Re: [ATM] Field Rotation and CCD Flat fielding

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

Then there are two components to flat fielding when using a focal plane
rotator. There is the fixed component that does not rotate and a rotating
component that changes with rotation.   I suggest a two part flat fielding
that separates the two flat fielding components when using a focal plane
rotator. This could be accomplished by taking a series of flats while
rotating the focal plane and a fixed flat with no rotation. The rotating
flat fielding component can then be subtracted from the fixed flat fielding
component. The fixed component could  be applied to all pixels. The rotating
component could then be applied as a weighted value algorithm to pixels
based on the calculations of the amount of rotation in the particular field.
Rotating diffraction spike smear could be reduced or eliminated with such a
method.



Don Clement

Running Springs, California




Andy writes,

> 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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