[Edited Message Follows]
Mike, Ted
If you have not already, I would start reading Dr. Robin Glover's (SharpCap author) post in the SharpCap forums to get a basic idea about CCD vs CMOS. It's a little tedious but helpful. SharpCap has a built-in exposure calculator, but I have not used it since I still image with MaximDL for extended objects and galaxies.
https://forums.sharpcap.co.uk/viewtopic.php?f=35&t=456
Understand that sub lengths really depend on your Sky Background Noise. There's no way to be specific without knowing that. Sky Noise is dependent on your sky brightness, camera pixel size & QE, filters and f ratio of your optics. But it's the same issue in dark vs the city skies.
I first use Dr. Glovers online Sky Background Noise calculator.
http://tools.sharpcap.co.uk/
I then use a spread sheet I made up that uses the basic calculation for all kinds of sky conditions and telescope/camera combos. The spread sheet is based on:
Sub Length= K(Read Noise Camera*2)/Sky Noise
Where K = Is a constant based on the percent of noise acceptable in a stack of images vs a single sub of total integration time. As a shortcut I use K of 5% and it's 9.76. What you learn quickly is how sensitive low read noise cameras are to Sky Noise. I use 1.5e Read Noise for my QHY268C & QHY410C because that's where I set the mode/gain/offset at. The QHY600C is nearly the same as the QHY268C. The 600C is just a full frame version. The Read Noise can go as low as 1.1e for the 410C and the absolute QE can be over 80%. Compare that to 9-10e for CCD sensors. Plug a few numbers in.
Also helpful is this link:
https://snrcalc.vercel.app/
I try to moderate the calculations with common sense because they can show some ridiculously short exposures.
If I'm doing a star field with any OSC CMOS camera and a good sky pollution filter, I never go over 30 seconds in my sky conditions. I've gone up to 4 minutes on subs with the same broadband filter targeting extended objects, (no gradients) but have not been satisfied because of the low contrast. In fact, I rarely image extended objects in the city with broadband filters because of my Bortle 8 skies. They simply wash out.
I have been testing broadband filters (IDAS P3 and equivalents) on Galaxies with some initial success at about 2 minute subs because they are typically bright. Not sure where this leads.
With the OSC narrowband filters like LExtremePro, Radian and IDAS NBX you can treat sub lengths like narrowband even in the city. I've exposed 8-10 minutes with decent results and contrast like the IC443 image I posted.
Please take my experiences and comments with a grain of salt. This is a pretty complex subject probably beyond my pay grade and I'm learning and testing all the time.
