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Old 30-05-2016, 11:05 AM
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Shiraz (Ray)
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Originally Posted by gregbradley View Post
I've been following this thread with interest.

It seems to me the following are critical with this camera:

1. Highlights will blow out if the gain is too high. I think gain is the same as ISO on a digital camera so then when you increase the gain you lower the dynamic range. Also typically there is usually a sweet spot in the ISO range of a digital camera. Where the signal is boosted enough to raise shadows but not introducing much noise and not lowering dynamic range too much.

So think in terms of ISO and you would aim for the lowest ISO to get the shot as noise is lowest at lowest ISO and dynamic range is greatest. If you look at dynamic range graphs of digital cameras you'll see its not a straight line drop with increases in ISO. It tends to worsen more quickly the higher you go.

From what I have seen so far this seems to be the most critical point as 2000x 1 second subs at high gain of M51 with a totally blown core and surrounds is never going to cut it against a high quality CCD camera.

2. Again the low read noise is probably down with the low gain as the higher gain probably amplifies all the noise including the read noise.

One way of working out the optimum gain (ISO) for this sensor would be to look at the DXO scoring for the EM1 this sensor came from. They measure what is the highest ISO you can use before image degradation starts. I haven't looked it up but its probably around ISO1200 for a small sensor like this. So trying to relate that to the gain settings you might be able to find a table of gain and ISO from someone who tests various cameras that way.
It would be valuable to know.

Most DSLR imagers use around ISO800 when stacking lots of images so whatever that equates to would be good to know also. Its probably quite low.

It seems that gain control is a potential image wrecker and should be treated with a little is a lot type of approach and less is better.

nope, the read noise goes just the opposite way Greg - it reduces with increasing gain and is lowest at highest gain. However, the effective wells are not very big at high gain, so you need short subs - which you can do because of the low read noise.

The graphs posted earlier and available on the ZWO website show exactly how the various parameters vary with gain. ISO is not used as a measure of gain with these cameras, actual amplifier gain is used. This chip has made a complete transition from the world of DSLRs to the world of CCD measurements.
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