I've had my Celestron 6SE since 2011 and it was never really that great besides occasional visual use. I had tried taking photos with it a few times with a canon DSLR, but the quality of the images and star shapes was always pretty terrible(even before I knew the technical details of what actually looked wrong about the images I was taking). Last year when I started trying to take astrophotography more seriously, I put in some solid effort to try and get a good photo or two out of it. I used it to learn how to shoot/stack/process image sets with the usual free programs available, but when I hit a point where I couldn't seem to get any more improvements out of it and the results weren't anything I was all that happy with, I just assumed that its performance wasn't ever going to be all that good as an entry-level go-to scope. Once I decided to purchase the GT81 for imaging, the 6SE got set aside for the better part of a year while I got familiar with all the new bells and whistles on that, but I kept thinking that the 6SE couldn't be that bad, like... I have to be able to get some better results out of it.

With the last couple months of non-stop rain, I decided I'd try to upgrade/improve anything I could before I attempted another actual target with it. I had a hunch the star shapes were caused by a corrector plate or secondary mirror misalignment, but I couldn't recall ever disassembling it and throwing things out of alignment in the past, but I decided it couldn't get any worse. I marked the current position of things and then disassembled the scope entirely to first apply Edmund Optics black optical flocking sheet to every reflective surface that I could inside the scope tube. I also used matte black acrylic paint mixed with graphite powder and thinned with acetone, which I applied with an airbrush to select surfaces inside as well to cut out every possible point of internal reflection. This worked extremely well for spraying down through the baffle tube to cut out that reflection source. I also brushed it onto the edges of both mirrors with the extra left over. Looking into the scope after all of the flocking is like looking into a black void now. I do see two more potential spots for reflections which I've circled in red in the photo below. The one right against the primary mirror will be difficult to darken with it being in such close proximity to the mirror, and the other is the secondary mirror retaining ring at the front of the scope.

After reassembly I painstakingly manually rotated, recollimated, and took a test image with the corrector plate in each 30 degree increment position while counter-rotating the secondary to keep it in the same position, and then fed those images into astap's aberration inspector to see if I could identify any signs of improvements. After a couple rotation positions I could see that the star 'tails'/triangles were moving around, shortening and lengthening, and having different effects in different quadrants of the starfield. I could also see that the lines of elongation in astap were slowly moving towards a circular/concentric shape rather than being skewed all over in random directions and I just assumed that was a good thing lol. After going through that process and finding the position that minimized their tail shape, I marked that position and moved onto the exact same process with the secondary mirror. I did have a photo from years ago where I could see the serial number sticker on the secondary so I knew the approximate angle it should be in and that didn't take very long at all.

After going through that whole process I was able to actually get ~90% round stars, and I could finally see the near/far star-shape-backfocus-pattern that you'd expect to see when trying to adjust backfocus which I was previously unable to use to dial things in. At that point I grabbed some extension tubes/ring adapters to thread the celestron T-adapter into the extension tubes at any position so I could find out what the best backfocus was from scratch(I ended up at 130.25mm iirc). I managed to get things looking really good compared to when I started, but I had also heard that the 0.63x celestron reducer wasn't really the best thing available, so on a whim I machined a custom thread ring adapter to adapt my william optics 6AIII to the 2" SCT threads on the 6SE which improved things even more. This had the added bonus of allowing me to adjust the backfocus, use the camera rotator from the GT81, and I can now simply unscrew the entire imaging train and swap it back and forth between the GT81 and the 6SE. The 6AIII plus the length of the rotator is much longer than the celestron t-adapter, so I'm well outside of the intended backfocus distance and my effective focal length without any reduction factor would probably end up being somewhere around ~1900mm now, but the 0.8x gives me a pixel scale of about 0.51"/px that places it right back down at about 1500mm~ but with much better stars than without it installed.

In order to balance this massive change in weight from the heavy 6AIII, the original dovetail would have had to be gripped by about ~25mm of remaining material, so I also machined my own extended losmandy-style dovetail plate to give the mount something to actually grab onto.

This was the first image I was able to process once I was "done" with all of my tweaking and modification, and I actually enjoy using the 6SE now after seeing how well it can actually perform. Hopefully this image of M51 along with the context of the process involved was interesting or informative to anyone else with a C6/6SE who is trying to get a decent shot out of it.