Re: ATM All Sky Instrument

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

> From: "John D. Gwinner" <gwinner@northnet.org>
 
>   On the TASS (The Amateur Sky Survey) list, they've been talking about an
> all sky instrument.  In this case, they are using 2k x 2k CCD's and widely
> available cheap camera lenses, but recently there's been some discussion of
> using a Schmidt camera or other instrument.  They want LOW F ratio; say F2
> or F1.4.
> 
>   What would be the best optical design for that?  Wouldn't a Newtonian
> have a lot of coma at this F ratio?  Schmidt cameras would have none, but
> could you accommodate the curved focal plane with the CCD?

For Schmidt cameras - Reading Rutten and Van V's book Telescope 
Optics, Evaluation and Design (Willmann-Bell publisher) they say you 
need a positive lens just in front of the focal plane to flatten the 
field.  (It would probably have to be an achromatic lens assembly for 
good color correction.)  That may be a reasonable approach for a 
large CCD mated to a fast, low aberration optical system.  I can't 
comment on cost but the starting point of a schmidt camera is hard to 
beat for wide fields, speed, almost zero aberrations - except for 
field curvature. 

>   A few people have suggested removing the secondary on a commercial
> (Meade) schmidt camera and converting to prime focus CCD imaging.  

Celestron is offering fast (f/1.95) CCD imaging 
putting a small CCD array at the focus of an 8 inch schmidt-cass (in
place of the secondary mirror) but that probably won't work for a
large CCD array because of field curvature and coma off axis.

Tom Krajci

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