[Aavso-photometry] Flat frame quality and itseffectonphotometric accuracy

[Lionel Catalan]

I use dew shield that extends about 2 ft in front of my SCT, so it also
provides some protection for scattered light. My sky flats are taken by
pointing the telescope to near the zenith, so I would think that the risk of
scattered light would be minimum during flat acquisition (except for sky
light itself, of course).

Lionel 

-----Original Message-----
From: Michael Newberry [mailto:mnewberry at mirametrics.com] 
Sent: Tuesday, August 12, 2008 2:17 PM
To: Lionel Catalan
Subject: Re: [Aavso-photometry] Flat frame quality and
itseffectonphotometric accuracy

There are two effects: scattered light is additive and flat fielding is
multiplicative. Assuming that the corrections are all correctly apllied,
flat field correction multiplies the image by a number that varies across
the image. Scattered light adds a diffuse background that varies across the
image. When you do an aperture photometry measurement, yes you subtract the
diffuse background. But what Arne is talking about is whether there is
scattered light that affected the flat field frames, which would then cause
the correction factor (which gets multipled) to be incorrect. In other
words, the flat should make a 1.1x correction near the corner but it
contained scattered light which meant that the correction numer ended up
1.2x. You can't compensate for that in background subtraction. Does this all
make sense?

But also consider the other issues I pointed out in my list of questions.

Michael

----- Original Message -----
From: "Lionel Catalan" <lcatalan at lakeheadu.ca>
To: <aavso-photometry at mira.aavso.org>
Sent: Tuesday, August 12, 2008 11:05 AM
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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