Picking the correct exposure for Deep Sky

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Borodog
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Joined: Fri Jan 01, 2021 7:25 pm

Re: Picking the correct exposure for Deep Sky

#51

Post by Borodog »

Hmm. How does that work mathematically?

Take for example my ASI183MC. Shooting in might light pollution (B5) at 4/f.8, this difference in optimal exposure at 0 gain and unity gain is about a factor of 3. The difference in DR is about 1.5 stops, about a factor of 2.7. The FWC at 0 gain is 15k; at unity it's just over 4k. So 3 unity gain exposures don't get me quite back to the same capacity.

Not trying to argue; just trying to understand.

SharpCap's Smart Histogram has 2 selectable gain targets, MDR & unity. But many people say that unity is not really a special or magical gain. Why not half unity? Or twice?

A separate consideration for my camera is that it is subject to banding that is worse at lower gain, so that alone might be a reason to choose unity over zero. Although I haven't tested SharpCap's banding suppression for the 2 different gains.
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Re: Picking the correct exposure for Deep Sky

#52

Post by admin »

Hi,

if there is a sharp drop in read noise as the gain is increased then the minimum exposure length needed can drop *a lot* - halve the read noise and the exposure length drops by a factor of 4, so you can take 4x as many frames in the same length of time. Each of them may have a bit less dynamic range than the low gain single frames, but the fact that you have 4x as many can overcome that.

This definitely happens for cameras that have a switchover from LCG (low conversion gain) to HCG (high conversion gain) at a particular gain value, since the step decrease in read noise is usually enough to cause this to happen. For cameras without that step drop in read noise, it all depends on the relative rates of change of the dynamic range and read noise as the gain changes.

chreers,

Robin
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