Distortion

[Tom Droege <droege@wwa.com>]

Elliot has brought up the question of how much distortion we can stand.
Sounds like now that he is deeper into the design he is finding the solution
more difficult than at first.  

I assume these are things like pincushion distortion.  This affects the 
astrometry.  I assume we are interpolating between the catalog stars.  Is 
this true?  Thus we usually have nearby stars for the position determination.
Or does the process use a mean of all the reference stars in the image??

He also asked whether the different filters needed to have the same focal 
length.  I assume that they do not.  Since we compute the positions for 
each filter separately, we could stand a large difference in focal length.
The
only loss I can see is the loss of stars seen in all filters due to different
plate scales.  

Someone comment so I can provide feedback.

Tom Droege