skrylten wrote:Thanks for your comment , John !
I think your suggestion is more or less what I did...
I did take 8 identical images for each focus step but instead of averaging them together I used the median pixels to create the single image.
Not sure if there is any noticeable difference between using the average or the median for the single image.
I will use this way of noise reduction for some stacks att full zoom ( it also worked really well with a "moonshot").
/Leif
A few comments on this general topic, hoping that they are somehow of use to others:
A technical consideration - the median of a set of random numbers is statistically more uncertain than the mean of the same set of numbers numbers. However, a key advantage of the median is that it will reject outliers that the mean will simply average in with the "real" data.
If you don't have or expect too many outliers from the replicated photos for each pixel, and especially if you don't have too many replicate slices to average, then the mean should be more stable and less subject to random noise.
If you wanted to try this out in a computer simulation, perhaps it would be interesting to create sets of random (Gaussian) numbers in Excel and compare the variability of the mean with the variability of the median for the sets.
Finally, there are components of camera system noise that are due to fixed pattern noise and also dark noise in the sensor. Averaging slices won't get rid of the fixed pattern noise, which may be somewhat spatially random in appearance. Instead, one should obtain a separate "dark noise exposure" using a very long exposure without light use this dark field image appropriately to correct one's photographs.
Amateur and professional astrophotographers cope with these noise sources routinely, and it may be helpful to Google "astrophotography noise reduction" (without the quotes) to obtain many more details and probably some useful software for performing noise reduction techniques on photographic imaging.
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As an aside, there are some remarkable technical discussions of quantitative photographic imaging, take a look at the Hubble Space Telescope's user manuals regarding image calibration and their image data pipeline. They are both very educational and amazing, IMO. Needless to say, their image processing techniques make those used by ordinary mortals look rather simple!
As best I can tell from a quick review, these are some useful links to the Hubble current technical manuals:
http://www.stsci.edu/hst/HST_overview/d ... ahandbook/
This related Hubble documentation page re their image processing may also be of interest:
http://www.stsci.edu/hst/HST_overview/drizzlepac
