Re: [APML] What's the Best Way to "Combine" Two Images ofDifferingTimes?

[Wei-Hao Wang <astrophoto@gmail.com>]

On 9/19/05, Juan Conejero <pleiades2004@pleiades-astrophoto.com> wrote:

If your images have different exposures, then you should apply correcting
factors when integrating them. Less-exposed images should be given lesser
statistical weights, since they have less signal and more noise. On the page
linked above you'll find also a description of how to apply such corrections
in PixelMath. Be aware, however, that film response is wildly nonlinear. In
practice, knowing the actual response curve of a particular film under some
particular conditions is not feasible. This means that you should derive the
correcting factors (weights) from actual measurements of pixel values. For
deep-sky images, I guess that the median values of the raw images could be
good starting values to establish relative weights between them, on a
per-channel basis. What do you think? Anyone has some thoughts with regard
to this?

Best,
Juan
______________________________________________________________________
Juan Conejero, Pleiades Astrophoto Team
PixInsight Home Page: http://pleiades-astrophoto.com/pixinsight/

Hi Juan,

I have created many stacked film images from different exposure times.
To reach the best S/N, the images should be weighted according to the 
inverse-square of noise.  To do this, the images must be first adjusted to 
have similar brightness and contrast.  This can be done manually in 
Photoshop or automatically in Registar.  Then we can measure the 
standard deviation (rms) in the images and use the inverse-squares of the 
rms as the weights.  Measuring rms can be done in both Photoshop and 
PixInsight.  The optimum way is to do this in R, G, B separately.  I am 
usually lazy and only measure the RGB combined rms values.  The 
question is where to measure the rms values.  If we want to optimize
the quality of the shadow, we should measure the rms in the background.
If we want to optimize the quality of the highlight (rarely the case), we 
should measure the rms in the same highlight area in each image.

For raw (linear and without any prior processes) digital images taken from
the same night and under similar conditions, they can be simply added 
together with equal weighting even if they have different exposure times.  
This is the simplest case.  

Cheers,
Wei-Hao

-- 
________________________________________________________________
Wei-Hao Wang  :)

Institute for Astronomy at University of Hawaii

Address:                      
2680 Woodlawn Drive         Personal Website:
Honolulu, HI 96822             http://www.ifa.hawaii.edu/~wang
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