Complicated topic, about which there is much misinformation.
Short answer. Both aperture and f-ratio matter. How much of each depends on circumstances. Like much in astrophotography it's complicated. And definitely not determinative when it comes to selecting a scope to start with.
The one thing that's certain is that, pretty much independent of aperture, higher F numbers require longer subexposures. Add that to the fact that the longer focal lengths found generally (generally, it's not a theoretical requirement) magnify tracking errors, and you start to really need an excellent mount ($$$) to make one work. And a lot of experience. It's easy to think that this is all about equipment, an it's not. Knowledge is crucial.
So here's what you're missing, and it's _big_. Not something I thought up myself. The fairly uniform recommendation of both beginners and experts. The best tool for _learning_ DSO imaging for a beginner is a small refractor on a good mount. A long, heavy, slow scope is quite difficult to learn on. Even on an expensive mount, much moreso on a typical beginner mount. More than you would think intuitively. Tracking is a major reason, but the big scope makes _everything_ harder, right down to locating your target.
The two big beginner mistakes, seen over and over again here, are an inadequate mount and too big a scope. Often combined. Not because they are dumb, but because this is _so_ unintuitive.
As I say, not something I thought up. Here's one sample (and they are representative) of an expert and a beginner.
Scroll down the webpage of this recommended book, to the picture of the author. That's a $1200 Sirius (aka HEQ5) mount with a $500 70mm refractor. The expert did not choose those because he had them lying around. <smile> That's the best way to spend $1700 on a starter setup. Yes, you pay more for the mount. It's more important. Yes, you can make fine images with a 70mm. A sample of one of mine below.
Here's a very talented beginner looking back on his first year. He came here looking for equipment advice.
"First and foremost is listen to the folks who have been there. The philosophy of 80MM APO and good $1500-2000 mount is great advice for beginners. Sure you can possibly image as a beginner with something that is larger or that you may have but holy cow its hard enough with something small."
Holy cow, it is. <smile> No one quits imaging because it wasn't challenging enough, especially to learn. You want a setup that gets out of your way, and lets you get on with the job. I have many more of those references.
Good mount, small refractor. The right tool for the job. Your intuition will lead you astray on this one, _particularly_ if you have a background in visual, something altogether different.
You seem to have a background in photography. Here's someone who also had that.
"A very short time ago I bought the 8" SCT thinking that heck, I've been into photography for 20 years so how hard can AP be? Good Lord, I wish I would have found this thread <recommending the good mount, small refractor approach> before I went the route I did. The SCT isn't too bad but not for starting AP <of DSOs, planets are quite different>."
Note that the posts above are essentially about imaging, not about learning imaging. That they (appropriately) didn't even mention the mount. The best equipment for an experienced imager is quite often not the best to learn on. A good analogy would be trying to learn how to drive with a Formula One car.
This is why you see people recommend APOs over SCTs. The subject is usually starting out. An SCT is not a bad scope for AP, but it's a tool for an experienced imager.
Edited by bobzeq25, 02 July 2020 - 03:30 PM.