[Aavso-photometry] Flat frame quality and its effectonphotometric accuracy

[Lionel Catalan]

I would like to start by thanking everybody who has been participating in
this discussion about flats. I'll try to answer questions as best as I can.

 

Arne: You mentioned that the problem is likely due to scattered light. I am
surprised because I thought that scattered light would be corrected by the
process of aperture photometry since the background level is subtracted from
the star signal. It seemed to me that as long as the aperture and annulus
for a given star are similarly affected by scattered light, then the net
star signal would be corrected. Am I wrong about this? My images usually
have some gradient in the background caused by city light pollution or
neighbor's lights, etc. 

 

The test proposed by Arne in which a star is raster scanned across the CCD
would be difficult to carry out at my location (northwestern Ontario)
because I almost never get perfect skies. Therefore, it would be difficult
to distinguish the effects of change in transparency from those of poor flat
fielding. Instead, I propose to do the following test. 1. Image an open
cluster both before and after meridian flip making sure that the same star
with good signal-to-noise ratio stays at the centre of the CCD. 2. Calibrate
the images. 3. Do differential photometry on stars distributed across the
CCD using the central star as reference. 4. calculate the difference in
calculated magnitudes of the same stars before and after meridian flip. Plot
the differences as a function of star position on the CCD image and look for
trends. Would this be a good test?

 

Regarding dust on the filters and the repeatability of the filter wheel, I
checked that the dust donuts are at different positions using different
filters (B,V, and I), so it is clear that most of the donuts are caused by
dust on the filters. Moreover, when I divide two flat frames taken minutes
apart with the same filter (and without turning the filter wheel in
between), I get a perfectly flat image with only random noise. On the other
hand, dividing flats taken with the same filter on separate sessions after
having turned the filter wheel gives an image that shows the outlines of the
dust donuts. So, I think that the problem is in a large part due to poor
repeatability of the filter wheel. If I remember correctly, my SBIG filter
wheel (CFW10) always turns in the same direction to change from one filter
to another. So the repeatability of the filter wheel remains a problem even
if the filters are always approached from the same direction. Therefore,
removing the dust from the filters may be the only possible solution here.
By the way, can somebody recommend a procedure for cleaning filters in order
to remove as much dust as possible?

 

George: Until now I have taken sky flats either at dusk or at dawn. Usually,
I can get 30 flats per session (10xB, 10xV and 10xI). I use the program ACP
in conjunction with Maxim DL to automate sky flat acquisition. The program
automatically adjusts the exposure time so as to get an average ADU of about
50% the linearity range of my CCD. In my case, I aim for an average pixel
value of about 25,000 ADU.  The exposure time varies between 2 and 12
seconds. I median combine the individual flat frames to get rid of stars.
Sometimes, I have combined flat frames taken at dusk with those taken at
dawn to improve the S/N of my master flat.

 

Michael: The amount of vignetting is about 5%. (The brightest spot near the
centre of the image is about 26600 ADU and at the darkest corner the signal
is 25200 ADU). Most of the time, my target and comparison stars are all
fairly close to the centre of the image, so they wouldn't be subject to that
much vignetting. I apply both bias and dark corrections to the raw flat
frames with Maxim DL. The average signal of the flat is usually about 25,000
ADU, which is close to 50% of the CCD linear range. Flats are filtered and I
use the sky at dawn or at dusk. Light pollution is moderate. I live at the
edge of a relatively small town in the boreal forest. I don't understand
what you mean by normalizing the flats to common signal level , but the
signal level is about the same for all flats since the software adjusts the
exposure time to keep the signal within a narrow range.

 

I apologize for posting such a long message, but there were a lot of
questions, and I wanted to be as rigorous as possible.

 

Lionel Catalan