Quote:
Originally Posted by Camelopardalis
As Ray says, I only use 2 gain values (rarely, a 3rd) and have built dark libraries up over time. I always use offset 50 also.
Yeah the background ADU is the K value in PI, if you hover the cursor over background sky and not nebulosity or any other light emitting object. It might help to change the probe size to 9 or more pixels (click on the "play" button at the bottom where you see the K values). I often use a gain of 139 (and offset 50) and aim for a background ADU of around 1200 or better (looking at uncalibrated light subs).
Your (very) low offset will skew your figures relative to mine, but at gain 139 offset 50, a short dark frame of mine will result in a mean/median of around 800 ADU. I'd imagine that yours would come out at ~120ADU?
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Thanks again guys
Well, I compared linear subs on Eta Carina (offset 21) and NGC3576 (offset 15) and their K values are more or less the same as low as 428 on Sii and 780 Oiii. All at Gain 139.
I've also looked at two dark frames taken at 180s and 900s. These achieve minimum K values of 16 and 224 respectively. I think they are taken at offset 21. Their mean values are 324 and 970 respectively. So I guess that explains why I've been sitting at 900s subs!
It sounds like it is a problem that, for example, the mean readout on my Carina sulfur exposure is 989? The same as a dark frame taken at the same duration of 900s. The minimum value is 32.
I'm going to try to up the offset first while I collect Ha on the target I've been imaging lately, if that looks good I'll re do the Oiii and Sii, and then perhaps take the plunge of upping the gain to 200 and starting again with darks and flats. It'll be worth it if I can increase the number of subs to integrate, and reduce the reliance on noise reduction caused by underexposed images.
Thanks again for stepping me through this.