[Aavso-photometry] Flat frame quality and its effectonphotometricaccuracy

Sun Sep 7 20:49:38 EDT 2008

What other programs can you do this 6th order polynomial fix on a flat. I 
don't have Mira yet. I have MaximDL. Can you do that there?

Steve
----- Original Message ----- 
From: "Michael Newberry" <mnewberry at mirametrics.com>
To: <lcatalan at lakeheadu.ca>; <mike at orionsound.com>; "Gary Walker" 
<bailyhill at aol.com>
Cc: <Aavso-photometry at mira.aavso.org>
Sent: Sunday, September 07, 2008 8:11 PM
Subject: Re: [Aavso-photometry] Flat frame quality and its 
effectonphotometricaccuracy

> Gary,
>
> Do you mean Mike Potter? He's the one who first mentioned the 6th order
> polynomial. Assuming yes, then I would answer that the coefficient values
> don't have to reproduce from night to night in order for the correction to
> be justified or valid. My understanding is that Mike is not trusting a
> specific set of values that he applies every night but, rather, he is
> computing the fit from the image of interest and then applying that to the
> same image. I am not addressing whether the coefficient values themselves
> would be of interest or value, but I believe his goal was simply to 
> flatten
> the image using whatever coefficients resulted. I don't mean to second 
> guess
> Mike here. I am just giving the general principle behind the process he
> used.
>
> The coefficient values and the order of the fit needed to correct or 
> reduce
> differential vignetting (multiplicative) and scattered light (additive) 
> vary
> because the sources of non-flatness come from all sorts of external
> variables. Here are some sources of non-flatness (I'm mixing the two types
> here): sagging of the primary mirror, repeatability or constancy of 
> position
> of an aperture stop like the primary baffle or filter cell, the amount and
> direction of light pollution, imperfect dark correction if the dark 
> current
> is not uniform over the CCD or if the cold finger does not uniformly 
> spread
> the temperature across the chip, the moon's phase and direction relative 
> to
> the FOV, and which particular bright stars are in or near the FOV. It is a
> complex situation! By "differential vignetting", I mean that the basic
> vignetting is corrected by the flat field correction and whatever is left
> over, is differential vignetting.
>
> Michael Newberry
>
> ----- Original Message ----- 
> From: "Gary Walker" <bailyhill at aol.com>
> To: <lcatalan at lakeheadu.ca>; <mike at orionsound.com>
> Cc: <Aavso-photometry at mira.aavso.org>
> Sent: Sunday, September 07, 2008 3:18 PM
> Subject: Re: [Aavso-photometry] Flat frame quality and its effect
> onphotometricaccuracy
>
>
>>
>> Hello All
>>
>> Mike Simonsen, I have a question for you.? Do you know the coefficients 
>> of
>> the 6th order polynomial?? If so, it would be interesting to try this on
>> several nights and see if they are stable, or change considerably each
>> night.? If they do change, then I would not trust them.? However, when 
>> you
>> do get a stable polynomial, after making changes to the scope, flat
>> routine or whatever, you have good evidence that they might be
>> appropriate.
>>
>>
>>
>>
>>
>> Thanks
>>
>> Clear Skies
>> Gary Walker
>> Maria Mitchell Observatory
>> 4 Vestal Street
>> Nantucket, Mass  02554
>>
>>
>>
>>
>>
>>
>> -----Original Message-----
>> From: Lionel Catalan <lcatalan at lakeheadu.ca>
>> To: 'Mike Potter' <mike at orionsound.com>
>> Cc: Aavso-photometry at mira.aavso.org
>> Sent: Sun, 7 Sep 2008 5:41 pm
>> Subject: Re: [Aavso-photometry] Flat frame quality and its effect
>> onphotometricaccuracy
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>> 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
>>
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>
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