[Aavso-photometry] Flat frame quality anditseffectonphotometric accuracy

Tue Aug 12 18:23:36 EDT 2008

Lionel,

Interesting,

I looked at my calibrated images again, and I stretched a couple that were 
pre-flip, and a couple that were post-flip. It seems like the pre-flip 
images had a much more evenly illuminated field. I wonder what would cause 
the post-flip images to be so unevenly illuminated. I think my flats were 
acquired at dusk and the scope was on the pre-flip side (East).

Steve
----- Original Message ----- 
From: "Lionel Catalan" <lcatalan at lakeheadu.ca>
To: "'Michael Newberry'" <mnewberry at mirametrics.com>; 
<aavso-photometry at mira.aavso.org>
Sent: Tuesday, August 12, 2008 2:32 PM
Subject: Re: [Aavso-photometry] Flat frame quality anditseffectonphotometric 
accuracy

>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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>
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