[Shiraz (Ray)]

hi

the new CMOS cameras allow inversegain/read noise to be varied, which means that there is no single measure of the most efficient sub length (here "efficient" means that the best possible image resolution and SNR will be obtained in the overall shortest possible time) - what sorts of sub lengths make sense then?.

The following tables have been prepared for two colour CMOS cameras using a fairly detailed astrograph model to suggest some starting sub lengths for a variety of environmental conditions and camera gain settings. To use, measure or estimate your sky brightness (eg use an SQM or a sky brightness atlas) and choose the table that best represents your sky. Then look at the column that best describes your scope Fnumber. Look down that column and choose the sub length that corresponds to your chosen gain setting for the camera - use lower gain for best dynamic range and higher gain if you want short subs.

There is also a column that shows the expected ADU value above bias for your gain setting. When you take a sub under the chosen sky and with the chosen gain, you should see an ADU value in the background sky that is the sum of this expected number plus the average camera bias value. If you measure a background sky value much higher that the expected value+bias, reduce your sub length and vice versa.

working through an example, if your sky brightness is 19, and you choose a gain of 150(asi) with an f8 system, the suggested sub length is 50 seconds. The background sky should be 450ADU above bias, so, if your camera produces bias images with an average value of (say) 350ADU, then you should see an ADU value of 800 (= 450 + 350) in the clear background sky regions of your lights. If you want to use gain or Fnumber values between those in the tables, guessing on the basis of nearby values should be good enough.

The published data for the cameras is a bit sparse, so some rather heroic assumptions had to be made on things such as quantum efficiency. However, the suggested sub lengths should not be too far wrong and will give a starting point that can be refined. The data were calculated for the ASI071, but equivalent gain points for the QHY168C were estimated by cross referencing (they are in grey). QHY claim slightly lower read noise at an equivalent gain than ASI, so shorter subs may be possible with that camera - as a starting point though, the suggestions could be OK for both cameras.

I do not have either camera, so cannot test the predictions. If you use the tables, I would be very grateful for any feedback on how effective the suggested sub lengths are. also, if you find any obvious errors, please say.

thanks for looking. cheers Ray