[Aavso-photometry] Flat frame quality and its effectonphotometricaccuracy

Sun Sep 7 21:29:16 EDT 2008

I don't know all the software options out there so you would have to look in
your software documentation. There may or may not be tools to do these kinds
of corrections. In Mira, which I do know, the command is called "Correct
Background".

Michael

----- Original Message ----- 
From: "Steven Orlando" <sorlando at sorlando.com>
To: <Aavso-photometry at mira.aavso.org>
Sent: Sunday, September 07, 2008 5:49 PM
Subject: Re: [Aavso-photometry] Flat frame quality and its 
effectonphotometricaccuracy

> 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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