Re: OPTICS FOR TASS: REFLECTOR OR REFRACTOR?

[aah@nofs.navy.mil]

We've got a nice set of 'lurkers' on the maillist...they caught my error
on solar system absolute magnitudes quickly.  Keep it up!  Like Herb said,
"TASS may be able to reacquire some asteroids with poorly determined orbits",
and I've noticed two recent emails on the ccd maillist from observers who
are doing just that.  Spacewatch says they've looked at 60K asteroids in
the 4-year reported period, and I'm sure many need better astrometry (though
whether that is true at 14th magnitude, I don't know).  I'm not a solar
system observer (pretty obvious, huh), but I do try to keep track of the
major projects underway and agree with Maury that several groups will be doing
a better job at finding faint near-earth objects that TASS could do.  I also
know that these projects are complex and may take time to get fully functional,
and they can't possibly cover the entire sky enough times to catch everything
and get good orbits on every piece of debris they detect.  Prove me wrong! 
  Chris -- I've never tried to use a projection lens for viewing. What is
the difference between a 'projection lens' and a normal refractor (that is,
can they be used the same way)?
  Merle Jephson-King wrote:
> 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.
  I agree that a reflector has no chromatic aberration, and if one were to
use a single objective for the entire passband, then a reflector is definitely
the way to go.  Coma is a serious problem with reflectors, and is one reason
why most wide field designs (2dF, 4m prime focus camera, our new 1.3m 2-degree
telescope) need expensive correctors and field flatteners.
  At any given aperture, reflectors are cheaper (fewer surfaces
to figure, non-clear and inexpensive glass can be used).  At any given aperture,
however, you get more light-collecting area with a refractor since there is no
central obstruction and the psf is likewise cleaner.  I don't know of any real
wide-angle reflector designs other than some modification of the classical
Schmidt.  I worry that the Pro-Optic Mak may not be well baffled, and we
would get stray light problems (which wreak havoc with photometry), and in
addition I'd expect serious vignetting with the field compressor.  Tom
has the right approach -- if someone has a system like Glenn's (500mm f/5.6
with field cruncher), it would be very instructive to see how well it
performs with a Loral 2kx2k CCD.  While that is going on, Tom can proceed
with a design and quote for a refractive solution and compare.
    Merle does have a good point -- perhaps some optics house would consider
the TASS project something that they would be interested in and would support
with in-kind contributions, such as a lens design or providing the optics at
cost.  I have no such contacts (and certainly no free time to make them!),
but I second Tom and would even make up the "hero of TASS" medal to anyone
who wants to take on such a fast-time-scale project.
Arne