From: "Tom Krajci" 
To: 
Reply-To: "Tom Krajci" 


>From: "Kristopher Barrett" 

>These commercial OTAs are Schmidt-Cass., and have a corrector plate,
>rAther than a simple window. They fix contrast problems caused by the
big
>secondary via a bigger focal length....

Diffraction effects (that spoil contrast) are caused by obstructions...like
the secondary mirror.  (I'm ignoring other effects, like bad figures,
scatter off optical surfaces/coatings/etc.)

Please explain how a corrector plate changes or overcomes these diffraction
effects?  (I don't think it does.)

Also, how does focal length play a role in diffraction/obstruction-induced
loss of contrast?  (I don't think it plays a role.)

I recommend folks check out this book: http://www.willbell.com/tm/tm5.htm
It's not the only source of info, but it covers pretty well the concepts of
diffraction from obstruction, and changes in 'contrast' (see the modulation
transfer function graphs and associated discussions).


Tom Krajci
Tashkent, Uzbekistan


-+-
Outgoing mail is certified Virus Free. Checked by AVG anti-virus system
(http://www.grisoft.com). Version: 6.0.443 / Virus Database: 248 - Release
Date: 1/10/2003

--- BBBS/NT v4.00 MP