[Aavso-photometry] Flat frame quality and itseffectonphotometric accuracy

[Steven Orlando]

Lionel,

I also use ACP, and I don't know how you can tell ACP to image a set of 
flats on both sides of the meridian.

You can contact SBIG about your CFW10. I had that problem, and they had to 
adjust it.

Steve
----- Original Message ----- 
From: "Lionel Catalan" <lcatalan at lakeheadu.ca>
To: <aavso-photometry at mira.aavso.org>
Sent: Tuesday, August 12, 2008 2:05 PM
Subject: Re: [Aavso-photometry] Flat frame quality and 
itseffectonphotometric accuracy


>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
>
>
>
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