Re: [APML] Long vs. Stacked Short Exposures

[Ray Butler <ray.butler@nuigalway.ie>]

Wei-Hao,

 > A single 40m exposure will give better S/N than 2x20m
> combined.   

That is true of imaging/photography where the same object appears in the 
  two (or more) images. However, it is not true of pictures of 
transients - like meteors. Once a meteor has been captured, any further 
exposure is eating away at its signal to noise. Or if it happens in the 
2nd half of the 40 minute exposure, the previous exposure of the sky has 
already eaten away at its S/N. The sky background penalty is 
approximately doubled over 40 minutes compared to over 20 minutes. If 
the Poisson error of the background is the dominant source of noise 
(dominating over film grain and scanner readout), then S/N would be 
sqrt(2) worse (1.4x worse). Whereas meteor S/N can be preserved at the 
higher level, by stacking multiple images with rejection of (in this 
case) the fainter background pixels at each point. By rejecting them, 
they play no part in the final image - it is the same as if the meteor 
was only exposed for the 20 minute period in which it occured. And fixed 
objects, like stars if the camera is tracked, will benefit from the full 
40 minutes worth of exposure. It's a win-win.

On the other hand, if you're at a really dark site or taking very short 
exposures, so that fixed-per-exposure noise (film grain and scanner 
readout) is dominant over sky background, you gain very little by using 
the multiple exposure with rejection approach. For the metors, you end 
up with the same grain and scanner noise as a single long exposure, and 
you save only a little bit on sky noise. But you certainly don't _lose_ 
any meteor S/N by doing so. For the tracked stars, you do lose some S/N 
(1.4x again), but who cares - the stars are incidental to the real 
targets, the meteors.

Ray "think DSP, shoot film...if the goddam weather lets you" Butler

-- 
Dr. Ray Butler (ray.butler@nuigalway.ie || ray@physics.nuigalway.ie)
Lecturer, Dept. of Physics || Computational Astrophysics Laboratory
National University of Ireland, Galway, University Road, Galway, Ireland.
Tel: +353-91-524411 ext. 3788   FAX: +353-91-525700

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