CMOS camera, exposure, gain and darks

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astrosharkey
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Joined: Thu Apr 04, 2019 6:52 pm

CMOS camera, exposure, gain and darks

Post by astrosharkey » Thu Apr 04, 2019 7:00 pm

I have been imaging for about a year now and always looking for ways to improve so recently started using SharpCap to work out my exposure and gain settings for my different filters for an image. Do I need to take a set of darks and bias for each filter as exposure and gain are different each time? I have noticed that a lot of CMOS imagers are still using long subs rather than lots of shorter ones contrary to advice, is there any reason for this or are people just sticking to what they know?

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turfpit
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Re: CMOS camera, exposure, gain and darks

Post by turfpit » Thu Apr 04, 2019 11:27 pm

astrosharkey

On a CMOS camera, if I change exposure, I would take a new set of dark and bias frames(otherwise they don't match the lights).
On a CMOS camera, if I change gain, I would take a new set of dark and bias frames (otherwise they don't match the lights).

Based on:
  • a dark frame needs same exposure/gain/offset as the light frame
  • a bias frame needs same gain and offset as the light frame but lowest possible exposure setting
For any change in the optics, such as a filter, I would take a new set of flat frames because there has usually been a re-focus, a change in optical characteristics, introduction of dust specs.
I have noticed that a lot of CMOS imagers are still using long subs rather than lots of shorter ones contrary to advice
If I want to get decent images from kit I own or am considering buying then I enter that equipment into the Search field on Astrobin and look at the resulting images, particularly the settings for Top Pick or Gold Cup winners. I prefer to let the images do the talking.

A question for you - what do you define as:
  • a long exposure?
  • a short exposure?
Don't forget, in your reply, to take into account the differences between a RASA or Fastar/Hyperstar running at f/2 versus a refractor running at say f/6.

Dave

astrosharkey
Posts: 5
Joined: Thu Apr 04, 2019 6:52 pm

Re: CMOS camera, exposure, gain and darks

Post by astrosharkey » Fri Apr 05, 2019 2:06 pm

Dave,

Thanks for that and it was what I was already thinking, might make up a dark library for generic settings and have to recapture darks and bias for anything else.

I already scout out Astrobin before any new target to get a starting point.

I am normally doing 30 subs unguided (F5.6) but looking to go further in the future, I found it strange when I ran the smart histogram it was suggesting very low exposure times (4.7 sec for L and around 20 for RGB) but I get that it is for a stacked image.

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