Let me start by saying that I have spent about 2 hours scouring forums looking for the answer before posting, and didn't find anything definitive. Perhaps that is a hint that one doesn't exist...
So anyway, I have a Stellarvue SV080T-25FT, which was Stellarvue's best 80mm triplet before the SVX series took over. It has a strehl rating of .98 according to the report given with the scope. I purchased it about 2 years ago, when I knew practically nothing about imaging. It came with the SFFR-80 .8x reducer, and the performance of that reducer is shockingly poor. I tested it last night with the 4/3 sized sensor in the ASI1600MM. The recommended spacing for that reducer/flattener is 55mm from the t2 interface on the back. Since I am using 3mm thick astrodon filters, I hear I am supposed to add an additional 1mm, bringing it to 56mm of spacing, which is what I went with.
The corner stars were absolutely horrible, as were the left and right sides. The stars in the top center were a little better than the bottom center, which suggests there is also a small tilt issue. All connections were threaded, no wobbling, so I may need to adjust the focuser slightly. I can try fussing around with the back spacing a little bit, but I am doubting it will help much with how bad the stars looked. My imaging scale was about 2 arcseconds per pixel, guiding was between .5 and .6 total RMS error, with RA and DEC error being very similar. I ran a drift alignment to .5 arc min error on each, and I was shooting only 30 second subs, so I don't want to discuss tracking issues.
I noticed that when Stellarvue put out the SVX series, they had a new dedicated reducer for their SVX80, claiming the old one wasn't all that great (no kidding). But NOW when I go to the website, they now mention "Please Note: We no longer offer a reducer-flattener for this system. This is the sharpest visual 80 mm f-6 refractor and when used with the included SFF4 photographic field flattener it is an outstanding, full frame 480 mm f-6 astrograph." This is on the 3 inch focuser version.... What gives Stellarvue?
In the past I have tried a TRF-2008 which had seagull stars in the sides/corners, the orion .8x reducer (rebrand of long perng I believe, used by astrotech, et al) which gave elongated stars in the corners, but surprisingly better than the TRF-2008. Does ANYONE make a decent reducer/flattener for an 80mm f/6 scope that is worth while? I am looking for one that will give excellent performance for at least a 28mm circle to fully cover APS-C sized sensors. One that does 42mm would be even better, but I won't hold my breath.
Edited by mistateo, 14 February 2020 - 06:08 PM.