Re: [APML] 1805 Redo with Mike's technique

[Gene Horr <genehorr@texas.net>]

Philip Perkins wrote:

> Has it ever been answered why CCDs cope rather better with light pollution?
> (I have observed this myself)  Is it because the linear CCD reponse ensures
> a better separation between the signal and the noise?

Well, this is going to sound funny, but the sky background is better thought
of as a "signal", not a "noise".  But there is a noise componant when recording
background.  And yes, the answer has to do with the linear nature, especially
at low signal levels.

At the low background level such as sky glow film tends to have far worse 
SNR.  In other words if you look at each pixel in a scan of film in a
"blank" area the readings will vary quite a bit.  So if the average 
background reading is 25 in an 8 bit image, just subtracting 25 from each
pixel will not eliminate all of the artifacts as there will be a large 
number of higher pixels.  But if you bump the low end cutoff too high 
you start throwing away too much of the DSO/star signal.

Since CCDs are much more linear, especially for low end signals, the 
variance between pixel readings in a "blank" area will be lower.  If the
average background reading is 25 you could subtract 25 or 26 from each
pixel and get very few pixels with a higher background reading.

(For you math types think of it as the standard deviation of the background
sky glow signal is much higher for film than for CCD).

There is another factor that is in effecting this, although it is smaller
and could theoretically be corrected.  Most CCDs produce 16 bit readings.
There are subtler recordings of "contrast" than many films/scanner 
combinations capture.

Gene Horr
genehorr@texas.net

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