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Unity gain of my confusion

ZWO ASI533MM ZWO ASI533MM Pro CMOS Galaxy
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Karl Theberge avatar
Karl Theberge avatar
Good evening 

After a First "not so good" galaxy season I am about to swap the Edge for the RASA and I found myself confuse "again"…

I was running the 533mm pro at gain 100 on my edge.  Never thought about it much, was good enough for my 1-2 min exposure doing galaxy on a slow system.

But now with the rasa I want maximum dynamic range.

What would be the downside tonuse gain of zero instead of unity ?

K
Ryan Génier avatar
Ryan Génier avatar
Running a lower gain improves the full well and dynamic range, at the expense of noise.

For reference, I have a RASA 8 with a 2600mm and I shoot from B7.

When shooting narrowband (Ha, Oiii),  I use Gain 100 and 300s exposures.

When shooting broadband (RGB), I use Gain 0 and 30s exposures. I also have a LPS-P2 which I use for Luminence. For this one, I use Gain 0 and 60s exposures.
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Wei-Hao Wang avatar
Wei-Hao Wang avatar
When you have a very high photon throughput (with RASA, for example), using a low gain can be a good idea without much penalty.

A low gain is accompanied with high readout noise. A gain lower than unity gain (1 ADU per electron) further brings quantization noise. These are the common disadvantages of low gain.  However, when you have a very high photon throughput, you are guaranteed to have bright image background (unless you use unreasonably short subs, or you use narrow-band filters). When the image background is bright, the noise would be dominated by photon noise. Under this circumstance, read noise and quantization noise are no longer issues.
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Marc-Antonio Fischer avatar
Marc-Antonio Fischer avatar
Readnoise will go up. With F2 and longer exposure you can however overcome that, so the background will limitate that. With narrowband you probably will not have a good result with gain zero. The images will have these "lines" and the read noise will be visable a lot. (At least at dim objects) 
to be honest the difference in fullwell and dynamic range is not very huge compared to gain 100, so its not very an adventage
Marc-Antonio Fischer avatar
Marc-Antonio Fischer avatar

as you can see at gain zero you got about 13.8 dynamic range and read noise at 3.8e. Compared to gain 100 you got dynamic range of 13.5 but much lower read noise at 1.5e. So 0.3 more dynamic range for pretty much a lot of read noise. Not a good trade
Marc-Antonio Fischer avatar
Marc-Antonio Fischer avatar
Btw you better use gain 101. I know some instances where the unity gain is shifted by 1 or 2 values. Just to make sure you really hit the hit where it shits into unity
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