OK, I’ve used the Robin Glover tables as a baseline for my calculations many times, so here is my full calculation based on Juggernaut’s configuration and situation. He’s talking using short exposures on a Goto Dob, very similar to mine, and I started with a similar camera, so I’ll configure it as I would if I was in his shoes:

Camera gain: 300 (gives the best combination of low read noise at ~1.1 and a large enough well depth to not be blowing out stars with Juggernaut’s likely range of exposures times from 5-10 seconds).

OK, from Robin Glover’s table for a mono CMOS camera and Bortle 5 and an F/5 scope:

17 seconds starting number. Now let’s put in all the modifiers.

1) Juggernaut’s F-ratio of F4.7 delivering slightly more light gathering power per pixel:

17 * (4.7/5)^2 = 15.02 seconds

2) Juggernaut’s use of 300 gain/1.1e read noise versus the 2.5e in the benchmark:

15.02 * (1.1/2.5)^2 = 2.91 seconds.

3) Juggernaut’s use of a camera with ~~70% quantum efficiency (just to use sharkmelley’s last number which is probably in the ballpark) versus the 50% in the benchmark:

2.91 * (50/70) = 2.077 seconds

4) Juggernaut’s 2.9 um pixels versus the benchmark of 3.75um pixels, so less light gathered per pixel:

2.077 * (3.75/2.9)^2 = 3.47 seconds

5) Juggernaut using a color camera, so only about 1/3 the light per pixel vs a mono camera of similar technology:

3.47 * 3 = 10.4 seconds

So I think if Juggernaut wants to keep read noise to about 5% of the total stack noise on this camera and scope at 300 gain, in B5 skies shooting broadband, he needs exposures times of roughly 10 seconds.

If he has to use 5 seconds, then his stack noise will have an additional 10% noise due to the read noise and to get back to the amount of noise that someone with very long exposures experiences with said setup, he would need to use up to (1.1)^2 = 121% of the imaging time, or about 21% more imaging time.

Hopefully I didn’t miss anything big here.

if anyone is interested in the fundamental equations used to generate the table they can watch Robin Glover’s video and he goes through all the equations and you can actually build a spreadsheet that generates the same results.

My only issue is Robin Glover‘s use of the term “optimal sub length“ when he really should have said “sub length required for 5% read noise”, because when doing astrophotography the optimal sub length involves balancing many different trade-offs and he doesn’t go into those so that just is an unfortunate term that he used.

Assuming my calculations are close to correct, here are the big hitters for impacting this number:

1) What your actual sky darkness is. B5 has a pretty big range, so it can be 1/3 shorter or 50% longer depending on what your actual sky scale is.

2) If the moon is out or you are shooting low in the sky: The Bortle scale is generally a good night straight up from what I understand. Actually shooting conditions are often less dark than that so exposure times can be shorter.

3) If you get that Nexus reducer you mentioned once before. That lets about 78% more light per pixel, so minimum exposure time is 1/1.78 as long.

4) If you get a larger pixel camera

5) If you get a dual narrowband filter, the number will increase dramatically: very roughly proportional to 1/(% of light that gets through to each sensor relative to broadband). For the IDAS NBZ at about 12nm bandpass per color channel versus about 100nm, it's about 1/(12.5%) or 8x longer. For the L-Ultimate at 3nm per channel vs 100nm, it's 33x longer. These are crude calculations, you can do a more precise calculation using the actual spectral sensitivity of the camera, filter bandpass %, etc...

**Edited by smiller, 08 December 2023 - 01:30 PM.**