[Aavso-photometry] Averaging images for optimal photometry

[Jeff Hopkins]

As I mentioned in a past message, the time of the exposure varies 
with the software used. With AutoStar I believe it references the end 
of the first exposure so you can stack or expose for an hour, but the 
time reported is the end of the first exposure. I have more 
information on this at
http://www.hposoft.com/SZHER/Mentor.html

See the "Timing Information" link.

Jeff

At 11:58 -0700 01/06/2008, Michael Newberry wrote:
>Generally, stacking (or "combining") is a good thing becasue it increases
>the SNR and beats down the artifacts and imperfect flat field corrections.
>Rotating and deforming is OK as part of combining. However, if you are going
>to do photometry on the combined image, image registration ("alignment") is
>best done with no resampling (in other words, the pixels are shifted by
>whole-pixel amounts rather than partial pixel amounts).
>
>If you want the prettiest picture or the finest detail, then resampling is
>OK but, if you want the most accurate representation of the noise structure
>in the image (used for calculating the internal errors of the magnitude
>estimate) then you want whole pixel shifts. Using whole pixel shifts for
>combining many images inflates the FWHM by about 0.3 pixels---which is
>acceptable for most photometric work.
>
>Another thing to consider for combined images is that the time you reference
>is the mid-time of the exposure, not the DATE-OBS keyword which gives the
>beginning time of some image used in the stack.
>
>Generally, speaking, the SNR of a combined image will grow by as much as the
>square root of the number of images that are combined. This is the best
>case, in which the difference in pixel values from one image to the next are
>*purely* random. Since that is not exactly the case, the SNR will grow a
>little less quickly than sqrt(n) would suggest.
>
>Michael Newberry
>
>----- Original Message -----
>From: <gianlucaros at gmail.com>
>To: <aavso-photometry at mira.aavso.org>
>Sent: Sunday, January 06, 2008 11:30 AM
>Subject: [Aavso-photometry] Averaging images for optimal photometry
>
>>I have read that combining (averaging) multiple images of the same
>>  variable imporves SNR. As this operation is carried out automatically
>>  by many softwares which use different algorithm I wonder whether there
>>  is a standard procedure in averaging images, or a preferred software,
>>  for photometry. My concern is that many softwares actually rotate and
>>  deformate images to get the stacking. Is there any risk for photometry
>>  or averaging just increases SNR and thus is preferable? Concerning CCD
>>  errors is the formula 2.5*Log (1+1/SNR) acceptable? Reading the AAVSO
>>  manual there is a table (4.6.1) which states the absolute error as a
>>  function of SNR which gives different results. Can you explain the
>>  difference between the two methods and which is the best?
>>  Thank you
>>  Gianluca (RGN)
>>  _______________________________________________
>>
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>>  Aavso-photometry at mira.aavso.org
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>>
>>
>
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-- 
Jeff Hopkins
HPO SOFT
Counting Photons
http://www.hposoft.com/Astro/astro.html
Hopkins Phoenix Observatory
7812 West Clayton Drive
Phoenix, Arizona 85033-2439 U.S.A.
(623)849-5889
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