Re: [ATM] Field Rotation and CCD Flat fielding

[Andy Saulietis <iss@pvtnetworks.net>]

Don writes,

>How does one apply a ccd flat field when using a field derotator? Each ccd 
>flat field is calibrated for a specific relationship with respect to 
>orientation of optics and detector in order to correct for vignetting, 
>optical imperfections, etc. A field derotator changes this relationship.

Field rotation flat fielding works to fix vignetting and the optical 
components, filters etc
that rotate with the focal plane rotator. It does not work for 
contamination and optical
non-uniformities upstream of the rotator; fortunately these are usually far 
enough from the focal plane that
the flat fielding effect is pretty small. Fixed obstructions like secondary 
spiders
do produce diffraction spikes, as the field rotates the diffraction pattern 
will rotate relative
to the focal plane and 'smear' the pattern dependent on the amount of field 
rotation.
This is significant only for bright stars usually.



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