Triangular stars on WO FLT 91 with 0.79x reducer.

Don't you hate that!

Could be tilt or collimation, which can be annoying to correct. But I suspect it can be fixed by slightly changing your back focus. I had similar artifacts in one corner that were corrected completely by moving the camera a bit closer to the scope body. That is made easy with the FL6AIII by just screwing it in or out a bit and checking. This was with the 2600, which is the APSC version of your camera.

The specs don't take into an account the broad range of variability in everyone's setup. So it's not unusual nor wrong to try shortening or lengthening the flattener a tad and see what happens.

I did just that one night. My scope is the GT71. Disconnected the camera and turned the flattener/camera assembly one turn, reconnected the camera, and took a shot. Doesn't take long to see results, better or worse, and then fine tune.

Try this before the other steps. Refractors and cameras are both closely calibrated at the factory and don't shift easily. It's not a great idea to change those settings until you know you don't have a choice.

Why one corner? Could in fact be a bit of tilt. But it also could be simply that the camera sensor isn't perfectly centered in the optical train and you are seeing the beginning of quality drop-off that occurs in all corners, but visible in the one your camera is closer to. Especially with the 6200, which is trying to deliver the scope's entire FOV.

Good luck! I'd enjoy hearing how the scope works out. It looks like a nice instrument.

I have a Pleiades 68, and it is very sensitive to back focus. The Askar back focus adjuster is nice if you have space for it. It allows you to very precisely adjust the BF without having to take the image train apart to add or remove spacers. It works great on my scope.

Here is a JPEG of the unprocessed, but stacked image. it is pretty evenly distributed that the stars are triangle shaped. If there was tilt i would expect problems on one corner more than the other.  There clearly is a back focus issue but I was not clear on the triangle shape. 

They are not "triangular", it is astigmatic coma with astigmatic field curvature at the extreme corners with, potentially, poor tracking (witness the elongated stars at the center of the frame). All it points at issues with either position of the corrector/reducer and/or the capacity of said corrector/reducer to do its job. And obviously there is tilt, as the rotation of the astigmatic lines imply, but to a much lower degree that the astigmatic field curvature.