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16" F/1.2 Baker-Schmidt

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#1 Paul Novy

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Posted 07 December 2010 - 03:18 PM

Recently I've found on ebay souch a weird piece of mirror:

http://tinyurl.com/38pgatg

It is an 16" F/1.2 mirror of unknown origin. Unlucky there is no correction plate which making I think would be too expensive. So my question is: would it be possible to use this mirror in another optical system usable as an astrograph? I'm thinking of following sollutions: no full aperture corrector but insted using an 3 element corrector near focal point, or some solutions including using of a Nasmyth focus or even some weird off axis cases.

#2 kfrederick

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Posted 07 December 2010 - 03:42 PM

A lensless Schmidt

#3 m31supernova

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Posted 07 December 2010 - 03:57 PM

I been eyeing that thing the last two times it's been listed. Think all you would have to do with it is remove the coating and give it the good o'le 72 strokes to turn it from spherical to parabolic for a no corrector lens system. But then with such short f ratio I assume a field flattener would be needed? Since it so thin a wood based grinding handle could be made to put on top for the polishing besides a ceramic based pitch lap.(watch some youtube videos to see what I talking about). Anyway I slice it this item belongs in the hands of a professional and will take hundreds of more dollars to correct. I'll await a professionals opinion now... :)

#4 Paul Novy

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Posted 07 December 2010 - 04:04 PM

I assume it would be extremely difficult to make it parabolic at a such extreme F-ratio in terms of ATM capabilities.

#5 Old Will

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Posted 07 December 2010 - 04:36 PM

make a Dall-Kirkham

#6 jeffg

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Posted 07 December 2010 - 11:21 PM

I really don't think even a lensless Schmidt is possible due to the short focal length of the mirror (~500mm). One could only use about an aperture of about 150mm if you still want blur circles from stars that are usable. So it would be a waste of big mirror.

#7 Gordon Rayner

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Posted 08 December 2010 - 01:39 AM

I have a Perkin-Elmer Cassegrain derived F 1.2 collimator, which uses a mirror of similar dimensions, a Schmidt corrector, and some correcting lenses . There is an illuminated target in the focal plane. The system is heavy, being at least 70 pounds, probably more. The secondary mirror is very large, and the obscuration is thus a large portion of the aperture.

I have not used it as a collimator, though I tested it for that purpose. Nor have I used it in reverse as a telescope or camera. Its aperture is large enough to be useful for aligning the two telescopes of a binocular telescope. The large secondary would not be a problem with testing most Porro prism binoculars, but could be a problem with some small aperture straight-through roof prism using binoculars.

#8 Achernar

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Posted 08 December 2010 - 02:31 AM

You'll need a corrector plate, which will likely either be a meniscus shaped lens or an aspherical corrector plate to cancel the massive spherical aberration the primary mirror will exhibit. A true Schmidt camera has the corrector and mirror separated by twice the mirror's focal length, with the film holder or CCD chip at the focal plane, and that often has a curved focal plane at that. Therefore a corrector lens will be needed. Unless you are a skilled optician, I would pass on this mirror, unless you want to take up the challenge of building your own Schmidt camera.

Taras

#9 m31supernova

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Posted 08 December 2010 - 05:58 PM

Paul, When I researched awhile back on what or where such mirror came from I tracked it down that likely it came from a Meade scope of the R (ACF)type and so that had led me to do some little leg work in trying to track down such a corrector from the scope I thought it came from. Think it was this one maybe: http://www.meade.com...16/lx200_16.php Can't seem to find the exact link just now that mentions one of theirs using an f1.2 but the info is out there somewhere. SLightly remember it not a consumer level scope though so maybe someone else here will know whether it applicable to the lx200 or lx400, etc. The lx400 is rinign a bell the more I think on it though.
I dead ended in considering it to buy upon realizing Meade don't sell correctors seperate from scope. Here's another listing for same or similar mirror on astromart: http://www.astromart...ified_id=325814
If someone that has that class of scope is smart/wise they would research this such that they end up with a spare mirror for their observatory class scope!.

#10 GlennLeDrew

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Posted 08 December 2010 - 06:09 PM

The Meade SCTs and their derivatives have slower primaries of f/2, or very nearly so. Moreover, I believe none has a fully spherical (convex) back surface of more or less constant thickness, but rather a more steeply curved rear face so that it tapers down in thickness toward the edge.

This mirror really doesn't strike me as being related in any way to a commercial SCT.

#11 m31supernova

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Posted 08 December 2010 - 08:28 PM

Not sure if I can find again Glenn but I've seen the info that spherical mirrors of that f number belongs to a Meade in relation to one of their versions of an aka-ritchey chretien and it wasn't via the astromart ad link I posted either. That said, what is your take on that astromart one?. There is pictures there showing the original Meade box and model number and if not mistaken it looks to have something of a shallow taper involved. So I don't understand what your saying. Are you just saying all consumer Meades use much deeper back taper or are you saying them two mirrors are different from one another or both them mirrors would have to be from some more obscure non-consumer level offering? I should state that I haven't read the ebay mirror description lately so unsure what they say about it or if it same as astromart one. Info I was going off was more about what qualifies actual rc or the other various scope designs and can't recall if any mention of flat back/taper or not. Do you know of a good source to get actual mirror specs of the myraid of various scopes out there?

#12 Old Will

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Posted 08 December 2010 - 09:58 PM

Regrind it

#13 Flybywire

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Posted 08 December 2010 - 11:13 PM

This mirror looks exactly like a mirror I purchased a while back on e-bay for womewhat less $. I believe it is a Meade mirror manufactured for Terabeam. I know there was a Terabeam laser reciever that had a 16 inch f1.2 sperical Meade mirror in it, and have seen a couple of the recievers come up for sale. They are loaded with mirrors and optical equiptment.

#14 Mike I. Jones

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Posted 08 December 2010 - 11:50 PM

Well, you could make an Ebert spectrograph with it. A 1"x1" diffraction grating from Edmund Optics, an entrance slit, and a CCD camera or even eyepiece and you're all set. This is that 16" f/1.2 (R=38.4") spherical mirror to scale with the 1" square grating.
Mike

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#15 Mike I. Jones

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Posted 09 December 2010 - 12:05 AM

The Edmund Ruled Diffraction Grating, 25mm square, 1200 grooves/mm, 500nm blaze, part number NT43-005 for $97.50 would be perfect for this setup.

Or, you could go with more dispersion with NT64-406, 1800 grooves/mm, 500nm Ruled Diffraction Grating, 25mm Square, also for $97.50.

Just a thought. I can provide dimensions for the plot above if anyone is interested.
Mike

#16 Benach

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Posted 09 December 2010 - 08:00 AM

Jon is indeed right: commercial SCTs have no primary focal ratio of f/1.2.
What you can also consider: make a Companar from it. By far the best telescope in the book of Rutten&Van Venrooij but also by far the most difficult one. Apart from that, I seriously doubt it is suitable for narrowband imaging because of the huge diffraction angles makes the narrowband filters useless.
If you want to consider making the real Baker Schmidt, google for Bob Pfaff or look at this link:
http://www.considine...th/schmath2.htm
Then you know how to make them. I have no first hand experience but I know folks that made them, and they said it was rather easy.

#17 m31supernova

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Posted 09 December 2010 - 06:08 PM

Benach, So then what does the lx400 acf f/8 or rcx have in it for it's primary? f/2's? And are you able to cross ref the model numbers found in the pics of the astromart ad to something for visual work?. The terabeam mention doesn't ring a bell as being what I had read about but there sure is alot of info out there suggesting it for the aborted terabeam project..Not sure at all why I found myself thinking it was of their rc offerings. The pic of the box label on the astromart ad says "Com" which we can assume stands for communication (aka- laser based communication mirror) :)

ps- anybody ready Russian? :
http://www.astronomy...p?topic=23093.0

pps- I translated it and they got really deep into technical speak and but in the end it seems the consensus was that the mirror would be useless in it's current form.

#18 DarkSkys

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Posted 09 December 2010 - 07:28 PM

It could allways be stripped of it's coating and regound to a usable focal length.

#19 Paul Novy

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Posted 10 December 2010 - 05:16 AM

ps- anybody ready Russian? :
http://www.astronomy...p?topic=23093.0
pps- I translated it and they got really deep into technical speak and but in the end it seems the consensus was that the mirror would be useless in it's current form.



Haven't read the whole thread yet but they seem to assume that it comes from device like a laser projector system or something similar. And it would be most useful for heating the water for a tea on a sunny day :lol: Also making a corrector plate for such F-ratio is a problem due to some manufacturing processes problems.

Personally I think that the best solution for this mirror would be regrinding it to elipsoid (which is very simple to obtain when we have a sphere in compare to parabolic shape)and try to make a corrected Dall-Kirkham.

#20 Benach

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Posted 10 December 2010 - 05:42 AM

M31: Could be that the newer ACFs have a even shorter primary focal ration. However, I wouldn't try to make a ACF lookalike. Simply because I have the strong impression that an ACF is a SCT-a-like and I am not very fond of Meade or Celestron's SCTs.

#21 m31supernova

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Posted 10 December 2010 - 12:27 PM

Benach, nah I think it safe to trust others here that know a heck of alot about this stuff and assume I read something wrong. The mirror on astromart is obviousley the terabeam mirror due to the box saying "com" on it.

Paul, Info here: http://www.rfroyce.com/cassegrains.htm on Dall-Kirkham's suggest f ratio would need adjusting to +f/4. Would you make the convex secondary via treating it as the tool of a regular concave grind?
This Wiki link: http://en.wikipedia....rkham_telescope mentions modified Dall-Kirkham using focal corrector making them more suitable than RC's so sounds like a worthy project. Maybe following their reference link to the french site they sited will help. Another translation event: http://translate.google.com/#fr|en|

#22 nytecam

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Posted 10 December 2010 - 03:27 PM

A lensless Schmidt

I've made a couple of Lensless Schmidt Cameras [see video link below] and the f/ratio, to produce acceptable images, has to be quite slow ;) Most of this mirror's aperture would need to be stopped-down thus just adding unneccesary mass :p

For an Ebert spectrograph try my practical webpage

BTW the advertise knows his stuff IMHO :rainbow:

#23 Ruster

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Posted 12 December 2010 - 01:52 PM

Terabeam was a startup Meade partnered with to develop inter-building gigabit optical datalinks. Sort of a solution without a problem once wireless got better. Meade did mod some 16" LX200 OTAs for this; interesting they're out in the marketplace now. The optics might have a tweaked (near-IR) coating.

#24 Owen

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Posted 14 December 2010 - 08:49 AM

From a rough calculation, you could make an optical train (ignoring SA), with this mirror using this acro from the shed, and a doublet from the shed.

such as
http://www.surplussh...tem/l13003.html
and
http://www.surplussh...item/l2148.html

Basic calc indicates that the lens will revert the light cone back to infinity focussed (or very, very nearly), which you could use then to drive a much smaller scope.

Such a jury-rigged exercise demands low costs though, and I doubt that mirror is inexpensive...

Owen

#25 Gene Hunter

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Posted 22 April 2011 - 10:25 AM

I have this same mirror that I bought from ebay for $75 which I consider a fair price. I have not decided what to do with it yet, but I considered using just as a blank. I do know they where from the terabeam project which was to use light to communicate as it is safer than radio and microwaves which can be intercepted. I believe they where to communicate with satelites, and other secret stuff.

bought mine for when I get bored. one day I will do something with it. I did find someone who said they would make a corrector for it and from there I was thinking I would try one of those celestron fastar correctors to get a fast astrograph, but its a lot of money to just try something out.






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