[Aavso-photometry] Averaging images for optimal photometry

[Michael Newberry]

Isn't the problem with AutoStar that the images are summed automatically and 
there's no way to save the individual frames for combining elsewhere?

Michael Newberry

----- Original Message ----- 
From: "Jeff Hopkins" <phxjeff at hposoft.com>
To: "Michael Newberry" <mnewberry at mirametrics.com>; <gianlucaros at gmail.com>; 
<aavso-photometry at mira.aavso.org>
Sent: Sunday, January 06, 2008 12:16 PM
Subject: Re: [Aavso-photometry] Averaging images for optimal photometry


> 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)
>>>  _______________________________________________
>>>
>>>  Aavso-photometry mailing list
>>>  Aavso-photometry at mira.aavso.org
>>>  http://www.aavso.org/mailman/listinfo/aavso-photometry
>>>
>>>
>>
>>_______________________________________________
>>
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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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> www.hposoft.com
>
>