Hi bobzeq25, thx for exposing this. I've found tutorial how to calculate minimum & maximum exposure time based on the factors highlighted by You. I did some math, It looks ~280s should be fine. For darker place (~3.5 Bortle) I receive 320-350s. Better sky means longer exposure time, why? Because of less LP?
All calculations done for narrowband filter.
There are a number of ways of calculating "optimum" subexposure. The basic idea (there are wrinkles, described below) is...
If you shoot too many too short subs, you get too much read noise, which accumulates per subexposure.
If you shoot too few too long subs stars tend to saturate, and saturated stars have RGB maxed out, so you get white. No processing can restore the color, it's been obliterated. Highlights may also be saturated (or "clipped"), which blurs them.
To get a good balance, one method is to work on the first problem. Your subs also have light pollution, "sky noise". If the sky noise is large enough, the read noise doesn't matter. It's been "buried" in the sky noise.
So, less light pollution, you need to expose longer, to get enough sky noise to bury the read noise with.
The problem is, if you have little light pollution, you need such long exposures to get enough sky noise, that the second problem appears. By the time you've buried the read noise, stars are white, and highlights clipped. Not good.
I used to use that method. Now, I just work on the second problem, too long subexposures, instead.
I shoot a test sub, measure the number of saturated pixels. Zero is bad, you're not using the full dynamic range of the camera. So then I increase subexposure. Thousands are bad. White stars, blurred highlights. I decrease subexposure. I shoot for a few hundred saturated pixels.
All these methods work. Because it's only subexposure, and not all that important.
But I've been on CN for years. Looking at images people post I often see white stars, and blurred highlights. I believe it's because people make the mistake of thinking you need long subs to capture dim detail. After all, if you want to shoot a terrestrial photo in dim light, you increase subexposure.
But that logic doesn't work for DSO AP, because we stack subs.
I've gone back and looked at whether or not I'm burying the read noise. Usually yes, at least somewhat.
Here's an example which shows all these things. An image I did of the Pleadies. Note the bright stars are pretty white. That's always going to happen, they're bright. But note the dim dust (and it's really dim). How long did I have to expose to get it? 8 seconds, I took 8 (not a misprint, eight) second subs, and I got it.
662 of them. <smile> Click on the mediocre CN thumbnail for a better version, and capture details.
Another factor was also at work. I was using an F2 C8 RASA. Which sucks down photons real fast. So optimal subexposure is always going to be a lot shorter than, say, an F5.6 scope. 8 times shorter.
See why what others use for subexposure may be meaningless for you?
One day, all this will be clear to you. Until then, just do the important thing.
Shoot more subs. <smile>
Edited by bobzeq25, 02 October 2023 - 11:50 PM.