Quote:
Originally Posted by Camelopardalis
Ray, would you mind refreshing my memory wrt TargetADU that you did before for CCDs using some example figures from your 1600?  |
Had been reticent to post anything on this approach for the 1600 because
- it doesn't have a single "gain"
- it also has a hidden gain that is not obvious from the ZWO data
However, it turns out to be not so bad after all, so thanks for the prompting.
The basic formula (slightly rewritten) is:
TargetADU = Bias + 10*RN*RN/gain
where TargetADU is the sky background value you should try to get in your subs if you want the optimum balance between dynamic range and signal to noise ratio.
From the ZWO data, the ASI 1600 has variable gain (actually inverse gain) from 5 electrons/ADU to about 0.5 electrons/ADU, depending on the nominal gain setting (somewhere from 0-300). Then the software (or hardware) changes from 12 bit into 16 bit data by filling the bottom 4 bits with zeros. This is in effect an extra gain term of 16x and the modified inverse gain actually varies from about 0.3 to 0.03 ( ie it looks much more like other cameras).
Plugging in these values and using the published read values, yields the results in the attached table, where the "sky ADU above bias" column shows the calculated values of the second term in the above equation. To use it, read off the "sky ADU above bias" for the gain setting that you are using, add on the measured bias ADU for the offset/gain you are using (measure it in a bias frame - it will typically be a few hundred ADU) and the result is the TargetADU value for the background sky in a well exposed sub. Measure the sky background in a dark part of a sub and if you measure a higher value than the TargetADU, you could reduce your sub length - or if the measured value is lower, consider increasing the sub length (although you might like to deliberately choose underexposed subs for RGB to keep some colour in the brighter stars).
in summary
for your gain setting > read the "sky ADU above bias" from the table > add the measured Bias ADU to get the TargetADU > compare TargetADU with the background sky ADU in a sub > adjust the sub length if needed
For example, I use gain 100, so my Sky ADU above bias from the table is 380. Adding on my measured bias of 310 ADU gives a TargetADU of 690. I measured the average background sky brightness in one of my subs to be 758 ADU in a dark sky region with no hot pixels (image below), so I could have used slightly shorter subs than 1 minute - but 60 seconds is close enough and keeps the number of subs down a bit.
And if you don't want to bothered with all that, just adding 400 to your measured bias ADU will get you fairly close to the TargetADU for normal gain settings.
Regards Ray