[Aavso-photometry] Flat frame quality and itseffectonphotometric accuracy

Tue Aug 12 17:52:12 EDT 2008

Lionel,

When SBIG did that correction (I think they just adjusted the allen screw 
that tensions the carousel to the housing. I also think they adjusted the 
screws that mate the motor to the rubber gasket in the carousel) I was not 
doing photometry at that time. Even when I did do photometry, the trouble 
just started a couple of days ago when I got a large mag drop on my images 
acquired after the meridian flip.

Prior to this problem, I was unaware that a meridian flip could cause this. 
I thought that even though the scope flips, the orientation of the camera to 
the scope still stays the same.

Steve
----- Original Message ----- 
From: "Lionel Catalan" <lcatalan at lakeheadu.ca>
To: "'Steven Orlando'" <sorlando at sorlando.com>
Sent: Tuesday, August 12, 2008 2:37 PM
Subject: RE: [Aavso-photometry] Flat frame quality and 
itseffectonphotometric accuracy

> Hi Steve,
>
> I just launch the autoflat script manually at the start of a session in 
> the
> evening and at the end in the morning. What kind of adjustment did SBIG do
> on the CFW10? Is it something that I can do myself? Did it completely 
> solve
> the repeatability problem?
>
>
> Lionel
>
> -----Original Message-----
> From: aavso-photometry-bounces at mira.aavso.org
> [mailto:aavso-photometry-bounces at mira.aavso.org] On Behalf Of Steven 
> Orlando
> Sent: Tuesday, August 12, 2008 2:25 PM
> To: aavso-photometry at mira.aavso.org
> Subject: Re: [Aavso-photometry] Flat frame quality and
> itseffectonphotometric accuracy
>
> 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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