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Hmmm
My summary version:
1. Bit depth is used by camera suppliers to "define" both the well depth and (say a unity gain where 1e=1ADU) the number of "shades"/ levels which can be obtained.
2. Bit depth is used in processing to maximise the "usable" number of "shades"/levels.
3. Bit depth in the final image is used to present the "best" resulting image.
Example:
The camera:
An "8 bit" mono camera, at unity gain would have a well depth of 256bits and be capable of handling 256 shades of grey, each shade would be 1bit.
A "16 bit" mono camera, at unity gain would have a well depth of 65000bits and be capable of handling 65536 shades/levels of grey, each shade would be 1bit.
The Processing:
If we use a stack (say 1000) of say 8 bit images (as above) then when summed the total ADU count would be 256,000. Each ADU would be a shade level, giving 256,000 different levels in the image (!!!)
To manipulate each level available we would need to process at 18bit (2^18 = 262144)
If we used 16bit processing, then the 65536 limit would mean that 256,000/65536 =3.9=4bit per level. Inferring that the "finer detail" contained in the 4bit step would be compromised(?)
A side note: Stacking either by summing (or more commonly by averaging) always improves the SNR - all things being equal -See Howell's "Handbook of CCD Astronomy", p 71)
If we "average" stack the 8 bit camera image, the result is still an 8 bit image (1000 x 256)/1000 =256. BUT the SNR is improved due to the reduction in "variation", which is also smoothed.
The Final Image
If we save a 8bit camera file to an 8 bit image format then it will have 256 levels of grey.
If we save an 8bit camera file as a 16 bit image, it will still have 256 levels. Due to the unity gain and each level can't have less than 1e. This means that not all the possible 65536 levels are used.
If we save a 16bit camera file to an 8 bit format, the number of levels is restricted to 256 - each level in this case would have 65536/256 =256 electron. Again inferring that fine detail could be compromised..
As always, open to comment/ correction and ridicule.
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