Re: [ATM] Field Rotation and CCD Flat fielding

["Don Clement" <clement.focuser@verizon.net>]

I wonder if there is an algorithm that could apply a weighted value for flat
fielding to the pixels affected by field rotation. For example, a weighted
value of flat field be used to compensate for spider smear in the rotated
field of pixels.

Don Clement
Running Springs, California

> 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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