Re: ATM schmidt camera

["Tom Krajci" <krajcit@det5wg57.barksdale.af.mil>]

> From: Peter Abrahams <telscope@europa.com>
 
> Why doesn't the diffraction around a large secondary cause loss of contrast
> in a photograph, like it does in visual use?

Unless you use fine grain film like techpan, most films will delivery 
star images about .001" (25 microns).  An f/1.5 system will deliver 
an airy disk about 2 microns in diameter.  Even if a big secondary 
throws lots of light in the first ring or two you're still packing it 
into an area probably less than ten microns.

Very few astrophotographers have claimed (and perhaps fewer have 
actually done it) to generate negatives with star images 10 microns 
or smaller.

It's a common design goal to make star images 25 microns or smaller 
for camera systems unless you plan to use fine grain film.

Note:  No matter what focal length, the airy disk of any/all systems 
with the same f/ratio have the same physical (not angular) airy disk 
size.  (I'm assuming perfect optics.)  To get an airy disk that's 
about .001" in diameter you need a system working at about f/20.  
Since most astro camera systems are much faster than this the disk is 
much smaller.

Usually the hard part of astro cameras is getting the following 
to happen:

 - Cover a good sized piece of film
 - Mnimum image aberrations over the entire film area
 - Flat field
 - External film plane

The Schmidt camera absolutely kicks butt on the first two but pays 
the price by not solving the last two.  Could someone tell me if 
there is a more simple, elegant design that does better for the first 
two requirements?

Tom Krajci

Capt Tom Krajci
B-52 Intelligence Officer
"In God we trust, all others we monitor!"
http://spur.barksdale.af.mil