I recently got a QSI683 with Astrodon filter set (narrowband + LRGB). From my understanding, whether or not they are truly parfocal depends on the optical train. In testing with my 11" Edge SCT + focal reducer, they seem to be parfocal. I used 1 minute exposures on Ha for AF, and the difference in focuser position was negligible.
Just curious what others have experienced. Also, is the best way to determine offsets simply to run AF with high exposure times on the narrowband filters to determine offsets?