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Reducers often don't give round stars to the corners.
I originally used a Tak .6X reducer meant for the FS102. It worked up to APSc sensors but not full frame.
The larger 4 inch Tal reducer and flattener worked better. The Tak flattener worked flawlessly and it worked on other scopes besides my FS152. It worked on a TEC180 as well. The reducer though worked reasonably but the corners with a 16803 sensor (36mm square) were a tad weak close to the corners.
Dedicated TEC flatteners worked flawlessly though.
The AP telecompressor for the AP155 that was also meant for the AP140 worked well but it has a tolerance of +/- 1mm. It has a decent backfocus though so you could use it with the usual image train of camera, filter wheel and OAG.
I have read the specs on most of these Teleskop Services Kunming based scopes and the backfocus for their reducers and flatteners including the Riccardi can be quite tight like 55mm in some scopes. 75mm would be one of the longer backfocus distances and TS usually specify the backfocus with different focusers, it varies.
TS also sells a tilt adapter so you probably need that as the first image shows tilt more than uncorrected coma.
The 2nd image I agree is a mess and does not bode well. I think TS sells a variable length adapter tube which may be the go to test various distances.
Greg.
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