Re: OPTICS FOR TASS: REFLECTOR OR REFRACTOR?

[Tom Droege <droege@wwa.com>]

This seems like as good an opportunity to review how I got to 
wanting to use a 400 mm f/4 refractor.

First, we welcome all here, whether or not they have a burr under
their tail.  

Merle writes:

>would be nice to benefit from someone's mass production.  Has any of 
>the manufacturers been approached?  If not, why not?  If so, what did 
>they reply?  I am sure they would love to talk about our project 
>because both sides can benefit.  The U.S. manufacturers will have 
>overseas contacts also.  I think we have barely scratched the surface 
>of our options.
>

Looks like you just got the job.  This is a free for all here.  If you can
find
someone who has a good design for this project,  and can talk them into
giving us a deal, great!   We will award you a "hero of TASS" medal.  Note
that I am not a University, so I would be happy to endorse anyone who 
is willing to give us stuff.   Of course I can only speak for myself, as there
is really no offical tass to endorse anything.  Still we could all agree to 
make some deal.  I have something in mind to try here anyway.  

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.  

1)  I am trying to build simple rugged systems that I just ship out by UPS
and people get them to work (somehow) at the other end.  But the Mark IV
is a big system, don't think otherwise.  Since the CCD cost $2000 for a
camera,
one should not be too cheap on the optics for a cost effective design.

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.
That camera
head is about 12" in diameter.  The chip in it is not much larger than our
chip.  
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.  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. 

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?

4)  There are also 400 mm f/4.5 35mm format camera lenses available around 
$1000.  These are probably marginal for us.  We know the lens I have looked at
loses light in the corners of the 30mm square CCD chip.  I see a 400 mm f/2.8
listed in Keh for $3959.  This is too rich for me.  But it would be very
nice if not
too fuzzy in the corners.  

5)  It occurred to me that we will always use a camera/lens with a particular 
filter.  This means that we don't have to do full color correction.   We can 
design a lens to cover only our filter bands.  These are about 1/3 or less of
the full visual bandpass.  Since it is a quadratic problem, one hopes that 1/3
the bandpass means 1/9 as hard to design.   This is why I have tried to
persuade
Elliot Burke to to look at this option.  I await (not so) patiently his
result.  

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 
light spread out over several pixels, In fact (correct me) one does not
want a 
perfectly sharp focus.  One would like about 2 pixels (30 microns) FWHM.

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
camera lens systems.  I don't know what the size of the production of 400 mm
f/2.8 s is?   But I doubt that it is in the 1000s.  So we might make a lens
that is
comparable to what we can buy used in a camera store, at a price I can 
afford.

8)  Since I am in small production, and I have to focus 50 lenses, I don't
want them
to all be different.   So I don't really want to collect assorted telephoto
camera
lenses.  

OK, there are some tasks for volunteers.

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

At 07:58 AM 1/6/98 -0600, you wrote:
>It is interesting how the debate rages regarding the optics to use for 
>TASS.  One begins to wonder if we are working in a vacuum.
>
>I am not an optical designer by any stretch of the imagination.  
>However, there are some "rules of thumb" which I have always 
>considered necessary when choosing optics for astronomy.  When you use 
>refractors (camera lenses included) you have to correct for chromatic 
>aberration.  The way that is done today is to use at least three lens 
>elements to focus three colors at the same point.  If you use two 
>elements, two colors can focus two colors while the third and more are 
>blurred (out of focus).  Is this what we want?  Is the subject of 
>chromatic aberration the reason Elliot is working on a 4-element 
>design?  With refractors you can have pincushion or barrel distortion 
>as well.  If we are doing astrometry and not photometry we shouldn't 
>worry about chromatic aberration.  History tells us that refractors 
>work well for astrometry.  Look at Yerkes and Allegheny.  And, as 
>stated before, simple distortion can be corrected for when solving for 
>positions because it is a constant term for the telescope.
>
>If photometry is our goal, then a reflector system is a better choice.  
>Chromatic aberration goes away and you have other problems to deal 
>with such as coma and spherical distortion.  These two terms are 
>functions of the figure of the mirror optics.  Some of the large 
>telescopes of this world have these errors but you never hear about 
>them because they become a serious problem toward the edge of the 
>image.  The solution is that you use only the center 75 to 90 percent 
>of any image.  Take another history lesson.  Over 90 percent of all 
>the photometry done over the past few decades was performed with a 16" 
>reflector at Kitt Peak.  The telescope has recently been taken out of 
>service.  Even smaller telescopes, such as we are contemplating, using 
>CCD cameras, can accomplish the job done with 16" of aperture and 
>photographic plates.
>
>Now for another subject.  PRICE!  I feel that we need to rid ourselves 
>of the "MacDonald's" attitude.  "If it doesn't happen tomorrow, it is 
>too late."  This attitude is also prevalent on Government projects.  
>"We never get a chance to do it right, but we always get to do it 
>again."  I doubt if we will get our best price within the continental 
>United States.  The major optical companies have gone to Japan, Taiwan 
>and Mexico for cheaper optics.  Europe is not a good bet because their 
>labor rates are like ours.  Another thing to consider, which has been 
>alluded to in our e-mail, is to adapt someone's existing design.  It 
>would be nice to benefit from someone's mass production.  Has any of 
>the manufacturers been approached?  If not, why not?  If so, what did 
>they reply?  I am sure they would love to talk about our project 
>because both sides can benefit.  The U.S. manufacturers will have 
>overseas contacts also.  I think we have barely scratched the surface 
>of our options.
>
>P.S.  I sat on this burr all night and had to spit it out this 
>morning.
>
>Merle
>
>
>