astrodon
Pooh-Bah
Reged: 04/07/05
Posts: 1347
Loc: Hillsboro, OR, USA
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Apparently these are elliptical mirrors or at least one of them is over on Ebay..
http://tinyurl.com/66o4f7
http://tinyurl.com/5z5u8s
Any chance of making a usable (and fast!) Dall-Kirkham telescope with these (and the right secondary)?
Perhaps I should get into the book "Telescope Optics"...
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Don Miller
Meade 16" f4.5 in a hot water heater tube!
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jdownie
professor emeritus
Reged: 02/24/06
Posts: 737
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A wee bit fast, I reckon.
Try rfroyce.com for a primer.
John
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ATM project - a terrible waste of good Pyrex.
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Glig
sage
Reged: 10/02/05
Posts: 270
Loc: Baltimore, MD
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I think you can get Roger Angel to figure the 16 inch for $100,000. What are they, F/0.8?
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Richard Caldwell
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jdownie
professor emeritus
Reged: 02/24/06
Posts: 737
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The first one is f/0.6. Good for photography, though.
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ATM project - a terrible waste of good Pyrex.
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nytecam
Post Laureate
Reged: 08/20/05
Posts: 4819
Loc: London UK
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If I recall discussions with late Horace Dall - DK and 'fast' don't equate . DK is good for slow final f/ratios with small corrected fov ideal for planetary work  And these mirrors are pricey 
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Nytecam 51N 0.1W
Meade 30cm LX200+ETX-70+DS-2090+C8+Ha+CaK PSTs SBIG SGS+homebuilt spectrographs
Starlight SXVF_M9/Lodestar/Canon 300D DSLR/Fuji E550
My observatory build-ETX-70 imaging-spectro page
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Glig
sage
Reged: 10/02/05
Posts: 270
Loc: Baltimore, MD
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No one can figure something that fast unless you want to sell your house to pay for it.
Otherwise all that speed will only get you the world's most expensive shaving mirror.  It must be reground first. And the substrate itself isn't worth the price.
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Richard Caldwell
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Sean Cunneen
professor emeritus
Reged: 08/01/07
Posts: 565
Loc: Blue Island Illinois
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It doesn'r look like a mirror in the photos, it looks like a condenser lens, maybe for a spotlight or something Navel. I looked at the photos a bunch of times, but I am pretty sure unless my eyes are getting fooled, that is a clear glass lens. Perhaps the seller means mirror blank, in his/her description, they recommend using the flat side...
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Sean Cunneen
Blue Island IL
8" f/8 Home built Newt, Discovery mirror, curved vane spider, CG5 Motorized head with Intelliscope DSC, Pier
C102hd CG-4
100mm F4 Cedar Tube RF Refractor
2,3,4,5,6mm Televue Plossls
27T3,33T3,44T3
9-22 Ethos Zoom
NO, I'M NOT SELLING!
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operascope
newbie
Reged: 09/03/08
Posts: 4
Loc: Canada
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Keep in mind that not all ellipses are the same, unlike parabolas and spheres. The eccentricity of the ellipse in a DK is dependent on the focal length of the spherical secondary. In turn, the focal length of the secondary determines the final focal length and the back focal length. It is unlikely that this will satisfy the "off the shelf" approach to telescope construction, especially with this F ratio.
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Mark Harry
Carpal Tunnel
Reged: 09/05/05
Posts: 2492
Loc: Northeast
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At F/.6, as far as I know, there isn't a Dall, or any other cass that is designed for anything much under F/2 for the primary; at least that is offered commercially. There is a penalty of sorts that has to be adressed with high secondary magnification. (last I knew, anyways.)
Mark
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Scopes in the works-
Too many for putting down here! Favorites- 8" F/6, 8" F/4.72, 4.5" F/5.4, 14" F/4.455, all completed.
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jeffg
member
Reged: 02/13/07
Posts: 43
Loc: Irvine CA
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I agree that DKs with really fast mirrors won't work well. If you run the numbers through a ray trace program like JODAS:
http://www.geocities.com/ivankrat/java/jodas.html
You'll see the imagery is pretty dismal no matter what final FL or magnification factor used.
My 6 inch DK uses an F/4 mirror. I don't think I'd want to go much faster than that. Final focal length is F/15.8
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Jeff
14", 10", & 4.25" Dobs
8" Schmidt Newtonian, C-8 SC, 8" LSC
6" Cave Student Model A, 6" Dall-Kirkham, 6" RV-6
5" Refractor & 80 mm Folded Refractor
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GlennLeDrew
professor emeritus
Reged: 06/18/08
Posts: 630
Loc: Ottawa, Ontario, Canada
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My day job includes figuring 12.5" f/2 primaries for our corrected DK. It is rather exacting work, and lots of glass must be polished away to arrive at the ellipse. I shudder to imagine what f/1 and shorter would entail (initial grinding of the figure, it goes without saying, but I'll say it anyway.  )
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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
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jeffg
member
Reged: 02/13/07
Posts: 43
Loc: Irvine CA
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F/2 Primaries!!!!!!!!!!!!!!
I stand in awe of your ability. 
You must get abrasive delivered by dump truck. I almost got
kicked out of an apartment for grinding a 12.5 inch F/5 and all the mess it created on the patio. For some reason #60 carborundum and bare feet don't mix.
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Jeff
14", 10", & 4.25" Dobs
8" Schmidt Newtonian, C-8 SC, 8" LSC
6" Cave Student Model A, 6" Dall-Kirkham, 6" RV-6
5" Refractor & 80 mm Folded Refractor
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GlennLeDrew
professor emeritus
Reged: 06/18/08
Posts: 630
Loc: Ottawa, Ontario, Canada
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Ahhh, but we don't hog 'em out. We get them generated and then fine grind to the correct radius. I was referring to the glass removal in polishing to the elliptical figure from the polished reference sphere's surface. With a 12.5" f/2, at the radius of 6" the amount of glass removed amounts to a depth of 21 fringes as seen with a spherical test plate in contact at the 70.7% radius.
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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
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gregj888
sage
Reged: 03/26/06
Posts: 210
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At least one of these came from a semiconductor "stepper." The price listed might be a steal in that case, but for a telescope, it's awfully high.
These would make nice Schmidt cameras... Maybe a schmidt Cas.
Greg
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Glig
sage
Reged: 10/02/05
Posts: 270
Loc: Baltimore, MD
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Quote:
These would make nice Schmidt cameras... Maybe a schmidt Cas.
Greg
From the book "telescope optics"
"...focal ratios of F/1.5 and F/1 represent the effective limit of the speed of a Schmidt camera...faster than this, several spherical and a aspherical lenses are usually used"
You guys tell me: what kind of telescope uses a primary mirror at F/0.6? By amateurs? Even the Baker - Nunn camera isn't that fast. I would guess this is even too fast a primary for the guys at Steward Observatory mirror lab - or Tinsley...  
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Richard Caldwell
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Mike I. Jones
Pooh-Bah
Reged: 07/02/06
Posts: 1103
Loc: Fort Worth TX
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There isn't enough data on the 16" mirror at http://tinyurl.com/66o4f7 to even figure out what to do with it. It says it's an ellipse, but gives nothing about the locations of the two foci. An expensive paperweight, good only for its original purpose, or re-generation to some useful curve.
The writeup on the 12.5" mirror at http://tinyurl.com/5z5u8s says it's a 1/20 wave sphere. That precision is interesting, perhaps as a return sphere for interferometry. But it's challenging to design a telescope with a 12.5" aperture, 17" ROC spherical primary.
Since it's a sphere, by definition it can't be in a Dall-Kirkham Cassegrain.
I tried a single-plate Schmidt and the blur circles were huge, like 400 microns. The two-plate system shown in the plot is a little better. 9.5" aperture, 0.45-0.7um spectrum, 10 degree field, EFL 8.75", focal ratio f/0.92. But the curved focal plane (ROC -8.53") is good only for film. A field flattener could be used to provide a flat field for CCD use.
Mike
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56 mirrors, lenses, 16" f/6 Newt, 6" f/10 refractor, TOA-130S, Tinsley 5" f/15 Mak, 6" f/4 RFT, Coronado PST. Still to build: 24" f/10 Modified Dall-Kirkham, 10" f/26 Mak, 8" f/12 apo, spectrohelioscope, Herrig, Schupmann, and a new design you'll like.
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gregj888
sage
Reged: 03/26/06
Posts: 210
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Something this fast would need a curved film plane or auxillery optics. If it's part of a stepper it either has a lot of lenses with it, or is part of a light source.
There is an f/0.8 4" aperture 5" mirror shown in ATM-3 (the original) that's listed as a special purpose camera.
No matter how you slice it, this is a fringe project.
Greg
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Mark Harry
Carpal Tunnel
Reged: 09/05/05
Posts: 2492
Loc: Northeast
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Maybe they were used as a test optic, sorta like Hindle spheres???
M.
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Scopes in the works-
Too many for putting down here! Favorites- 8" F/6, 8" F/4.72, 4.5" F/5.4, 14" F/4.455, all completed.
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Stefan Rostyne
scholastic sledgehammer
Reged: 10/19/04
Posts: 940
Loc: Assenede, Belgium
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Don't know if anyone has read the comments underneath the ad :
>> Comment - Use Flat Side
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Stefan Van de Rostijne
4.5" F4.5 newt 5°widefield/finderscope
8" f/5.6 travel dob
old 12.5" F5 dob (used to look better...)
30 cm f/30 Classic Cassegrain (polishing primary)
23" f/4 dob project
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Mike I. Jones
Pooh-Bah
Reged: 07/02/06
Posts: 1103
Loc: Fort Worth TX
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I read the "use the flat side" comment, and it made zero sense to me. So why the ellipse? I don't think an optics-experienced person did the eBay writeup on it.
I thought about the 12.5" sphere at http://tinyurl.com/5z5u8s being a Hindle sphere, but it's not perforated, so a third fold flat at an angle would be required to get light into the test path. Could be a Waineo or other null-testing mirror, but it's just too fast. Could also be the primary mirror for an Ebert spectrograph, but again, unless it was a long-wave IR spectrometer, the spherical aberration at that low focal ratio would give too much blur at the slit image.
The only thing I can see it was intended for, and made to 1/20 wave (if it really is) would be a return sphere for double-pass interferometry. But why such a large diameter? Our Wyko return sphere is f/0.8, but it's only 1" aperture, and is placed very close to the focal point.
I would pay $250 for that 12.5" mirror just for the blank, but at $2500 they can keep it and load it up with trail mix at their next party.
Mike
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56 mirrors, lenses, 16" f/6 Newt, 6" f/10 refractor, TOA-130S, Tinsley 5" f/15 Mak, 6" f/4 RFT, Coronado PST. Still to build: 24" f/10 Modified Dall-Kirkham, 10" f/26 Mak, 8" f/12 apo, spectrohelioscope, Herrig, Schupmann, and a new design you'll like.
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