I normally use Atik Horizon on Hyperstar or ASI294mc at f/6.3. But I have two Celestron x6.3 focal reducers that I have yet to try in tandem due to incessant cloud ever since purchase of the second (sorry if my purchase has affected others).
Has anybody worked out the best optical train for dual stacked FRs? What back focus from the second focal reducer to camera sensor is ideal (e.g. without diagonal)? I have a T-adapter, Baader Varilock, and assorted spacers so I can experiment, but if somebody already has the accurate number, I would be grateful.
My enquiry embraces 2 x 0.63 Celestron reducers. If others can append data for other combinations of stacked FR then this thread might become a useful resource.
BTW, I am aware that increased vignetting is likely with larger camera sensors, so let's keep this thread focussed on backfocus required. As an EAA observer rather than an EAA photographer that doesn't trouble me.