[vsnet-chat 2095] Fw: faster read-out is really better ?

["Paul Warhurst" <p.warhurst@auckland.ac.nz>]

Ulisse Munari wrote:


>...There is one additional aspect to consider, among many others:
>if you leave the telescope unattended, I suspect the field of view
>drifts across the CCD, with the variable and the comparison stars
>migrating over regions of higher-and-lower response of the chip
>(them being caused by intrinsic pixel-to-pixel variation in CCD
>response, by vignetting effects and by randomly distributed
>shadows by the dust grains sticked to the CCD's window or on the
>filters). Unless you have FULL CONTROLL over the flat-fielding
>of your CCD, the expected errors may be QUITE LARGE.
>If you cannot properly and timely flat-field your data, one possible
>way out is to guide so accurately to maintain your stellar field
>FIXED on the same pixels, and therefore the variations you are going to
>see in your photometry will possibly look more intrinsic than spurious
>
>
>  Ulisse Munari


I think that it is better to concentrate on getting high quality flatfields
than trying to keep the stars on a specific set of pixels. I say this
because although one can achieve high precision using that technique, the
accuracy attained is not so good as there is a zero point problem which is
not being properly accounted for.
Good flats should allow a CCD user to get better than 0.01 mag phot.
(assuming good poisson stats) as a matter of routine.
Working on objects close to the sky background requires flats that have the
same characteristics as the sky, in particular the colour distribution. If
one examines flats made from various sources it becomes apparent that the
global variation in gain across the CCD is strongly wavelength dependent.
Here in downtown Auckland (population 1 million) the background is dominated
by the Na and Hg light pollution, so that blue twilight and red dome flats
are of no use. Sky flats, and if desperate cloud flats give the best
results; with 13th mag stars, SDs of 0.007mag and sometimes better are
achieved using sky flats and 25sec white light integrations with a 35cm
telescope. Basically the wider the bandpass, the more trouble one needs to
go to in order to achieve a good flatfield.

Cheers

Paul Warhurst

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