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Re: [APML] What's the Best Way to "Combine" Two ImagesofDifferingTimes?
Hi Wei-Hao,
>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.
This is very interesting. So after equalizing the images to achieve similar
brightness and contrast, you measure the standard deviation s on background
areas of each channel, to take 1/Sqrt( s ) as weighting factors. This makes
sense assuming a Gaussian distribution of noise.
Jean-Luc Stark describes an iterative method to calculate the standard
deviation of Gaussian noise that could be very helpful here (Astronomical
Image and Data Analysis, pp 37-38). A simple tool using this method should
be very easy to implement, for automatic calculation of relative weighting
factors.
How many samples of rms do you take on your images?
>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.
I don't understand this. I assume you're talking of images taken with linear
sensors (CCD). Linear images with different exposure times have
proportionally different signal intensities. Discarding noise, we should
take into account the total amount of signal when we are averaging a set of
images with different exposure times. For example, if I have a linear image
A exposed by a factor 1, and another linear image B exposed by 0.8, the
correct average should be (A+B)/1.8. Furthermore, if exposure time improves
the signal-to-noise ratio, this also should be taken into account. Or am I
completely wrong (I reserve that right :)?
Regards,
Juan
______________________________________________________________________
Juan Conejero, Pleiades Astrophoto Team
PixInsight Home Page: http://pleiades-astrophoto.com/pixinsight/
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