On long-focal-length telescopes like Schmidt-Cassegrains (and especially Schmidt-Cassegrains, with their floppy mirrors), off-axis guiding can provide better guiding than a guide scope. I've been using an off-axis guider with my C8 since 2018, and despite some troubles, it has still largely been a better solution than when I was using a guide scope.
I initially paired it with my QHY5 (the original red puck), but found it to be not sensitive enough to pick up guide stars, even at f/6.3 (1280mm). I picked up the more-sensitive QHY5L-II CMOS guide camera not long after, which typically gets just barely enough signal-to-noise ratio to hold onto a guide star on my setup. Of course, it performs better under dark skies, but any drop in transparency, and I'm barely holding onto a star, especially in the spring when the density of stars around out-of-galactic-plane targets tends to be a lot lower.
Focusing the Guide Camera
The first night I tried it, Wednesday, didn't go well at all. There were low clouds running across the sky, and once I got a bright star in my main camera and started slewing around to see if it would show up as a big unfocused blob in the guide camera, a cloud would inevitably cover the star. I gave up that night and went to bed.
Saturday night was much better, and had better transparency. Bonus points, the waxing crescent Moon was high enough above my lemon tree to see with my C8. A super-bright object is a lot easier to land in the guide camera because you can see it coming from off the edge. I slewed the scope over to the Moon, centered and synced it in the main camera, and then slewed above and below the main camera image to see where it would show up in my guide camera. Pretty quickly I could start to see the guide camera image lighting up, so I adjusted the exposure time down for seeing the Moon's surface, and got the guide camera roughly focused. I also created a field-of-view indicator element in TheSkyX where the Moon was in my guide camera compared to the main camera so that I could more easily land a star inside of it.
The center rectangle is the main camera; the smaller one above is my old guide camera; the rectangles above and below are the E and W side of the pier positions of the guide camera, respectively.
Next, I slewed the main camera to Regulus and centered it, synced the mount to it (so that it would be in the correct position with respect to my camera FOVs on the map), and then slewed the mount so that Regulus would show up inside the guide camera box. And bang, there it was! Now time to critically-focus.
Now, I use two different brands of filters: Astronomik CLS-CCD & RGB, and Chroma narrowband filters. They have different thicknesses, and thus adjust the main camera's exact focus point a bit. ("A bit" on my PrimaLuce Esatto focuser is still like 20,000 steps). So to set the guide camera position so that it's mostly in focus for both focus points of the two sets of filters, I take the CLS-CCD filter's in-focus point and the H-alpha filter's focus point and split the difference. I set the focuser there in the middle of the two, and then moved the camera in and out until the star was as small as it would get. Now, since we're so far off-axis and this is a Schmidt-Cassegrain, the star shapes are pretttttyyyy yucky, so "in focus" is hard to determine. But I got it about as small as it would appear in the camera, and called it good.
Unfortunately, that focus point has the Lodestar just barely inside the tube! I put on a C-mount extension tube, but it has a lip on it that won't let me insert it far enough to get the camera in focus. So I need to hunt down an extension tube that doesn't have a lip. We'll see if I can find one.