Re: TN89 and hints on optical vignetting and stray light

[Tom Droege <tdroege2@earthlink.net>]

Seems to me the way to do this is to start with sky flats.  This gets the 
pixel to pixel variations.  It also allows optimum star finding as the 
result of applying it to the dark subtracted image is a pretty flat image.

Now once the stars are found, use the point source data to apply a general 
correction.  I think this is what Michael currently has in mind.  The only 
problem is to determine whether such a correction is constant in time.  So 
we will just have to take data and see.

First I will try the flocked paper to see if this is a result of internal 
reflections.  If not, then we will just have to design a correction 
procedure.

Tom Droege

At 11:11 PM 1/1/03 +0000, you wrote:
>Just one additional point re. the following comment of mine in the foregoing
>note:
>
>"However, applying the methodology
>  used in TN89 but applying it to only the dark-subtracted images entirely
>avoids this approximation of averaged sky flats since it is based directly
>on point-source signal data.  Furthermore, if the TN89 methodology (applied
>to only dark-subtracted image data) can utilise an even greater number of
>frames and star measures, then coverage may be sufficient to even correct
>for response variations having relatively high spatial frequency such as
>caused by dust located near to, or on the window of the CCD chip."
>
>The point is that if CCD chips show increasingly large variations in
>intrinsic pixel-to-pixel response then this will increasingly compromise the
>proposed methodology for flat-fielding since it is not really possible to
>calibrate down to the pixel level.  (Am not sure what this pixel-to-pixel
>characteristic is as used in the MkIV camera.)
>However, it can in principle get down to a level comparable in size to the
>stellar images themselves.
>
>Richard Miles