[Aavso-photometry] Flat frame quality and its effect on photometric accuracy

[Arne Henden]

Flatfielding is an interesting exercise.  Since I've written a chapter about it,
plus numerous emails, let me just answer the current problem.

Since you indicate errors both with the meridian flip *and* with drifting across
the CCD, it does sound like your flat is the source of the error.  Most likely
the problem is scattered light.  To test, take a clear night and pick a field
with a reasonable star density, like an open cluster (this time of year, perhaps
M11).  Pick a star with good signal/noise on a fairly short exposure
(say 10-30 seconds),
and raster scan the star across your CCD (that is, a 5x5 pattern -
start with the star
in the center, move it to all 25 positions and then return to the
center position; a
total of 27 settings).  Then flatfield all frames and do photometry.
See if there
is a systematic trend (your star is fainter at the corners than at the
center, for
example).  With a perfect flatfield, you should get perfect
repeatability (if your
sky was perfect!).  You can usually see trends in the few percent with
this test,
and by using an open cluster, you can do the test on multiple stars.

Regarding the donuts that don't repeat - here the problem is likely
your filter wheel,
if you have one.  SBIG, for example, has a repeat setting accuracy
that is pretty
poor (many pixels), so any dust on the filter will cause a donut in a
different spot
on each rotation of the wheel.  You can test this by taking several
flats without
moving the wheel, then do random cycling and see if the donuts move.
Arne

On Mon, Aug 11, 2008 at 4:25 PM, Lionel Catalan <lcatalan at lakeheadu.ca> wrote:
> I've noticed that the main source of systematic error in my photometric
> analysis is now due to the quality of calibration flat frames. I take sky
> flats at dusk or dawn with an exposure time adjusted to achieve a pixel
> value approximately 50% of the linearity range of my camera (SBIG ST8XMEI).
> I usually combine 10 to 25 individual flat frames to make master flat
> frames. I make master flats for each filter that I use. Because I use a
> German equatorial mount, the stars change position in the CCD image after a
> meridian flip. This change sometimes causes jumps or drops of up to 0.015
> magnitude in the target or check stars. I attribute these jumps to
> less-than-perfect flats. I reason that an error of just 1.5% in the master
> flat pixel values would cause a systematic error of 0.015 magnitude, and
> this error only shows up during meridian flips or when the stars slowly
> drift in the image (I try to avoid that drift by using an autoguider to keep
> my target star centered in the image). I've also noticed that if I try to
> calibrate a flat done on one night with a master flat done a previous night
> (without changing the camera orientation in between), then I don't get a
> perfectly flat image having just random noise. Instead, I can see the edges
> of dust donuts, and these patterns in the calibrated flat represent about 1%
> variations above or below the image average pixel value. So, in summary, I
> don't know how to further improve the quality of my flats to avoid (or
> reduce) these errors.  I'd be grateful for any suggestions.
>
>
> Lionel Catalan
>
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