[Aavso-photometry] checking flatfields

[arne]

Arne Henden wrote:

> I've told the list before how to check for scattered light problems,
> both internal checking of the telescope as well as on-sky tests
> with large calibrated regions like M67 and NGC7790 or raster
> scans of your own regions.  These tests really need to be performed
> before you do much serious photometry.
> 
I should mention that the SRO system has been tested and has less
than one percent flatfielding error across its field.  Since it has
a 21x21arcmin field, and since we've started a couple of recent
campaigns based in its calibrations, you can use the photometry in
those fields to test your system for scattered light, at least out
to the 21arcmin field size.  All of the calibration stars are in
ftp://ftp.aavso.org/public/calib/master_sro.dat
(a file that is quite large, containing about 100K stars).  Download
it and separate out just those stars corresponding to the field
of interest.  The names are usually close to the AAVSO name; GJ436
and ASAS_1826 for example.

Sort first on magnitude and delete the faint stars,
and probably the brightest one or two (might be saturated - look
at the photometric errors).  Then sort on (B-V) color and remove
those stars that are very red or very blue, leaving kinda a group
that might have (B-V) from 0.4 to 0.9.  With most transformation
coefficients, that will give you a maximum error of +/- 0.02mags
from transformation (and that will be random with respect to the
field, so will just make your data noisier).

Get the instrumental magnitudes from your frame for all of the
remaining stars.  Choose a star near the center as your zeropoint
and set its difference between the instrumental and standard values
to be zero.  Use that zeropoint for all other stars.  Plot the
residual differences as a function of position on your chip.  You
will see any systematic trend, at least at the 0.02mag level.
Do this for a couple of the campaign fields to ensure repeatability.
(And remember to report the photometry of the target!)
Arne