Re: OPTICS FOR TASS: REFLECTOR OR REFRACTOR?

[hjohnson@pluto.njcc.com (Herbert R Johnson)]

I hope my comments are useful:

On Tue, 06 Jan 1998 20:43:41 -0600, Tom Droege <droege@wwa.com> wrote:

*>Lets review how I got to wanting a 400 mm f/4 refractor.
*>
*>0)  Folding in the fact that this is a survey and we want to measure all the
*>sky at short intervals, one needs a short focal length.  The sky brightness
*>and field crowding pushes for longer focal lengths.   Ever getting the survey 
*>done pushes for shorter focal lengths.  While 400mm is not gospel, we are
*>pushed to something in the 300-500 mm range.  

*>6)  We can also stand distortion if that makes the design easier.  I don't
*>know 
*>anything about optics, but I think are 15 micron pixels are somewhat easier
*>a problem than a first class optical design.  We really can stand to have the 

Since you are starting with the basics, Tom, it would be useful to
say what the size of the chip is and what field of view you expect to
obtain with these short focal lengths. [using 15 micron pixels]. Past
edge effects, some of the choice comes down to resolution per pixel.
A twentyth-wave optic costs a LOT more than say an 8th wave optic.

*>2)  The camera head is 4 1/2 inches in diameter.  It has a lot of stuff
*>hanging off of it.  This is really small compared to say the MT for the Sloan.
*>I think it would not be useful to put our head in front of a mirror much
*>smaller than 10", and that would be pushing it.  For 400 mm focal length, this
*>means an f/1.6 mirror.  Try to buy one.

Why not use a diagonal? Why should the entire camera block the optical path?

*>OK there are ways to shorten the focal length, but
*>now you are back to a lot of lens elements and color correction.   It is
*>also something that would probably have to be erected on the spot by me.
*>Remember there are 4 lenses in a Mark IV installation.
*>But a small Schmidt might be ideal.  It
*>is just that I think of that as the next generation system. 

If one optical element is devoted to one passband - one color - the color
correction problem is greatly reduced. Use the same elements but adjust
them for each color (or refocus for commercial optics). [Later on, you
confirm you will attempt this design.]

*>3)  OK, we could look at existing folded optics.  Glenn Gombert suggested a 
*>500mm f/5.6 Maksutov at $209.95.  This is a pretty good suggestion.  It is one
*>of the ones to beat.  We don't know how well these cover the corners of the 
*>CCD.  Perhaps someone will step forward and make some tests.  Benoit are
*>you listening?

Does the manufacturer have any specs for this? Are there no comments in
the newsgroups? I'll Web search for info and comments on the Pro Optic Mak.
Surely someone has taken film photos with it. To some extent, I'd say to
hell with the corners.

*>7)  If I can keep the cost down, (and con Lockheed out of a good price for 
*>the CCDs)  then I hope to build a dozen or so Mark IVs.  This is 50 lens
*>systems.
*>At this quantity perhaps we are close to the production economies for large

Tom, you have to ask yourself: are you in the optics business? I think
Tom is calling out this point:

Presume a 4-element lens system, in some kind of aluminum barrel, with
some kind of adjustments. Rough out the time to manufacture, assemble, and
test one of these. Then double it, and allow that for every bit of speedup
you get with practice you get some problem that cancels it out. Figure a
15% damage loss - now you need 57-58 systems. Run the numbers.....this
is a major project in itself, and it can't be built by email.

*>Does anyone out there want to open a camera store and buy for tass in
*>quantity?
*>Does anyone know what the mark up is on camera optics?   Does anyone want to
*>test the Pro Optic Maksutov for our CCD size?
*>
*>I have been thinking and learning about the problems now for a few years.  I
*>don't know that my thinking is correct, and would welcome a better solution.  
*>For now as big a refractor as I can afford designed to just do what we need
*>seems right.  What do you all think?
*>
*>Tom Droege

I think, as you've said, it's hard to beat a decent and available reflecting
design with an untested and unavailable refracting design, except maybe in
expectations. I also think it's about priorities: is optical construction your
priority?

Herb Johnson

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Herbert R. Johnson                      voice/FAX 609-771-1503 day/nite
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