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Probably an old discussion but lets review it with some measurements:
The dark noise should only have a small influence on the total noise of the final image. Most noise is generated by the sky background. Under good conditions SQM = 20.4, I measure using my ASI1600MM-Cool the following noise (standard deviation) in a dark and in a light for an area where no stars are visible (local measurement using ASTAP):
Dark 1 x 200sec, σ = 15 (range 0..65535)
Light 1 x 200sec, σ = 130
The noise in the dark is roughly 12% of the light, which seems acceptable to me. That would argue for about the same amount of darks as lights. With a worse SQM, you can probably do 2.5 times less darks for each (magnitude) step. So under light polluted sky you can do with much less darks than lights.
If you are going to photograph with the H-alpha filter, it will be super dark. In a single H-alpha (7nm) light I measure a σ = 25r. Of these, 15 are self-noise and 10 of the incoming light. In good conditions and using an H-alpha filter, this is an argument to make much more darks than lights
Above for a monochrome camera. To measure with an OSC (color) sensor I think it is better to first split the 4 Bayer pixels into 4 files and then measure them separately.
Some measurements with my ASI1600MM-Cool, monochrome:
1 x 200 seconds, σ = 16
1 x 200 seconds - master dark, σ = 15
4 x 200 seconds combined - master dark, σ = 6.8 This is approximately 15 / square root (4)
41 x 200 seconds combined, σ = 5
90 x 200 seconds combined, σ = 3.8 This is a limit value that arises mainly from unevenness of the pixels. The noise will be smaller, approximately 15 / square root (90) is 1.6
STACKED LIGHTS noise (lights corrected with darks and flats):
11x200 seconds, σ = 70 (measured at a star free area, standard deviation in 0..65535 range, sky conditions could have been different)
18x200 seconds, σ = 36
18x200 seconds, σ = 40
40x200 seconds, σ = 26
42x200 seconds, σ = 30
44x200 seconds, σ = 25
58x200 seconds, σ = 20
95x200 seconds, σ = 16
Apparently the light noise decreases considerably while stacking more lights and I reach σ values up to 16 a 20. You do not want to stack these images with a single dark having a σ = 15. If you want to keep the dark noise added below 10% of σ = 16 then you need 100 darks because they give: 15 / square root (100) = 1.5 noise.
So this confirms for a good suburban site (SQM=20.4) you will need about the same amount (or more) darks then lights. For a more light polluted area you can take less darks since the noise from the skybackground will be abundant. For H-alpha work you better take more darks then lights.
this little active region put on quite a show and i captured till i ran out of drive space.
160 frames x 40ms delay. (220 frames in each stack) (8 seconds per video capture) (160 captures)
Animated with https://gifmaker.org/
Cropped with avidub. Logo applied with avidub . Levels adjusted with avidub.
Files converted with PIPP and registax 5.1
Three pass Processing done in ImPPG (.xml files attached)
127mm x 1200mm explore scientific first-light achromat with Meade 2x tele-negative barlow. Basler aca720-520um camera.
Baader planetarium 36mm B-BCCD filter for energy rejection
1 angstrom calcium filter from Apollo Lasky @ http://calcium.solar
Thanks for watching!
1.xml 2.xml 3.xml