Getting good data - Looking for guidance on exposure times

[ionia23]

Hello,

I'm going to do my best with this as I'm still learning all the correct syntax and terminology. If this question is stupid, I'll take my Stupid Cookies with milk this time

When it comes to exposure times for images and building an image stack, assuming a camera shooting in prime focus, and we'll make it an actual astro imaging camera, no fiddling around with ISO or anything like that, which one of these two would show more usable data, or would they be identical? If a "test object" is needed, let's say it's Messier 13.

20 exposures at 30 seconds each = 600 seconds = 10 minutes total time
40 exposures at 15 seconds each = 600 seconds = 10 minutes total time

Would there even really be a difference between the two against a globular cluster like that? I've been wondering if I'm seriously overexposing mine.

Thanks much everyone.

(note to self: Maybe I should try it out myself and see what happens if the clouds ever break out here).

[Mike in Rancho]

Hi Ionia,

Well this could probably be talked about all day, depending on how deep one wants to get about read noise and all that. Also more acquisition than ST processing, so you might run it by a more general forum like CN or the Stargazers one (?) if that is closer to you.

In general you could consider them the same. The one with more subs might deal with more read noise due to more images, and may take more pre-processing, also due to more files. That said, sometimes longer subs can go awry - no big deal if an airplane flies through a 60 second sub, but a 10 minute sub? Ugh!

Also, astrocams (not that I have one) deal with gain and offset, so you're not off the hook there. Depending on the model there will be recommended gains and offsets to use.

Very general rule of thumb is to expose away from black clipping and noise on the left, but not so much that you oversaturate and white clip too many star cores.

[ionia23]

Hey, thank you for the information on this. I imagine it's a pretty broad subject. I got going on this when I saw that I think the guy behind the Fitswork application did a shot of M42 using 1800 1 second frames and it looked incredible.

The advise I was given is, for your light frames, a minimum of 20 usable exposures using whatever exposure time you deem appropriate. The law of diminishing returns kicks in after that.

What I try to do, time permitting, is pick my object to shoot and I gradually start increasing the exposure time in 15 second increments until background skyglow starts threatening to wash out finer detail. I'm not quite ready to get into narrowband imaging yet (can't afford good quality 2" sulfur, O3, HA/HB filters yet).

Where Startools really shines for me, though it takes practice, is how smoothly it handles differentiating between fine detail and background. In many ways, I think Wipe might be the most critical module in the whole system.

[Mike in Rancho]

Hey, thank you for the information on this. I imagine it's a pretty broad subject. I got going on this when I saw that I think the guy behind the Fitswork application did a shot of M42 using 1800 1 second frames and it looked incredible.

The advise I was given is, for your light frames, a minimum of 20 usable exposures using whatever exposure time you deem appropriate. The law of diminishing returns kicks in after that.

What I try to do, time permitting, is pick my object to shoot and I gradually start increasing the exposure time in 15 second increments until background skyglow starts threatening to wash out finer detail. I'm not quite ready to get into narrowband imaging yet (can't afford good quality 2" sulfur, O3, HA/HB filters yet).

Where Startools really shines for me, though it takes practice, is how smoothly it handles differentiating between fine detail and background. In many ways, I think Wipe might be the most critical module in the whole system.

Hmm, I don't quite know about that advice.

M42 is also a special target, exceptionally bright. Very short subexposures can indeed be impressive. Sometimes even one exposure. But my first M31 was about 1200 1 second exposures, static tripod. It wasn't very impressive.

20 seems awfully low for diminishing returns to begin, unless each such exposure is particularly long. Regardless, there have been some where they really don't begin to shine until I am getting near to 10 hours of total integration. Depends on the object. And everyone's mileage may vary based on optics and camera.

I don't quite understand the background skyglow washing out finer detail. Fine detail either rises above the skyglow, which means you can capture it, or it doesn't, which means you pretty much can't. Also in most cases one would need to stretch in order to even perceive the faintest detail.