SgtBilko

4 hours ago, andrew s said:

The reason I would not use scaled darks is because of amp glow. This adds additional signal and has shown to be non linear. 

The reasons I don't think gain change is likely are two fold;

Firstly, the big advantage of CCD and CMOS is their linear response to light so to deliberately subvert this with a time dependent gain would be perverse (although it might be unintended).

Secondly, where careful study of the ASI 1600 has been done its response was measured to be linear. This would normally be done with a constant light source and increasing the exposure. If gain changed with time the result would be non linear. 

Have a look at these two studies by Christian Buil who is a well respected for his work in this area and the premier amateur spectroscopist. http://www.astrosurf.com/buil/CMOSvsCCD/index.html and http://www.astrosurf.com/buil/atik_vs_zwo/

From my own experience of measuring the ASI 1600 MM it behaves very well and as I expect. I suspect based on issues I have had in the past that taking multiple short exposure lights, barks or bias frames without any delay between them can give erroneous data. I always input a delay to ensure (as far as possible) that each frame is taken under similar conditions. 

Regards Andrew

Thank you Andrew, just to be clear the information I picked up was not in respect of the linearity of gain / light.  It was that the build up of dark current is not linear over time due to introduction of some by the camera itself.  I read both the articles you cite (thank you), but I can't see that Christian measures the build up of dark current over time.

Anyway, I tried to go back over my browser history to see if I could find the reference, but I'm afraid I failed.  I did come across this from Jon Rista, which you may find interesting.  https://jonrista.com/the-astrophotographers-guide/the-zwo-asi1600-guide/the-zwo-asi1600/preliminary-analysis/

PLEASE NOTE:  Anyone reading my previous post should ignore it.  Whilst I picked the information up from avid reading I can't check it.  In any case the the fact that you shouldn't scale darks due of the potential build up of amp glow is so widespread and backed up by respected sources that (in my opinon) it should be taken as "fact"...... but then my advice to anyone would be not to take anything you read on the internet at face value 🙂

12 hours ago, andrew s said:

This seems very unlikely to me. Can you provide a reference to support this that has any evidence rather than anecdote. 

Thanks Andrew 

I'm afraid I can't as I didn't make any notes.  I wasn't intending writing a scientific paper about it, nor do I particularly want to defend it.

It is mentioned a few times on Cloudy NIghts.

Out of interest I'm puzzled why you think it is "very"  unlikley, given it fits with your theory that you can't use scaled darks becasue the chip / firmware exhibits a non-linear  response whilst at the same time producing stable bias frames...... on that basis it has to be at least "possible", don't you think ?   

My undersatdning from hours of reading is that some ZWO cameras (1600 included) have firmware which under certain circumstances adds gain thus making dark noise non-linear over time.  Given the dark noise can be non-linear it is best practice to make a library of master darks rather than let software (e.g. PixinSight) scale a single long exposure master dark.  If you have to make a library then there is no point in making separate bias frames because the darks already contain bias noise.