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

[Mike Potter]

     I know it's been a while since this thread was active, but it's been
nagging at me ever since I read through it.     

     I've frequently noticed exactly what you described in my own data.
After following the discussion here I decided to try a very simple
experiment.  A prime example of the problem you describe showed up in some
data I gathered early this week of the newly-minted WZ Sge star currently in
outburst in Andromeda (Var 08 And @ 020025.5+441019).  A plot of the
difference in brightness of two of my comparison stars showed a jump of
about 0.05 magnitudes after a meridian flip.  I'd seen that happen before,
but never 0.05 magnitudes - more typically 0.02 mag at worst.  I use a C14
with a SBIG ST8xme.  The chip is pretty small, and I use the AstroPhysics 2"
visual back that attaches directly to the 3.25" tailpiece of the C14 OTA.  I
did a very quick sketch of the optical path to the chip on the camera and
figured there must be very little or no vignetting across such a small chip.
Still, my flat fields show a decrease towards each corner of about 4-5% in
brightness.  So I essentially made the assumption that there was no
vignetting occurring, and that all of the low-frequency variation in the
flat had to be due to scattered light.  I used the "Correct Background" task
in Mira to "flatten" the flat field using a 6th-order curve for each
dimension (depending on the size of the largest "real" features (dust motes)
in the flat you may want to use a lower-order curve).  Re-processing the
data using the "flattened" flat completely solved the problem.  It also
resolved a problem I had wherein differential magnitudes between pairs of
non-varying stars changed from night-to-night, typically at roughly the same
level (few %).

     Basically, in my case, the scattered light appears to be coming from
internal reflections inside the OAG and, most significantly, from the baffle
tube inside the C14 OTA, producing a "bright" spot near the center of the
optical axis.  My flat-fielded images do look less "pretty", but at first
blush it appears I'm getting better photometry.

Mike Potter



-----Original Message-----
From: aavso-photometry-bounces at mira.aavso.org
[mailto:aavso-photometry-bounces at mira.aavso.org] On Behalf Of Lionel Catalan
Sent: Monday, August 11, 2008 4:25 PM
To: aavso-photometry at mira.aavso.org
Subject: [Aavso-photometry] Flat frame quality and its effect on
photometricaccuracy

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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