[Aavso-photometry] Averaging images for optimal photometry

[Michael Newberry]

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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