving about Carl's mirrors! Everytime
I look through my 10 F6, I can't believe the views it provides. Buying that
mirror was the best purchase I've ever made in amateur astronomy. Spending
time with master Carl at the Riverside Telescope Making Conference in 2000
was an experience I will never forget. He obviously cares deeply about his
profession and wants to put out the finest quality mirrors. Carl also
highly respects other mirror makers and will not talk down to others less
knowing than him. What I am trying to say is that Carl is a truly class act
and we amateur astronomers are very, very lucky to have him doing what he
does.
Chuck Gulker (Dublin, Ohio)
Del Johnson
theoretical limits and are manufactured with unquestionable quality control.
Your constant complaining that Carl has not thrown away all of his
hard-earned profits into unnecessary expensive testing equipment sounds like
sour grapes to me and comparing Carl's work to fake ED's only belittles
yourself.
Del Johnson
"ValeryD" <ar...@mercury.kherson.ua> wrote in message
news:5c4a4ee7.01101...@posting.google.com...
Chris1011
What is a theoretical limit?
Roland Christen
Raycash
An oxymoron?
Or a redundancy like "ST7E-NABG"? <G>
--Ray Cash
How to Build a Dobsonian Telescope:
http://members.aol.com/sfsidewalk/dobplans.htm
My Deep-Sky Page:
http://members.aol.com/anonglxy/deepsky.htm
How to Build a 13" "Travel Scope"
http://members.aol.com/radcash/travelscope.htm
Sjoplinh
>>I talked with a professional optician this weekend. He stated that the only
way
to truely document would be to use a phase shifting interferometer. I found
that to be an interesting statement.
--Mike Spooner <<
I'm wondering if y'all are talking about surface accuracy, or smoothness on an
ultra - micro scale.
Texereau and Twymann (big book) both discuss the Lyot method of checking
smoothness on an optic. Uses a 1/4 wave retarder on the main light; the light
scattered by micro - roughness goes to the film unimpeded. The interference
makes any micro bumps stand out like mountains.
It sounds only moderately complicated to do, and is something I'd try if I had
unlimited time(!).
In any case it would at least semi - quantify mirror smoothness.
Scott
Greg Crinklaw
All earthbound telescopes are limited by the earth's atmosphere. At the
eyepiece you can throw those theoretical limits that people here on SAA
like to quote and argue about right out the window. All the claims you
read about marginal percentages of increased contrast mean very, very
little in practice.
It seems to me that there is a big market these days for astronomical
equipment for equipment sake; the more "perfect" the optics can be said
to be (and the more expensive) the better. I suppose that for those
people for whom money is no object that's fine. But it needs to be said
for those newbies who may be listening that the amount of money you
spend on your telescope is not to be found among the important factors
which will lead to success and enjoyment at the eyepiece.
Clear skies,
Greg
--
Greg Crinklaw
Astronomical Software Developer
Cloudcroft, New Mexico, USA (33N, 106W, 2700m)
SkyTools Software for the Observer:
http://www.skyhound.com/cs.html
Skyhound Observing Pages:
http://www.skyhound.com/sh/skyhound.html
Chris1011
for those newbies who may be listening that the amount of money you
spend on your telescope is not to be found among the important factors
which will lead to success and enjoyment at the eyepiece>>
Well, you're a curmudgeon as usual, but you're probably right. On the other
hand, spending a lot of money for those who have it also works well and can
bring lots of enjoyment.
Roland Christen
Brian P. Murphy
If one needs an interferometer to determine exact correction, and you didn't
use an interferometer how do you _know_ they were not 1/40th wave?
Brian
"ValeryD" <ar...@mercury.kherson.ua> wrote
> BTW. I a past I made myself a lot of mirrors, include Dall-Kirkhams and
> Cassegrains without any interferometers and they worked just fine - no
> complains from me and their owners. But, of course, that were far from
> 1/40 wave fronts. I can say this straight and honestly.
> Valery Deryuzhin.
Derek
> for those newbies who may be listening that the amount of money you
> spend on your telescope is not to be found among the important factors
> which will lead to success and enjoyment at the eyepiece.
>
> Clear skies,
> Greg
>
good. Anything over ~180X was bad, not matter how good the night. It was
not very fun to have a mushy Jupiter while everyone else is seeing all kinds
of stuff in their scopes - why have a scope if you have to mooch views off
of other people. So I spent the money and had Steve Swayze fix the
correction of the mirror(he did an awesome job) and my enjoyment at the
eyepiece dramatically increased. It is amazing how much better everything
looked with a good mirror. So, does a person need the nth degree of optical
perfection to enjoy looking at the sky, probably not, but you won't have
much fun with bad equipment either. At least I didn't.
Derek
Nate Perkins
was a very entertaining read, and I appreciated his time in posting it for
us.
I have also enjoyed numerous other posts from Roland, Valery, and others
describing their techniques and products. However, as a consumer I would
like to suggest that you might want to spend less time arguing about
quantification of (practically) perfect wave surfaces and more time
manufacturing the great products for which you are known ... I have a small
hope that as your volumes increase your prices may drop ;-)
ar...@mercury.kherson.ua (ValeryD) wrote in
news:5c4a4ee7.01101...@posting.google.com:
(preceding messages cut out)
Del Johnson
Del Johnson
"Chris1011" <chri...@aol.com> wrote in message
news:20011016212949...@mb-fo.aol.com...
Stedystate
Thank you for your input, I appreciate your opinion. I don’t know you well,
but from the type of work I have heard you are engaged in I suspect that the
case must be that in the craft of opticianry you are a consummate professional.
Because of that fact I must regard your opinion, as I do all professionals that
I correspond with. I will consider what you have said as we continue to conduct
our project study of interferometry and zonal Foucault testing.
One reason I understand your viewpoint is that I know that of a necessity you
have to provide objective evidence that best as possible defines the total
system quality, as I understand that you provide complete optical systems,
perhaps more so than single surface reflective components. Interferometry
appears to be the best option for that purpose, so much so that perhaps you
even have no choice, so I do appreciate where you are coming from.
Our criteria are different for two main reasons. One, our customer base is
amateur astronomers rather than commercial industrial customers, and two, our
production item is single-surface paraboloids which are easily measured at the
radius of curvature employing no auxiliary optics. The method is profoundly
elegant in its simplicity.
We of a necessity have to consider total cost to our customers. Mirrors that
are documented objectively at the level of performance that we provide (such as
an industrial shop would do) historically have a cost of at least double our
prices. Case in point, LOMO mirrors, for example. To date we have not found a
way around that. I seriously doubt that amateur astronomers are wanting to pay
that kind of money for verifiable objective evidence when in fact it appears
that on the whole they do trust our reputation.
So for the time being we will continue to turn our micrometer dials and provide
all the information to our customers that is used to conduct the test, as we
have always done historically. I will note that every aspect of our process is
included in the information package that accompanies each piece, even down to
zone parameters. In that case there is nothing that we can hide, as all of it
is completely verifiable given same method of measurement. I do agree with you
that from an industrial (for lack of a better term) perspective this
documentation cannot be taken seriously. I suppose I will have to live with
that for the time being. It is my hope that our customers will be understanding
in that regard.
Thank you again for your input. Perhaps we can have yet another discussion when
our project results have been finalized.
Sincerely,
Carl Zambuto
----------------------------------
Ed Erbeck Jr.
<Snipped the Whole thing>
Ok I have a Vested interest in this subject and really I'm not shy
about it. So as to any comments beyond this point feel free to ignore or
take with massive grains of Salt.
Ok Valery said and said, And said, well frankly alot of stuff, And he's
justified in his Opinion,
But Carl NEVER claims that the numbers on his documentation are beyond
reproach. The only thing he assures is that his Optics will not degrade
befor reaching 50X per inch given the Atmosphere is willing to cooperate and
the Optics are properly mounted - Period end Carl's Data.
And as to his Gurantee (taken from the info that comes with his
Mirrors):
" I often observe people stating final views of their mirrors in public
forums. First, it should always be stated along with the test method, that
being zonal Foucalt, if it is going to be stated at all. I personally were
not stated, as one it causes confusion with other methods to the uninformed,
and two the numbers are not included in my actual gurantee. What I guarantee
is found at the bottom of the document for each mirror, accompanied by my
signature. I gurantee performance at the eyepiece, (where it really matters)
to beyond 50 power per inch with no breakdown in the infocus image under
appropriate conditions. Although I have observed much debate over numbers on
paper over the years, I have yet to observe argument at the eyepiece when an
optic is a superior performer. So I would rather it were stated a coustomer
owns a "50X per inch mirror" that an "x-wavefront" mirror. Besides, once the
RTA is less han 1.0 accompained by a smoooth surface and a good edge, the
differance is negligible."
There you have it (BTW if there are misspellings or missing words -
well it's my fault, as I type using the Marco Polo method_ I know it's out
there somewhere and I'll find it if I keep looking....)
Ed Erbeck Jr. (Owner Crazy Ed Optical)
Online Catalog:
http://www.crazyedoptical.com
Snail Mail:
Crazy Ed Optical
P.O. Box 446
Pearce, AZ 85625-0446
Phone / Fax 520-826-1484
Mike Spooner
news:5c4a4ee7.01101...@posting.google.com...
>
> Mike,
>
> I do understand. Tell this to Carl. :-) May be YOU can convince
> him?
>
>
> Valery Deryuzhin.
Hi Valery,
Actually Carl and I have talked about this (and other optical concerns). I just
was wondering how many opticans use phase shifting interferometry. I have star
tested a few Zambuto mirrors and accept his measurement Foucault methods that
produce these consistantly superb mirrors. I might question the software
reduction routine of the slope measurement data. I think some of the confusion
stems from the old Texereau data reduction that derives a wavefront rating from
slope readings. Perhaps we need to differentiate slope derived wavefront
ratings from interferometric wavefront ratings. Both methods have been around
for a long time but perhaps not well correlated with each other. Foucault slope
measurements may be subjective but many interferometric tests I've seen have
mirrors standing on edge with resulting astigmatic effects that are a bit
pessimistic. I use double pass autocollimation to test paraboloids and while it
is doubly sensitive compared to testing on a star, a center of curvature test
is effectively done at twice the f/ratio thus increasing sensitivity
comparitively. Yet some opticians have championed the doublepass over C of C
testing due to increased sensitivety! (Being a null test I personally prefer
the doublepass but the sensitivity issue is mentioned in some optics
literature.) Always seem to be something to be at odds with in our beloved
field. :) --Mike
P.S. I hope I haven't posted this several times but I'm having some problems
with the reader reply.
--
Posted from pg1-line12.az.net [209.145.196.19]
Chris1011
provide
all the information to our customers that is used to conduct the test, as we
have always done historically>>
I would like to jump in here for just a minute and clarify something about the
two different test methods. Knife edge testing is done with zone masks, and
measures the focus points of 4 or more areas of the mirror. These areas are
thin rings (depending on the type of zone masks used) and the resultant focal
point of each ring is an average of where the light from the entire ring or
zone comes to a best focus. These focus points can be measured to a very high
degree by a competent master optician using sensitive micrometers. However,
this reading is an average of the entire zone or ring, not a point by point
focus of each part of the mirror. The interferometer does provide a very fine
point by point reading of the whole surface at once, so it can measure not only
the overall spherical correction, but also subtle higher order effects
including astigmatism which is totally missed by the knife edge. The final P-V
is always much lower because these effects are all included. You can separate
them out of course, and if you want to know only the overall spherical
correction, as you would see in a knife edge test, you can display that. For
instance, a well corrected optic may have the following numbers (this is a real
optic):
RMS - .015
P-V - .08
Strehl - .99
Astig - .06
Coma - .03
Spherical - .01
This optic then would measure in at 1/100 wave for spherical correction, but I
could not call this a P-V wavefront error. The total P-V error is 1/12 wave
(.08 wave). When tested with a knife edge in 4 zones, this optic will show no
variation in the average knife edge position from the theoretical position.
Nevertheless, it is still only 1/12 P-V.
Before I started using an interferometer I would have argued up and down that
my optics had to be at least 1/20 wave because I could see no variation in
focus on the double pass knife edge test. The interferometer sobered me up real
quick. As Peter Ceravolo would say, the interferometer takes no prisoners.
Roland Christen
Chris1011
measurements may be subjective but many interferometric tests I've seen have
mirrors standing on edge with resulting astigmatic effects that are a bit
pessimistic>>
The same is true of lenses tested on edge. I have a vertical interferometer for
both lenses and mirrors. The optics are held in a special flotation cell to
eliminate any astigmatism induced by the holder. Companies such as Tropel who
produce 1/20 wave surfaces routinely have a whole room full of custom built
vertical interferometers. The only other way might be to suspent the optics in
a sling, but sitting them on edge does not produce good results.
Roland Christen
Bratislav
> With all respect to you, your work level and to Markus, let me still
> say, that it is impossible to speak seriously about optical quality
> "documented" by eye. Really, all this is far from serious.
Have you SEEN those foucaultgrams on the Portaball site ? I totally
agree with Carl's estimate of better tnah 1/50 wave microripple. I
can't say I have seen many mirrors that smooth, and can only wish that
all of my mirrors are that smooth as well. Frankly I thought one can't
get surface THIS smooth on ordinary Pyrex.
So I'd say that this ISN'T 'far from serious'.
And you'd agree that there aren't many interferometers capable of
quantifying that amount of ripple ?
> I don't remember even one case, when Markus, ordering any mirror optics
> from us was agree with such sertificate of quality like you do mean.
But I also don't remember Markus praising ANY other optics he's seen
as much as that 8". Regardless of certification, quality will always
speak for itself. I do admit that I would have a hard time buying
something that is only certified on a piece of paper after zonal
measurements by Foucault, but I think I would be willing to make
an exception with Carl Zambuto.
> We all know very well, that even 1/8 wave spherically corrected mirror
> (with smooth surface) can deliver an exceptional image quality. No any
> question about this. Another side is that we don't know how typical
> 8" mirror (which Markus tested) is for your porduction. It is quite
> obvious, that if a master _really_ can achieve such quality which was
> constantly claimed, he will be VERY MUCH INTERESTED in its confirmation
> for EACH his optical set. In this case any doubts will leave us and
> any another persons who know optics.
> So far, I never saw ANY optical laboratory where interferometer was
> forgotten and a very old and inaccurate knife method was in use as a
> main test and certification method.
Old, yes, but 'very inaccurate' ? If the mirror is as smooth and free
from zones as one we've seen on Portabal site, and f-ratio is moderate,
Foucault will be DEADLY accurate in capable hands. I'm willing to bet
that mirror of this quality will surpass the limit of a typical
'low cost' interferometer reference element (1/20 wave). Maybe those
mirrors aren't true 1/47 wave or whatever, but how many observers would
really care ?
> So, all what were said here is far form serious. May be for amateurs?
> I personally doubt even in this. And the only way to go is to use an
> interferometer. It is not too expensive if make it and use a professionally
> made reference sphere and large flat mirror.
I agree with this statement 100%. For any serial production interferometer
is the only way to go if you want to assure consistency and impartial
quality control. I'm sure companies like GalaxyOptics and AstroPhysics
would confirm this, in fact both have said something to that effect one
way or another.
But let's be realistic - there will be maverics like Carl Z and John Hall
whose skill is REALLY hard to question. The volume of optics they produce
maybe doesn't warrant the investment for the interferometer (for Christ's
sake, Carl didn't even use machine until fairly recently), and in reality
the quality of stuff they produce would not significantly improve if they
used the interferometer. Because they are already making mirrors as
good as they (and dare I say ANYONE) can get them (short of access
to an ion beam machine).
Bratislav
PS many thanks to Carl for the best essay on making optics SAA has ever
seen ! I hope this post makes it one of many ATM sites. Reference stuff.
Markus Ludes
you say , my testing is invalid and not acceptable for you, fine, no problem
for me. For me and customers its good enough to see how the mirror performs and
how good it is made.Smallest local area surface which decrease numbers on
reports are not interested because they show nearly 0% influence in the image.
Startest against collimation can show you clearly deffects, like spherical
aberration, turned edges, zones , anything which can trouble the image quality
at the eyepiece.
A well made startest give to the user more informations than any other testing
method, the startest show you all possible errors at once without excaptiopn,
right ?
The Startest cannot print numbers for reports, but it can tell you is the
optics, poor, good, very good or near perfection. Near perfection means your
eyes cannot detect any error.
My own Zambuto 12.5" is one one such supreme mirrors. Philipp keller, your
friend, the LOMO dealer for expensive proffesionell optics was interested in
Zabmutos work and claims. He used testing methods like Carl plus Laser
interferometry to test my mirror.
When i asked what he thinks about this mirror, he answere me the following:
This is the best non lomo mirror he ever tested. Your mirrors from CAO and LOMO
are handled as equal by proffesionell customers who tested both modells.
If I ask you or Phlipp/LOMO , can you do me such mirror : i getting always the
answere : No.
Why no ? Because the mirror is from pyrex and very thinn. There might be a
chance that i can buy from you such mirror, but than I have to pay maybe 10
times or more money than to Carl, right ?
Lets forget the numbers from Carl you claiming maybe wrong. The important point
is, that retesting show an quality which is equal to the very best LOMO and CAO
mirrors.
As you and philipp claiming: LOMO and CAO mirrors are difficult to surpass in
quality by any other proffesionell company even for 10 times more money.
Now Carl showed that he stay at the same steps on the leather for his smaller
mirrors ( the diameters he product are limited as you know)
as CAO and LOMO, at least for his parabolic mirrors.
How Carl can reach such level without the expensive controll equipment ? Many
proffesionell think , this is impossible.
Usual I would need to agree, but we all forget one thing: Carl Zambuto is not
only an extraordinäry Talented mirror makers, no he also have magic fingers.
His magic fingers doing art of work, for which most others need extremly scill
and proffesionell testing equipment, Carl not, he needs only his magic fingers.
Until you have not seen his magic fingers, you cannot understand how he doing
such good optics with such simple amateurmethods
best wishes
Markus
"Stedystate" <stedy...@aol.com> wrote in message
news:20011014165952...@mb-fw.aol.com...
> Mr Kord,
>
> I apologize for causing you tears with respect to your "loving old telescope".
> I agree that once such a view is had it is difficult to go back. Perhaps
> Starmaster needs to put a warning label on their scopes...
>
> I don't have any advice at this point, except that when I was a beginning
> mirror-maker I had little money for the hobby and more time. I used to pride
> myself on how little my complete scopes cost. Of course i had weeks worth of
> labor into any one of them. Perhaps mirror-making/refiguring would be a hobby
> of interest to you, in conjunction with a local mirror-making group or the ATM
> list. That is a way to learn a new thing and gain great satisfaction in the
> result.
>
> Best regards,
>
> Carl
>
> ---------------------
>
> << From: ted...@excite.com (Ted Kord)
> Date: Sat, Oct 13, 2001 8:08 PM
> Message-id: <7c0be594.01101...@posting.google.com>
>
> Mr. Zambuto,
>
> Thanks for a very informative explanation. I've never tried my hand at
> mirror
> making, but it sounds interesting.
>
> Just let me say that I once had an opportunity to look through a
> Starmaster
> with one of your mirrors in it. Now I'm not an optical expert, and
> know nothing of focault tests, interferometers, etc... but the view
> brought tears to my eyes. Not only for the clarity/contrast that I
> saw, but because I knew my old dob would never satisfy me again.
>
> If I had the money to spare, I'd snap one of these up in a
> heartbeat...
> >>
--
Posted from p3ee25b4d.dip.t-dialin.net [62.226.91.77]
ValeryD
Hi Bratislav,
You know, I value your opinions. But let me escape of any comments of
THIS your post. I totally disagree with all what you said, and I can
only agree, that Carl's mirrors are well worth to use them for observing,
but IMO, they are not any better than simply good mirrors. They can't have
a claimed quality (R.C. clearly explained this in his last post).
They, probably, are much more consistent in quality than were mirrors from
another amateur-like companies and from most commercial sources of Dobs.
Valery Deryuzhin.
ValeryD
Mr. Zambuto,
I can't agree with your thoughts, that Intefrerometry will double
price
of your mirrors. Of course, if you will use it frequently at final
stages
of figuring - then, yes, you are right - each mirror will cost at
least 2x
higher. But as a benefit, you will escape situations very clearly
described
by Roland Christen, when spherical aberration is corrected to a very
low
value, say, 1/30 - 1/100 wave, but other aberrations are much higher
and
their participation in an image forming is much higher than for
spherical aberration. I the described (by R.C.) case a total P-V error
was formed by several independent aberrations. With knife test it is
impossible to detect
them and try to fix.
In pros optical shops, when RMS factor is the main in the requirements
to an overall optical quality, opticians and engineers MUST use an
interferometer after _each_ session of figuring - they need to control
an overall optical quality, not only spherical aberration.
In your case, when a low cost of mirrors is one of the main goal, I
can
advice you to build a very simple interferometer, buy a certified
large
optical flat, cheap CCD TV camera and frame grabber + appropriate
interferogram processing software. With modern (and inexpensive!)
software
you will be able to create a test certificate within a few minutes -
not any
longer than micrometer measurements.
This procedure will cost quite low, because it will be used only once
- at
certification of a complete mirror. This will allow to escape any kind
of
discussions about your mirrors quality and will make your customers
quite
confident in quality of optics in their scopes.
All in all _any_ RESPECTABLE optical laboratory MUST have an
interferometry
in its arcenal. Who can argue with this?
Sincerely,
Valery Deryuzhin.
William R. Meyers
> Someone wrote:
>
> If no interferometer in his process, then forgot to use a wave length
> as units in his "certification" data shits. "
The unconscious speaks both Russian and English?
Bill Meyers
>
>
>
Stedystate
setup and astigmatism. Interferometry is not so simple as some would purport it
is. In our evaluation of test methods and what we would use to test and certify
our mirrors we did certainly evaluate interferometry as one possibility. I am
pleased (at this point) that my mentor saved me the trouble of building one as
he had already done it himself. (it now sits idle in his shop) Had he not, we
might be a long distance behind where we are now in our ability to produce
high-performance optics. The fact is, significant astigmatism is induced to any
mirror (even full thickness) in any horizontal test setup tried to date. That
atigmatism is only removable by multiple interferograms (such as dozens) and
complex mathematics to remove the factor that does not change. Another
professional optician (bob Royce) with whom I correspond regularly and whom I
regard very highly has done an excellent study on this subject. He has photos
of interferograms demonstrating this phenomenon on his webiste at the following
page:
http://www.rfroyce.com/ast/testmethod.htm
On that page he provides the following conclusion:
"I hope I have demonstrated that one cannot simply hand out a single
interferogram and an analysis of that interferogram and call it a mirror. This
should also help to make it clear as to why the precise certification of optics
for amateur telescopes would place the cost of such optics, were the
certification done correctly, much higher than they currently are. The present
certification required the production of 40 interferograms, all of which had to
be individually analyzed. Even though I possess modern, interactive software
that speeds up this process considerably, it still requires considerable time
and effort. Therefore, I've chosen to produce my mirrors by method which
ensures that they will meet my advertised criteria without having to engage in
extremely time-consuming and expensive interferometric analysis for each
optic."
Another point, and one reason why we use what we use: the knifedge has the
ability to see right through this astigmatism, as the phenomenon is vertically
weighted, and we read the mirror across the horizontal axis. The test does not
even know it is there, however that does NOT make it blind to real astismatism
that is contained in the glass itself. We simply rotate the optic to another
axis and take another set. (That practice is identified in our documentation
package). We then read the slopes in each axis and compare them to the ML
envelope. Because of interactive field study from our professional customers,
we now have it down to the point where we are able to determine at what point
the astigmatism will be seen in the star test!
So now I suppose it will be suggested that we build a vertical interferometer
facility to certify our 20" mirrors. Delightful. Anyone want to build that
building for us, or should I just tack the cost on for our customers?
Carl Zambuto
--------------------------
Chris1011
facility to certify our 20" mirrors. Delightful>>
If your present knife edge testing works for you, I would hardly suggest that
you change your testing methods. If, however, you find that you need to do
interferometry sometime down the road, you can indeed build a vertical
interferometer where the mirror is properly supported on its back, not side,
and the rest of the instrument is horizontal. My own setup is a folded system
only 5 ft high, yet I can test optics up to 100 inches long.
Roland Christen
Toddwx
standardize this stuff. You all quibble on saa, and yet you all have done
business with one another, and have had ample opportunity therefore to discuss
a mutually agreeable standardization of test method or methods. It is very
frustrating for the amateur. Please come up with some sort of consensus if you
would be so forward thinking and kind.. to help us out here as users.
Respectfully,
Todd G
Toddwx
years of this debate. However, I am appreciative of this public discussion as
it educates the amateur, big-time. :)
As for cost, this is very important to the amateur, and if there are two
differing methods of testing needed to produce two differing priced mirrors, so
be it. The cost of the 8" zambuto mirror was something near 1,000 I believe,
which is already a strain for the amateur, but worth it for the fine optic. To
go the next step would break the average amateur's budget, but perhaps be
appropriate for an industrial/professional application.?
tg
Toddwx
>equipment for equipment sake
There are enough very serious observers here though that really DO want and in
fact need the smaller aperture scopes, in particular 4-8" range, to be as good
as possible to produce results that otherwise would be unattainable without
going to the next step up in aperture, which may have other drawbacks.
As an example, my 5" A/P EDF f/6 was able to produce exemplory Jovian detail
and yet be light enough to be used on a mid-weight mount, and fast and
widefield enough to be used for sweeping, widefield photography. Had this scope
not been so carefully figured, then the solar system detail would not be quite
what I needed for the "all around instrument" .
tg
Toddwx
would like to reitterate that the Zambuto mirrors, are from an observers
standpoint, quite outstanding. (They do not appear to have a perfect parfocal
star test, perhaps the system is limited by the secondary?), but they do
perform better than any reflective system I have viewed through. I currently
have a Zambuto 7" and 14.5". The 7" shows a bit more planetary detail than a 6"
Takahashi FCT-150 (but less sky-planet contrast). The 14.5" appears to have
something closing in on double the resolution to the eye (I know, this is an
estimate at best, but does follow the theoretical limits, I believe) on Jovian
detail of the 6 and 7" scopes above when skies allow.
tg
Bill Foley
shit.
--
Clear, Dark Skies
Bill Foley
P.S., do NOT believe the reply-to address, it is set up to
prevent some spam. The mail address is also bogus but does
contain my real e-mail address.
Ratboy99
>they [Zambuto mirrors] do
>perform better than any reflective system I have viewed through. I currently
>have a Zambuto 7" and 14.5".
I can't wait for Jupiter to get higher in the syk in the evening. I've done a
number of improvements on my 10" F6 Dob w/ Zambuto mirror and I want to see how
it does. I'm hoping for the best. I did get a good look at Saturn and the views
were very promising. Jupiter is my real test object though.
rat
~( );>
email: remove 'et' from .com(et) in above email address
ValeryD
Markus,
You missed the point. I didn't said, that Carl's mirrors are poor. No.
I just said, that it is not serious, when Carl questioning interferometers
internal calibration errors and speaking about such precisions like 1/40
wave, to appel to your eyes capability to prove such precision.
First - even if your eye is capable to see the difference between 1/35 wave
and 1/40 wave - fine, but an atamosphere above you will not allow to prove
such precision.
If you do use an "in-lab" test, then, I am sure, your test optics is worser
than 1/40 wave.
Closed circle. No win for Carl's and your sentences. Phillip's opinion is
the same valid for me as my for him. I know, that Phillip will never accept
any similar to Carl's "certificate of quality" .
This is not serious and even advanced amateurs well know what is going on.
From many private e-mails to me (during about last two years), from reading
some groups, I can conclude, that Carl's Zambuto mirrors are:
1. far tot as good as they claimed (like 1/40 wave ).
2. really more consistent in quality than from other similar firms.
3. best CZ mirrors are not any better than best from competitors.
They are about the same.
4. They are well worth money peoples pay for them.
The story with first 8" you returned to Carl to re-work is a once more
confirmation, that all is not as perfect as claimed.
Valery Deryuzhin.
CHASLX200
>From: tod...@aol.com (Toddwx)
>Date: 10/17/01 5:36 PM Eastern Daylight Time
>Message-id: <20011017173601...@mb-bk.aol.com>
I can tell ya for a fact my 14.5" Starmaster with Zambuto mirror, was by far
the best mirror out of over 50 newts i've owned!
I was able to use over 1100x last march on Sat and Jupiter, and i could of
gone higher.
But try hand tracking, over 850x, it's not easy...
Chas P.
ValeryD
> So now I suppose it will be suggested that we build a vertical interferometer
> facility to certify our 20" mirrors. Delightful. Anyone want to build that
> building for us, or should I just tack the cost on for our customers?
>
> Carl Zambuto
It is not that difficult and not too costly. But such equipment will
allow you to expand your horizons. Orders from pros customers are much
more profitable than making optics for dobs. However, as you know well,
such customers always require interferometrical confirmation of quality.
For less critical cases such equipment will allow to use interferometry
only for final certification.
I bet, that earlier or later you will have such equipment. It is always
better to stay on a realistic positions.
Valery Deryuzhin.
Stedystate
that
you change your testing methods. If, however, you find that you need to do
interferometry sometime down the road, you can indeed build a vertical
interferometer where the mirror is properly supported on its back, not side,
and the rest of the instrument is horizontal. My own setup is a folded system
only 5 ft high, yet I can test optics up to 100 inches long.
Roland Christen >>
-----------------
that is a -nice- suggestion!!! thank you, Roland, I will remember that.
Carl
Stedystate
more profitable than making optics for dobs. However, as you know well,
such customers always require interferometrical confirmation of quality.
For less critical cases such equipment will allow to use interferometry
only for final certification.
I bet, that earlier or later you will have such equipment. It is always
better to stay on a realistic positions.
Valery Deryuzhin.
>>
----------------------------------
Yes, I do understand all your points, and that they are true and correct.
I want to say here for the sake of the amateur community that I am an amateur
astronomer and an amateur mirror maker at heart. My passion began with amateur
astronomy and I hope and believe it will end there as well. Lest anyone think I
may be proselytized successfully into industry by the lure of greater profits,
I don't see that as a serious probability at this point. What we are in fact
doing is spending what profits we can scrape together on expansion of machines
and shop space to have capability for larger mirrors. We currently have a new
shop being built. Not a large shop, but with substantially more capability than
the back half of our house.
I mean, I for one am frankly DYING to look through a Zambuto 24" or 30". I
can't IMAGINE what its going to look like!!!
Carl Zambuto
----------------
ValeryD
Todd,
Yes and yes! Most amateurs always forgot about this relatively hidden
fact, that smaller optics made at its best will allow to see more vs
average optics without jumping to larger apertures which are much less
portable, require much more efforts to transport, to set-up, etc and
often require more expensive mount. You underlined a really importnant
side of problem.
Valery Deryuzhin.
Greg Crinklaw
> Todd,
>
> Yes and yes! Most amateurs always forgot about this relatively hidden
> fact, that smaller optics made at its best will allow to see more vs
> average optics without jumping to larger apertures which are much less
> portable, require much more efforts to transport, to set-up, etc and
> often require more expensive mount. You underlined a really importnant
> side of problem.
>
> Valery Deryuzhin.
Oh give us a break. This really rankles me. You might be able to make
up something on the order of 1/2" to 1" of aperture with "quality
optics" but that's about it. Unless you purposely choose a telescope of
unacceptably low quality to compare against, the effect of "quality
optics" can't compete with aperture. You and Roland should be ashamed
of yourselves for perpetrating this myth. These are nice telescopes
with lots to say for them. I do not understand why you feel the need to
make exaggerated performance claims -- claims that I'm now seeing in the
advertising of telescope stores, even though there is no objective
evidence to support them.
I'll give you a similar challenge to the one I gave Roland earlier this
year. Prove your claim scientifically with a specific case. I'm not
interested in anecdotal evidence -- show me a peer-reviewed paper or
some other respectable, quantifiable, research that substantiates the
claim that a 4-inch telescope with "quality optics" will perform
similarly to an 8-inch with "typical" optics. I chose an eight inch
because it seems reasonable to me that you would have to go to such an
aperture before the sort of complaints you make about larger instruments
become apparent (after all, a 6-inch isn't significantly larger and
heavier than a 4-inch). If you have another specific example, fine.
Unless you can substantiate your claim then I respectfully suggest you
stop misleading people.
Greg
--
Greg Crinklaw
Astronomical Software Developer
Cloudcroft, New Mexico, USA (33N, 106W, 2700m)
SkyTools Software for the Observer:
http://www.skyhound.com/cs.html
Skyhound Observing Pages:
http://www.skyhound.com/sh/skyhound.html
Jeff Morgan
ar...@mercury.kherson.ua (ValeryD) wrote:
> Yes and yes! Most amateurs always forgot about this relatively hidden
> fact, that smaller optics made at its best will allow to see more vs
> average optics without jumping to larger apertures which are much less
> portable, require much more efforts to transport, to set-up, etc and
> often require more expensive mount. You underlined a really importnant
> side of problem.
>
>
> Valery Deryuzhin
I wouldn't buy the arguement that an excellent 6" scope can perform up
to a 10" scope. Limiting magnitude isn't any fainter. DSO's aren't any
brighter. Globulars aren't any more resolved. Etc. etc. etc.
However I would buy the arguement that a mediocre 10" scope could
perform down to a 6" scope.
--
Jeff Morgan
email: substitute mindspring for nospam
Bill Foley
In the case of Zambuto mirrors, as in the case of AP objectives, I think
that worrying about the difference between 1/35 and 1/40 wave is sheer
niggling. I agree that the eye cannot detect the difference between an
extremely good objective and one only slightly less good. But if the
eye cannot distinguish this difference, then all the test results in the
world will not make any difference, and will only add to the costs of
production and therefore the cost to the buyer. Please, folks, take
your pot-shots at each other on your private mail and leave it out of
SAA, now that your points (however meaningless in practical use) have
been made.
--
Bill Foley
others involved in this sniping make great scopes, but this sniping is
also detracting from your reputations, as far as I am concerned. Please
happily sell your documented stuff to those who want it, and let others
sell undocumented and cost-effective stuff to those others who care less
about documents and only want the evidence of their own eyes....
--
Chris1011
of yourselves for perpetrating this myth. >>
some other respectable, quantifiable, research that substantiates the
claim that a 4-inch telescope with "quality optics" will perform
similarly to an 8-inch with "typical" optics>>
Hey, Uncle Crinky, LEAVE me out of this stupid debate. I do NOT and never have
claimed that a quality 4" will equal or beat any typical 8". If anyone calls
here at AP and asks if he/she would gain anything by getting a 4" refractor
over their present 8" scope, I say absolutely NOT. The reason I make 4"
refractors is for traveling (Traveler, get it?) AND wide field imaging, period.
You got me confused with the other 4" refractor peddlers who claim all kinds of
outlandish things for their small scopes.
Roland Christen
ValeryD
Greg,
At first of all, at such diameters and difference in aperture it is
difficult to satisfy you with a real examples. But let sepak about
7" perfect aperture vs average 9" or 8" vs 10" or popular sample -
6" highest quality APO vs good 8" MCT.
A lot will depends of atmosphere conditions - as worser, as better
chances in smaller apertures with perfect optics. This is not an
anekdot.
I just don't like to spend my time to argue in obvious questions.
I personally prefer an 8" perfect APO vs very good 10" MCT or SCT
or reflector or even average 10" APO.
Valery Deryuzhin.
Greg Crinklaw
I'm going to ignore your grade-school-level name calling. I included
you in my comments for a specific reason. I recently ran across an
advertisement by a telescope store that claimed on their web site:
REFRACTORS PERFORM AT ABOUT 1.5 TIMES BETTER THAN REFLECTORS AT
DETECTING
FAINT OBJECTS. THIS MEANS THAT THIS 4 INCH REFRACTOR WILL DO AS WELL AS
A
BIG 6 INCH REFLECTOR IN SHOWING DEEP SKY...
I don't believe this to be true (the 1.5 figure is not supported by
objective data) so I emailed them and asked them about it. Part of
their reply was "I personally know Roland Christen and I am certain that
he would not insert reference material that is questionable on technical
issues. As a matter of fact Roland is quite vehement about the
superior performance of his scopes over larger reflectors."
This is why I included you in my comment. If this person has
misconstrued your position, I apologize. I don't have any problem with
what you say below; it is quite reasonable and I would like to restate
that I think these are fine instruments.
Clear skies,
Greg
Chris1011 wrote:
> Hey, Uncle Crinky, LEAVE me out of this stupid debate.
>
> I do NOT and never have
> claimed that a quality 4" will equal or beat any typical 8". If anyone calls
> here at AP and asks if he/she would gain anything by getting a 4" refractor
> over their present 8" scope, I say absolutely NOT. The reason I make 4"
> refractors is for traveling (Traveler, get it?) AND wide field imaging, period.
> You got me confused with the other 4" refractor peddlers who claim all kinds of
> outlandish things for their small scopes.
>
> Roland Christen
--
Toddwx
through seeing conditions better (non-obstructed optics too)
Toddwx
Greg Crinklaw
> Greg,
>
> At first of all, at such diameters and difference in aperture it is
> difficult to satisfy you with a real examples. But let sepak about
> 7" perfect aperture vs average 9" or 8" vs 10" or popular sample -
> 6" highest quality APO vs good 8" MCT.
>
> A lot will depends of atmosphere conditions - as worser, as better
> chances in smaller apertures with perfect optics. This is not an
> anekdot.
No -- it's not an anecdote; it's a popular misconception. It has become
clear in recent years that larger aperture telescopes are not
significantly more susceptible to bad seeing. In fact, what happens in
practice is that the instrument with superior resolution is relatively
more degraded by poor seeing because there is more to lose in the first
place. This is in no way an advantage for the smaller instrument. And
what happens when the seeing is good? Which one wins?
Another popular misconception: large aperture is wasted under light
polluted skies. This is also quite false.
> I just don't like to spend my time to argue in obvious questions.
I'm sorry, but it isn't at all obvious to me and it would be a mistake
to dismiss my understanding of the subject. Do *you* understand the
subject well enough to prove your assertion? If so, let's hear it. It
can't be that difficult. Tell be about objective measures of point
spread functions or CCD exposure times, for instance.
> I personally prefer an 8" perfect APO vs very good 10" MCT or SCT
> or reflector or even average 10" APO.
As do many people, but that's not the issue here. You made a claim that
I believe to be quite exaggerated. Either provide objective evidence to
support it or stop making the claim. I can't be the only person to
question your assertion. So please help us understand.
CHASLX200
>Message-id: <20011018162314...@mb-fo.aol.com>
>
>jupiter is high in the sky, wake up early my friend!
Wished i could even see a star!
Been cloudy for over 5 months here in Tampa FL..
I've had my 12" LX-200 for over a month, and only got to star test it so far..
But maybe 2nite it will stay clear, a line of clouds, are about 50miles to the
south, and looks like they may creep up back north...
Chas P.
Chris1011
advertisement by a telescope store that claimed on their web site:
REFRACTORS PERFORM AT ABOUT 1.5 TIMES BETTER THAN REFLECTORS AT
DETECTING FAINT OBJECTS.>>
In my experience with my scopes I found the figure is between 1.3 to 1.4 times,
i.e., a 6" high quality object glass will show deep sky objects about the same
as an 8" obstructed mirror scope. I have many times set up side by side with
these two instruments and can say this quite honestly. A local amateur and
myself used to observe quite regularly from a dark site near Broadhead
Wisconsin, he with an 8" SCT, me with a 6" F12 triplet refractor. He did NGC
studies to plug up some holes and mistakes in the catalog (this was back in
1980's). We regularly tested the limits of our two scopes on very faint obscure
galxies, and came to the conclusion that there was no difference in the ability
to detect them with either instrument. Occasionally, he would point his scope
at Mars or Jupiter and try to see similar detail that I was seeing in the
refractor. Well, in that case, there was NO contest, the refractor wiped the
floor with the cat. So, which instument had the visual advantage overall?
I also have set up in my own observatory a nice 10" Mak with very high tech
coatings and a 7.1" triplet refractor. In this case the advantage is 1.4:1, but
in my light polluted surroundings, the refractor shows every deep sky object
just as easily as the reflector, and the reflector is no slouch!
In your original statement you alluded that I equate a 4" refractor with an 8"
reflector, well that's an advantage of 2:1, and no way that a 4" can equal an
8. This is where you raised my ire!
If you want to study further how a smaller instrument CAN have an advantage
over a larger, you can peruse one of my essays on Robert Provin's web page:
http://voltaire.csun.edu/roland/coating.html
The article shows that the efficiency of two optical systems can be 1.7:1 if
the components are chosen either poorly or wisely. So, if you want the most
bang out of your instrument, choose your components wisely. It does make a
difference. And frankly, I don't care if you agree with me or not.
Roland Christen
Greg Crinklaw
Neither of these claims are true. As this sort of claim is now making
its way into advertising I think it high time somebody debunked it.
Maybe *that* would make a good article for S&T.
Clear skies,
Greg
Toddwx wrote:
>
> also.. according to Roland and others (I believe) higher quality optics cut
> through seeing conditions better (non-obstructed optics too)
--
Bob Midiri
> Hi,
> In the case of Zambuto mirrors, as in the case of AP objectives, I think
> that worrying about the difference between 1/35 and 1/40 wave is sheer
> niggling. I agree that the eye cannot detect the difference between an
> extremely good objective and one only slightly less good. But if the
> eye cannot distinguish this difference, then all the test results in the
> world will not make any difference, and will only add to the costs of
> production and therefore the cost to the buyer. Please, folks, take
> your pot-shots at each other on your private mail and leave it out of
> SAA, now that your points (however meaningless in practical use) have
> been made.
>
I agree Bill. As a mirror maker my self, I have to add that once a
mirror is around 1/15th wave, the eye will not be able to detect any
difference between that and one claimed to be 1/100th wave, when
observing! The atmosphere just will not allow us to make that very
fine visual distinction, when looking at objects. Sure it probably
will be differentiated by doing a comparitive star test, or bench
test, but hey, the purpose of a fine mirror is in visual observing,
and gaing the enjoyement of what you see (at focus). Bob Midiri DVAA
Greg Crinklaw
I'm sure that you can find specific cases of specific telescopes that
agree with your conclusion. But it is a big mistake to try and draw
general conclusions from anecdotal evidence. There are all sorts of
real-world factors, not the least of which is the quality of the
telescope you are comparing to, which can come into play. Few if any of
these factors have anything to do with the quality of the optics (read
expense) of the refractor.
Your 1.3 to 1.4 factor simply makes no sense physically. The idea that
there exists a factor that scales linearly with aperture is highly
suspicious. After all, the differences between different telescopes can
and has been quantified objectively; you should be able to point me the
objective evidence (physics or direct measurements) which support your
claim. After all, these are optical instruments. I do not think it too
much to ask for objective data to support your claims.
Greg
--
Ratboy99
>
I work at night, I can stay up late, but observing at 2:00 AM is pushing it.
Quite alright actually, I look forward to spending the next month looking at
Saturn. As Jupiter leaves it behind in its orbit over the next few years, the
planet observing season will beexpanding. That's a good thing...
Brian Tung
better than low quality optics, Greg Crinklaw wrote:
> Neither of these claims are true. As this sort of claim is now making
> its way into advertising I think it high time somebody debunked it.
> Maybe *that* would make a good article for S&T.
It might. I remember having this discussion on SAA a couple of years
ago (?), and it was not resolved to my satisfaction. I'm sure there
are people who are convinced one way, and others who are convinced the
other way.
It's not even clear to me what the assertion really says. Does it say
that higher-quality optics outperform lower-quality optics under poor
seeing conditions? Well, yes, I agree that they do. But they also
outperform them under good seeing conditions. Big deal! So that
interpretation doesn't tell me a whole lot.
Can someone tell me, clearly and quantitatively, what the assertion
really means? That would give us at least a solid basis for discussion.
Brian Tung <br...@isi.edu>
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt
Brian Tung
> Your 1.3 to 1.4 factor simply makes no sense physically. The idea that
> there exists a factor that scales linearly with aperture is highly
> suspicious.
Here are some possible bases in theory.
The MTF of an obstructed telescope is, at low frequencies, roughly
equivalent to that of an unobstructed telescope, with an aperture
roughly equal to the aperture of the obstructed telescope minus the
diameter of the obstruction. Thus, an 8-inch reflector with a 25
percent obstruction has an MTF similar to a 6-inch refractor, all
other things being equal. This is slightly pessimistic for small
obstructions, slightly optimistic for large obstructions.
At high frequencies, the MTF is not affected very much by a central
obstruction.
If we consider light-gathering ability, a refractor may typically
consist of one or two groups of elements, which collectively have
a transmission of 97 percent or so. Add to that perhaps a star
diagonal; these are small and can afford to have, also, a high
reflectance coating applied. The total light loss might be around
5 percent. On the other hand, a reflector consists of one large
mirror and one small one, and it is obstructed. The obstruction
alone might constitute about 5 percent, the reflectivity of the
large mirror perhaps another 10 to 15 percent. The small mirror
can also have a high-reflectance coating applied. All in all,
the light loss might be about 10 to 15 percent *more* than the
refractor, all other things remaining equal. This is about equal
to a 5 to 7 percent decrease in aperture.
Both of these factors scale linearly with aperture, and are not
suspicious at all. On the contrary, I would be *more* suspicious
of a factor that did not scale linearly with aperture, and yet was
inherent in the telescope manufacture and/or design (as opposed to
being, e.g., atmospheric in origin). I similarly disagree that the
factor makes no sense physically. It has yet to be demonstrated to
my satisfaction, but it is not--in my opinion--absurd on the face
of it.
Ratboy99
>really means? That would give us at least a solid basis for discussion.
>
Qualitatively (as it appears to be the custom of SAA'ers to reply indirectly),
I thought it had to do with the size of the spot. If all of the light is being
packed into a smaller spot, it could potentially be more affected by
atmospheric defocusing before any of the rays would be pushed out of the airy
disk. Eh? I prefer your assertion, so what, a good telescope will beat a crummy
same-sized one no matter what anyway. Maybe it is the same thing.
Greg Crinklaw
The large factor that has been claimed can't be accounted for by the
difference in transmission and obstruction alone, at least according to
the algorithm published in S&T by Bradly Shaefer some time ago, which I
belive was based on an excellent paper in the PASP. Unfortunately I
can't find that reference and I'm missing that particular issue of S&T.
Given this I have to assume there is some unknown quantity at work and
it seems "suspicious" to me that it would be linear with aperture. Of
course, that doesn't mean it can't be. The real problem, as I see it,
is identifying what this factor might be.
My suggestion is that the rest of this factor is the result of any
combination of the following factors affecting the larger-aperture
"comparison" scope:
o lack of thermal equilibrium
o poor collimation
o dirty optics
o optics that need recoating
o low-quality or inappropriate eyepiece
o no tracking
o lack of objectivity by the participants
o etc.
Clear skies,
Greg
--
Bill Foley
about better testing reflected (!) more his own desire to produce the
very best he is able to produce - Roland, Carl, and some others, IMHO,
have carried on and extended the philosophy and ethic of the great
objective-makers such as Herschel, Fitz, and others, and their testing
is to check on themselves, not to use as a sales gimmick...
--
Bill Foley
to their real life experiences in comparing reflectors and refractors of
very high quality, and fax them to you? The factors coming into play
include contrast and resolution as well as light-gathering ability. And
some small difference can be attributed to the percentage of light which
passes through a refractor objective vs. the percentage of light
reflected by a primary and secondary mirror, not to mention the small
impact of the obstruction required in most reflecting telescopes. Or
perhaps it would be necessary to use a device similar to automatic
cameras, which measures the amount of light as well as the contrast of
the image. Would you believe that??
--
Chris1011
Most Newtonian open tube scopes that I run across at star parties need
recoating. Here's what my professional coating source tells me about coatings.
He does this for a living, has reflectance measurement instrumentation and is
an unbiased source. I believe him because he is not in the astronomy business,
knows what he is doing, produces some of the best coatings I have ever seen,
plus does not really care what coatings you specify:
1. Pure aluminum coatings with no overcoat, fresh out of the coater measure 92%
peak reflectivity, which drops in a few hours. Depending on atmospheric
conditions, the coating degrades into the mid 70s after a year or two. I asked
him about aluminum alloy, and he said it made no difference - 92% fresh, drops
same amount after time.
2. Overcoated aluminum, SIO overcoat, also known as protected aluminum -
reflectivity peaks at 90% in the green, 88% average white light reflectance,
drops to 86% after some time depending on atmospheric conditions. SIO overcoats
are porous, and the underlying aluminum coating can degrade with dew since
there are acids dissolved in dew particles.
3. Enhanced aluminum with multilayer oxide coatings - are the most expensive
and most durable. If done right, they will not affect the figure of the mirror,
if wrong, it can alter the figure. Reflectivity peaks at 96% in the green with
94% average white light reflectivity. Multi-layer oxide enhanced coatings are
impervious to most atmospheric attack and can be cleaned without introducing
major sleeks (carefully cleaned).
Conclusion, if you have two mirror surfaces with old unprotected coatings, the
efficiency of your scope could be as low as 50%, with average protected
coatings probably 77% (not counting the secondary shading), and with the best
hi-tech coatings as high as 88%. Add a 35% secondary shadow as is typical in
SCTs, and you have to subtract another 12%. Add a typical Newtonian secondary
shadow, and you subtract maybe 4% to 6%.
So what's the great mystery about not being able to figure out why reflectors
might have lower efficiency? You did say typical reflectors, did you not? Is it
typical for reflectors to have high tech coatings fresh out of the box, or is
it more typical that they are a bit aged and perhaps have some atmospheric crud
and degradation on them? Stuff which really can't be easily removed by cleaning
- especially open tube Newtonians.
Roland Christen
Chris1011
> agree with your conclusion>>
Well, in my case of the 10" Mak and 7" refractor, I believe they are both quite
optimized. If you can find any 10" reflector that can put significantly more
photons down into the image than my big Mak, well I'll buy you lunch.
Roland Christen
Greg Crinklaw
physical basis for the argument will do just fine. Contrary to popular
myth contrast can be quantified. Unfortunately it has become a catchall
explanation for "voodoo optics" directed at everything from the size of
the secondary mirror to the performance of refractors.
Greg
Bill Foley wrote:
>
> Perhaps Roland needs to get signed affadavits from himself and others as
> to their real life experiences in comparing reflectors and refractors of
> very high quality, and fax them to you? The factors coming into play
> include contrast and resolution as well as light-gathering ability. And
> some small difference can be attributed to the percentage of light which
> passes through a refractor objective vs. the percentage of light
> reflected by a primary and secondary mirror, not to mention the small
> impact of the obstruction required in most reflecting telescopes. Or
> perhaps it would be necessary to use a device similar to automatic
> cameras, which measures the amount of light as well as the contrast of
> the image. Would you believe that??
Chris1011
I forgot one more coating type, multi-layer dielectric. This is an extremely
hard coating with 99% or greater reflectivity over the visual range. Drawbacks
are extreme cost for anything larger than about 3 inches, and it will change
the figure at the edge of the mirror. The reflectivity drops rapidly to nothing
past about 700nm, so is not recommended for CCD imaging. Once applied, it
cannot be removed except by grinding it off. I would not recommend this coating
for primary optics under any circumstance. The best application is for star
diagonals where only the center of the mirror is used.
Roland Christen
Greg Crinklaw
It is not at all fair to use an old reflector with degraded coatings in
your comparison. I should not have to tell you this. Clearly, most
reflectors come out of the box with coatings in excellent condition.
After all, we are talking about the buying decision for a new
telescope. There are many fair comparisons to be made between
refractors and reflectors. A reflector must be recoated from time to
time; a reflector may take longer to come to thermal equilibrium;
reflectors require accurate collimation; etc. Yet clearly in terms of
cost effectiveness, "DSO's for the buck", if you will, the reflector
design is superior. Turning these specific factors into the general
claim "refractors are ~1.4 time better" is neither accurate nor
appropriate, particularly when it finds its way into advertising.
Greg
Chris1011 wrote:
>
> >>o optics that need recoating>>
>
>
> Roland Christen
Greg Crinklaw
Perhaps we should meet in a dark alley (it would have to be dark).
Telescopes at ten paces then?
Greg
--
Chris1011
myth contrast can be quantified.>>
How so?
On another note, you still didn't answer me about a 10" reflector that can
outdo my 10" by any significant amount. Are you sidestepping the question.
Roland Christen