It is kind of funny. Amateurs say you can double stack two F/.63 reducers atop one another to achieve F/3.7 Focal ratio. I tried that a few times, it produces a lot of vignetting on the outer 1/4 of the FOV. I only use that when I want magnified images at F/3.7 with my Nexstar C8. I also can get F/3.78 with my AT60ED, native F/6, using one Meade F/.63 focal reducer on my little APO refractor and it produces a nice 4.4 degree x 3.84 degree FOV.
Not sure what other folks tolerance for vignetting is or what F ratio they were achieving but I do recall a few folks being very happy with their double stacked .63 reducers. I never tried it because my configuration @ F/3.7 worked so well without any appreciable vignetting .
Edited by neaptide, 28 January 2019 - 12:37 PM.