# 29 Nov, 2019 21:12
Hello all and Happy Thanksgiving !|
Now that I think I have a handle and cmos camera gain, I could sure use some pointers on camera offset. I’m a ASI 1600 mono cooled user and I can’t find how offset should effect my images or how they might be better with a higher or lower offset .
# 30 Nov, 2019 02:27
|I've been wondering the same thing!|
# 30 Nov, 2019 04:20
|Hi , I image with the qhy 163 m, which is the same as the asi 1600, and I use different gains depending on the target or filters used, but I use the same offset for all my different gain settings, and I haven't had any problems. Hope this helps, clear skies!|
# 30 Nov, 2019 11:55
Offset is just a constant (DC) current added to your signal before it gets recorded. Whereas gain is voltage amplification, i.e. a
In very simplified and somewhat inaccurate terms, the end signal that gets recorded by the sensor is:
end signal = (incoming signal X gain) + offset
Actually the bit that says (incoming signal X gain) is a gross oversimplification because real gain is decibels, not absolute number.
Doesn't matter here, let's just assume gain is expressed as the equivalent multiplication constant instead, and let's focus on offset
which is just an added constant.
So, offset is the minimum signal which will be recorded by your sensor if the incoming signal is zero. Basically, open any picture in any image editor and try adjusting the brightness,this is what offset does and as long as you don't clip in either side the transformation is completely reversible. Why don't we set offset to
zero, or as close to zero as possible? Because noise (random +- in your signal) can result in negative values. Also, if the lower values you record are e.g. around 5, a noise of +-2 affects them a lot, almost 50% of the signal. If you add an offset of 50, suddenly the noise becomes 2.5% of the signal.
I haven't really touched the default offset of my 183MC, but from the above analysis (and the fact that I've never had to touch it) I am assuming any moderate value is equally good, does its job which is to keep your lowest data values at a reasonable level, and can be subtracted in post processing. Stick with that value
so that images from different sessions are easier to combine, although most modern integration algorithms will equalize offsets for you anyway.
The only case where offset will have an important impact is if you set it so high it will blow parts of the picture to white. This is
something you don't want.
Gain is a far more important setting because it directly affects the dynamic range of your image.
# 30 Nov, 2019 14:06
|Great info, thank you very much !|
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