[Aavso-photometry] Flat frame quality and its effectonphotometricaccuracy

Sun Sep 7 18:07:33 EDT 2008

Lionel,

I hope Mike Potter doesn't mind me jumping in here because I'm sure he would 
give you the very same answer I am about to give.... which is, that there is 
nothing special or "physical" about the 6th order multiplicative polynomial 
correction he uses, other than the fact that he had a vignetting 
(multiplicative) problem and the 6th order in both directions was found 
adequate to flatten the image. He flattened his flats, but he also could 
have flattened his data frames after final reduction. Vignetting requires a 
multiplicative correction whereas scattered background light is additive, 
and both types of corrections might be warranted in your data. The kind of 
correction you apply depends on the nature of the non-flatness, and you 
would need to analyze the results of making such corrections to assure 
yourself that you are improving, rather than actually degrading, the 
scientific value of your data. It's not that hard---you just need to do it 
and analyzethe results.

Mike could have been 3rd or 7th order or something else, and different 
values in the other direction, but he is using what he found to work. People 
often speak (incorrectly) about "gradients" in the images. Technically, that 
means a slope. But that is actually just the simplest approximation since it 
is seldom, if ever as simple as a slope.

Michael Newberry

----- Original Message ----- 
From: "Lionel Catalan" <lcatalan at lakeheadu.ca>
To: "'Mike Potter'" <mike at orionsound.com>
Cc: <Aavso-photometry at mira.aavso.org>
Sent: Sunday, September 07, 2008 2:41 PM
Subject: Re: [Aavso-photometry] Flat frame quality and its 
effectonphotometricaccuracy

>
> This is very interesting if it consistently works to eliminate these jumps
> in magnitude after meridian flips. Do you know why "flattening" a flat 
> with
> a 6-th order (polynomial?) would result in a better flat? This sounds like
> magic to me at this point, but there may be a perfectly reasonnable reason
> of which I am not aware. Since this thread started a few weeks ago, I
> noticed that there were differences between flats taken at dusk and dawn.
> Michael Newberry suggested that these differences may be due to slight
> changes in the position of the primary mirror in the OTA as the mount does 
> a
> meridian flip. But I suppose that changes in scattered light patterns as 
> the
> telescope changes orientation might also account for these differences. In
> addition, there is the problem of slight lack of repeatability of the 
> filter
> wheel positioning. Nevertheless, it would be great if the flat processing
> technique that you mention did the trick. I just wonder what are the
> physical reasons for it to work.
>
> Lionel CTE
>
> -----Original Message-----
> From: aavso-photometry-bounces at mira.aavso.org
> [mailto:aavso-photometry-bounces at mira.aavso.org] On Behalf Of Mike Potter
> Sent: Sunday, September 07, 2008 2:40 PM
> To: Aavso-photometry at mira.aavso.org
> Subject: Re: [Aavso-photometry] Flat frame quality and its effect
> onphotometricaccuracy
>
>     I know it's been a while since this thread was active, but it's been
> nagging at me ever since I read through it.
>
>     I've frequently noticed exactly what you described in my own data.
> After following the discussion here I decided to try a very simple
> experiment.  A prime example of the problem you describe showed up in some
> data I gathered early this week of the newly-minted WZ Sge star currently 
> in
> outburst in Andromeda (Var 08 And @ 020025.5+441019).  A plot of the
> difference in brightness of two of my comparison stars showed a jump of
> about 0.05 magnitudes after a meridian flip.  I'd seen that happen before,
> but never 0.05 magnitudes - more typically 0.02 mag at worst.  I use a C14
> with a SBIG ST8xme.  The chip is pretty small, and I use the AstroPhysics 
> 2"
> visual back that attaches directly to the 3.25" tailpiece of the C14 OTA. 
> I
> did a very quick sketch of the optical path to the chip on the camera and
> figured there must be very little or no vignetting across such a small 
> chip.
> Still, my flat fields show a decrease towards each corner of about 4-5% in
> brightness.  So I essentially made the assumption that there was no
> vignetting occurring, and that all of the low-frequency variation in the
> flat had to be due to scattered light.  I used the "Correct Background" 
> task
> in Mira to "flatten" the flat field using a 6th-order curve for each
> dimension (depending on the size of the largest "real" features (dust 
> motes)
> in the flat you may want to use a lower-order curve).  Re-processing the
> data using the "flattened" flat completely solved the problem.  It also
> resolved a problem I had wherein differential magnitudes between pairs of
> non-varying stars changed from night-to-night, typically at roughly the 
> same
> level (few %).
>
>     Basically, in my case, the scattered light appears to be coming from
> internal reflections inside the OAG and, most significantly, from the 
> baffle
> tube inside the C14 OTA, producing a "bright" spot near the center of the
> optical axis.  My flat-fielded images do look less "pretty", but at first
> blush it appears I'm getting better photometry.
>
> Mike Potter
>
>
>
> -----Original Message-----
> From: aavso-photometry-bounces at mira.aavso.org
> [mailto:aavso-photometry-bounces at mira.aavso.org] On Behalf Of Lionel 
> Catalan
> Sent: Monday, August 11, 2008 4:25 PM
> To: aavso-photometry at mira.aavso.org
> Subject: [Aavso-photometry] Flat frame quality and its effect on
> photometricaccuracy
>
> I've noticed that the main source of systematic error in my photometric
> analysis is now due to the quality of calibration flat frames. I take sky
> flats at dusk or dawn with an exposure time adjusted to achieve a pixel
> value approximately 50% of the linearity range of my camera (SBIG 
> ST8XMEI).
> I usually combine 10 to 25 individual flat frames to make master flat
> frames. I make master flats for each filter that I use. Because I use a
> German equatorial mount, the stars change position in the CCD image after 
> a
> meridian flip. This change sometimes causes jumps or drops of up to 0.015
> magnitude in the target or check stars. I attribute these jumps to
> less-than-perfect flats. I reason that an error of just 1.5% in the master
> flat pixel values would cause a systematic error of 0.015 magnitude, and
> this error only shows up during meridian flips or when the stars slowly
> drift in the image (I try to avoid that drift by using an autoguider to 
> keep
> my target star centered in the image). I've also noticed that if I try to
> calibrate a flat done on one night with a master flat done a previous 
> night
> (without changing the camera orientation in between), then I don't get a
> perfectly flat image having just random noise. Instead, I can see the 
> edges
> of dust donuts, and these patterns in the calibrated flat represent about 
> 1%
> variations above or below the image average pixel value. So, in summary, I
> don't know how to further improve the quality of my flats to avoid (or
> reduce) these errors.  I'd be grateful for any suggestions.
>
>
> Lionel Catalan
>
> _______________________________________________
>
> Aavso-photometry mailing list
> Aavso-photometry at mira.aavso.org
> To change options or unsubscribe, goto
> http://www.aavso.org/mailman/listinfo/aavso-photometry
>
> _______________________________________________
>
> Aavso-photometry mailing list
> Aavso-photometry at mira.aavso.org
> To change options or unsubscribe, goto
> http://www.aavso.org/mailman/listinfo/aavso-photometry
>
> _______________________________________________
>
> Aavso-photometry mailing list
> Aavso-photometry at mira.aavso.org
> To change options or unsubscribe, goto 
> http://www.aavso.org/mailman/listinfo/aavso-photometry
>
> 