Let me give your questions a shot...
I have a CEM40 mount - given the HD8 weight + weight of my T3i (likely will go to an ASI camera as well), etc - I think I should be OK, yes?
It will certainly work fine for visual or planetary/lunar imaging. I've never used a CEM40, so I won't make any comment on its suitability for deep sky work with an EdgeHD 8. For what it's worth, I usually recommend an EQ6 class mount as the bare minimum for deep sky with an 8" SCT.
I've read that DSO AP can be very difficult with the HD-8, given the long focal distance, and difficulty in keeping it steady (even potentially with a CEM40). My plan is to get a reducer - either the NightOwl (out of stock), or Corrector III, to make it feasible. Is that a reasonable assumption? Ultimately, I likely will get the Hyperstar and learn the joys of F/2 wide angle AP.
And EdgeHD 8 will present challenges far beyond what you are used to with your ED80T. It's not just the weight, but the focal length. This is true, even with a reducer. As for Hyperstar, imaging at F/2 won't be a problem with the focal length, but F/2 presents a different set of challenges. The issue is that the critical focus zone is razor thin at F/2, so any tilt or collimation error is immediately obvious.
Having imaged with a C8 Hyperstar and also with an 80mm APO at F/4.8 (and similar focal length to the Hyperstar), I much prefer doing deep sky work with the APO. Yes, it takes more exposure time, but the quality of stars in the field is better with the APO.
I currently have a 50mm guidescope with a ASI120-MM guide cam. Will that be reasonable if I'm using one of the reducers or the hyperstar?
I have not had success with a guide scope on my EdgeHD 8. Even with the mirror "locked" and with a reducer, I found that the stars were unacceptably elongated at subs longer than 2 or 3 minutes. I consider an OAG to be mandatory with these scopes.
If I wanted to try AP without a reducer, and go after smaller DSO targets, is there any chance of that working with my mount / guidescope setup?
If you have the gear, give it a try! It certainly won't hurt. If you haven't bought the EdgeHD yet, I would feel a bit uncomfortable about recommending you spend a bunch of money on the experiment, though.
I'll add that after imaging for many years with my EdgeHD 8 at F/7, I've recently switched to a 130mm F/6.3 refractor. I love the refractor! Yes, it's got a shorter focal length than my EdgeHD, but with my camera I am still at 0.88 arc seconds per pixel. The seeing at my location is such that I'm basically seeing limited on both instruments, so I'm not giving up any significant resolution by changing scopes.
Dumb question, but when using the hyperstar, and the camera is mounted at the far end of the OTA, does cable routing matter at all - to avoid any kind of occlusions?
The cables on a Hyperstar system will cause diffraction effects that are visible in the image. There are a couple of ways that people deal with this.
If you bring in the power cable and data cable at right angles to each other, the resulting diffraction spikes will be very similar to what you get with a 4 vane spider. Some folks have also 3D printed curved frames to route the cables, which diffuses the diffraction effect so that they are almost invisible.
I've spend the last 2 years doing DSO AP with the EDT-80 + CEM40. Coming from that, is there anything else I should know, good or bad, before jumping into doing the same w/ an HD8?
The EdgeHD is far less forgiving than your 80mm scope and will present lots of new challenges.
If you are comfortable with your results on the refractor, though, you should do fine as long as you are patient.
Edited by WadeH237, 07 May 2021 - 07:59 PM.