[Aavso-photometry] [Norton AntiSpam] Re: Light box question]

[Michael Newberry]

----- Original Message ----- 
From: "Ben Davies" <ben at davies.net>
To: "Aavso-Photometry" <aavso-photometry at mira.aavso.org>
Sent: Friday, June 15, 2007 3:39 PM
Subject: Re: [Aavso-photometry] [Norton AntiSpam] Re: Light box question]


> Guys,
>
> Thanks for the replies.  It was the S/N ratio that I couldn't see how to
> resolve.  I assume that in this situation you are not going for the 1
> million electrons/pixel that Arne was mentioning in a previous post, and
> are trading signal for accurate frequency matching.  What is reasonable?
>
> And Michael, you mentioned a "chip flat".  Could you explain what that is?

    A "Chip Flat"---what I personally have always called a "Pixel Flat"---is 
just a correction frame that corrects sensitivity variations of the CCD over 
the scale of one pixel to another. The Pixel Flat does not correct large 
scale variations such as vignetting (illumination falloff and shadows). A 
Pixel Flat adjusts each pixel to the average sensitivity of its close 
neighbors. In separating the Flat Field correction into an Illumination Flat 
and a Pixel Flat, you are providing two corrections that really are 
independent: the pixel-scale sensitivity variations have nothing to do with 
how the optical system illuminates the detector or how guider probes and 
such may cast shadows on the detector. The Pixel Flat is wavelength/color 
dependent, so you would have one for each filter. Once you create pixel 
flats for each filter and so long as you don;t make gross changes to the 
detector's quantum efficiency by operating at a significantly different 
temperature, then the pixel flat can be re-used. Another way to look at it 
is that you might have a library of pixel flats for temperature and filter 
just as you might have a library of dark frames for different temperatures. 
You can see the complexity increasing here, can't you? And note that by 
getting into all these gory details, you are also making it more difficult 
to black-box calibrate a night's data and have great confidence that 
everything was done right. Complexity goes way up. Said another way, if you 
want truly millimag results, you need to do a lot of interactive processing 
and sweat the details.

>
> I too, have never had great success at completely removing the ghosts of
> stars in combined images.  I should see Dr Kao soon.  I will ask him
> what software he uses and report back.

    Iraf and Mira Pro both have the tools that are needed, provided it is 
possible to do with the given data set. One requirement for doing it well is 
to have many frames taken at randomized positions spaced more than a few 
FWHM distances offset from one another so the star profiles don't overlap.

Michael Newberry

>
> Ben Davies
>
>> Hello Ben
>>
>> Mike did an excellent job of explaining the dither technique.
>>
>> Like everything in photometry, it has its limitations.  It is very
>> hard to collect enough images to get real high signal to noise ratio
>> in the flat.  As Mike said, you can spend more time doing flats than
>> observing.
>>
>> The other trick, is to remove the stars from these flats.  For me, I
>> have NEVER been able to make them really go away.  I can always see
>> residual stars at a low level.  Perhaps there is a trick that I do not
>> know able.
>>
>> I guess the bottom line is that you must match the flat technique to
>> the problem you are working to get the optimum results.  What this
>> probably means is to try several techniques, and see which answer you
>> like.  This technique will not always be the same.  I can tell you
>> that I spend some time almost every day working on flat fielding
>> techniques.  Its that important.
>>
>>
>> Clear Skies
>> Gary
>>
>>
>>
>>
>>
>> ------------------------------------------------------------------------
>> See what's free at AOL.com
>> <http://www.aol.com?ncid=AOLAOF00020000000503>.
>
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