I have an astro-physics focal reducer (telecompressor) which is a nominal x0.67 with some latitude for adjustment by varying the spacing. I’ve been working through the dimensions in my set-up according to the instructions here:
and you might find that interesting.
I also noted that François mentioned a range from F/4 to F/5 for the Alpy, but the manual says F/5. I wonder what it actually is ?