The book is best, here's an overview.
As Stelios says 1-2 arc sec per pixel is a typical range.
Closer to 1 can give better resolution, _if_ a number of other things (seeing, tracking, ...) cooperate.
Closer to 2 always gives a better signal to noise ratio, with equal total imaging.
Both are important for good images.
Some (very) experienced imagers will go numerically below 1. Things like deconvolution can work better.
Imaging at short focal lengths (particularly imaging with camera lenses) frequently goes numerically over 2. Nothing wrong with it.
I would suggest that a beginner go in the vicinity of 2, it's more "forgiving" of typical beginner problems.
THE most important thing to consider. The two most common beginner mistakes are skimping on the all important mount, and/or getting too big a scope. Visual astronomy experience often misleads people, imaging is completely different. That's more important than details like image scale.
Your 9.25 would be far too big, the typical SCT fork mount far too little mount.
Here's another good (but more basic) book. Scroll down the page to the picture of the very expert author. That's a $500 70mm refractor on a $1200 Sirius mount. It's just about the ideal setup for getting into DSO AP. The right tool for the job.
Here's where you do not want to go.
"I started out with a CPC 800 on a heavy duty wedge and a Canon 450d. In hindsight, I'd have started with an 80mm refractor <on a good mount>. I would have saved a lot of money and gotten up the learning curve a lot quicker."
Edited by bobzeq25, 20 August 2019 - 10:00 AM.