I understand there is a way to get an idea where to set the offset by looking at the histogram of a bias or dark frame. Now I am looking to find an answer to find the right set of exposure/ gain settings by looking at numbers and not shapes and visual clues. I figured, that staring at the histogram or stretching the image can be very misleading by just eyeballing it and not consider the numbers that should go along with it. I know it's a rookie thing, coming from DSLRs, where most of the info people seem to use is just the graph's position on the scale and try to "eyeball" things and hope for the best.
Here is what I'm really asking: taking 1 single exposure - let's say gain 101/ offset 30 for asi533mc pro - 120 seconds and stretching the image, what information I need to be looking if the star shapes or the object i'm imaging isn't telling me the whole story? I learned FWHM can help me with focusing and that's what the autofocuser use to calculate optimal focus, but does it help with exposure time?
I know stars on images grow larger by more exposure time, but what's the starting point?
What size are they supposed to be under "optimal" amount exposure time? Or is there another number I should be looking out for?
These are probably very beginner questions I realize and probably funny too, for people doing this since the beginning of time, but consider that I've been doing this for less than 6 months and out of those 6 , I've been only using CMOS cameras for 4 and there are limited number of clear skies and out of those clear sky nights, and not all available for scope time (I have a wife and an 8yr old daughter).
Also, various software auto-stretches the image at various ways and I think the curve (without actual numbers around it ~ like N.I.N.A's cartoonish histogram ) isn't the most precise information if you ask me.
What' I've been doing is starting out with Sharpcap nearly every single time and try to get some info by turning on the histogram and do a measurement. Most of the time it tells me a very low offset settings, which I never follow, because it seems too low ( 3 or 5 etc) and I'm afraid I'm gonna be clipping data, so I keep that offset 30, regardless what sharpcap tells me. This is just coming from my limited amount of experience, because sometimes I just follow what looks right, not what's written down. That doesn't necessarily mean I'm correct though, I also realize that.
But I do consider the exposure time Sharpcap tells me (interestingly it's never more than 52 seconds) , but I end up adding time on top of it, because the single subs I get via suggested exposure/gain/offset just look very faint and many times and the suggested number of subs to take is something I can't reach anyway, due to trees, clouds, guiding issues etc.
It's the same issue with NINA's exposure calculator, which gives me obviously too low numbers (13 second exposures??) , despite my light pollution isn't that bad (bortle 5) and my scope is at the darkest possible location in my backyard with no floodlights shining on it and I trip over stuff if I don't bring a flashlight with me.
Also to consider, while the measurement gives me the "best dynamic range" possible with my current setup and sky quality, that doesn't always translate into the best image (In my experience) , especially if you won't have enough subs to stack.
I also put into consideration, that I probably trade the dynamic range for more noise (I guess?) , but at least I have something showing up once I stack the images. That means, that I deviate from the suggested settings by bumping up the exposure time, but I usually leave the gain alone, or actually lower it a bit, regardless what the unity gain or read noise charts telling me.
Just yesterday I double checked my older subs and I do have a set of 300s, 160s, 120s, 60s, 30s of exposures made a while ago, from the same camera and taken with similar settings and the same scope and while the measurement info may indicate, that my best settings should have been the 60seconds, the post processing was much easier and revealed a better stacked/adjusted image with the exposure taken with 120 seconds. 300 seconds also wasn't too bad, although star bloating was obvious, the stacked image looked acceptable, it just needs some cosmetic work via Photoshop.
Ok so I'm rambling too long now, my question still about using information (in numbers) gained from taking a picture at a certain exposure and how is that helping me to figure out if I'm over exposing or there is room for longer exposures and I won't be "over filling" my photon capturing well of the camera sensor?
Edited by unimatrix0, 06 May 2021 - 11:43 AM.