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Hello. I have been having a bit of a cull lately and have some really good gear for sale - it is just not being used.
I have for Sale:
(1) Sky Watcher 50mm finder scope and bracket (AS NEW). £30 + £5 P+P
(2) Celestron 8-24mm zoom eyepiece. Has thread for camera under rubber eyecup. (AS NEW). £40. + £5 P+P
(3) Omegon CC 154/1848 scope BOXED. Dovetail rail. Back-focus adapters. (COMPLETELY NEW). £310. Collection only due to weight from DE55 (respecting distancing of course).
Please see pictures and thank you for looking.
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.
My Celestron 127eq telescope's RA setting circle isn't moving properly with the mount when I try and adjust my RA. It lags behind, seemingly sticking onto the bottom of my mount. What should I do to fix this and get my setting circle right?
I'm trying to get a photo of Jupiter and Saturn with my Nikon D3000 DSLR but I can't get anything clear. I attached the kind of photos I'm getting with my camera. They end up being too bright and no distinguishable features show up. Is it even possible for my Nikon to get photos of planetary objects like Saturn and Jupiter?
Jupiter with two of its moons, Ganymede and Callisto, imaged with a Skyris 618C CCD through a 8" SCT at f33.