I've jumped into this hobby only recently, and whilst I have managed to get out a few times for imaging, typically I have not had a lot of time either due to cloud blowing in, or technical & user issues delaying commencement of imaging. Anyway, after an imaging session last night I am left wondering what the best way to determine an optimal exposure is, particularly when shooting under changing conditions (eg sometimes I will be in a dark sky site with little to no moon, other times in Sydney with high levels of LP and potentially moonlight as well.) Over various imaging sessions I have been testing out different exposure times but am still scratching my head a bit at the best way to determine that a given exposure time (assuming shooting at unity gain) is 'best' for the conditions.
For example, a couple of weeks back I briefly imaged Carina Nebula (around 16m only, due to cloud blowing in) using 240" subs, no filter. Last night I imaged the same target again, from the same location but with a lot more moonlight, however this time I used the Optolong L-Enhance filter and dropped to 180s subs. I imaged the target for 90 minutes in total. The individual subs, and the final stack, show a lot less of the nebula before processing - although during PP, one can see that a lot more data was captured. I suspect that my subs were too short last night, but can't say for sure. I will show what I mean with some screenshots of the unprocessed subs below, from both last night and a couple of weeks back.
Here is a screenshot of a single untouched sub taken last night. ASI533MC Pro, SkyWatcher Esprit 80, 180", Unity gain, Optolong L-Enhance, ~70% moon, Sydney LP skies.
Here is a screenshot of a single untouched sub taken about two weeks ago. ASI533MC Pro, SkyWatcher Esprit 80, 240", Unity gain, no filter, no moon, Sydney LP skies.
And one more, the same night and settings as the above image except a 180" exposure:
Do any of these look correctly exposed? Clearly there was a significant loss of light using the Optolong, which is fine, it's all a learning process for me. Does it matter if you are shooting shorter subs but stacking more of them? (eg is 10* 3 minutes = 30 minutes the same as 6* 5 minutes = 30 minutes from a data acquisition perspective?)
If we then look at the stacked files, after stacking in DSS using identical settings, but no further processing or adjustments the first stack is the 16 minutes (4* 240") from two weeks ago and the second stack is the 90 minutes (30* 180") from last night.
What I observe in the 2nd, 90 minute stack: less nebula is visible in the unedited stack despite much longer integration time. The sky is far less washed out despite having a 70% moon (v 0% moon during the 16m stack) and heavy Sydney LP in both. Stars look like they have retained colour better, potentially over-exposed in the 16m stack using 4 minute exposures, and there are a lot less stars visible now too - due to the filter, lack of exposure, or both perhaps?
What I did find with the data gathered last night is that if I made any adjustments to the image itself within DSS (saturation, luminance etc) it would introduce horrible colour banding artifacts - as though the files were shot in 8bit - but when putting the untouched stack into Photoshop, no colour banding even after significant editing. I don't recall having this problem on previous images in DSS, even with far less integration time, which makes me wonder if DSS is struggling due to subs being too short or if something else was causing it?
Here is a quick edit of the 90 minute session. Considering this was really only the third time I've gotten out to actually shoot (several more attempts spoiled by weather) and really have no idea what I am doing on processing these shots I am quite pleased with the output of the Optolong shot, but do question whether I had long enough subs to begin with.
I will keep working on the edit, but would love to hear some opinions on how to really nail the right exposure time in camera. Cheers