Hi, I have been doing autofocus runs with my mod. Canon 550D and Robofocus (on an FSQ) without any problem (I usually get quite good V shaped curves). The settings I use are 2 sec. @ 1600 ISO. I have recently bought an EOS clip-on H-Alpha 12 nm filter. I have tried many settings (from 2 sec. to 10 sec. exposures and ISO 1600 to 6400; I also use darks) but continually get zig-zag curves (the HFR values are very inconsistent). I eventually have to focus “visually” (take one > check frame visually > change focus position…etc.) but even so, often it is difficult to decide what the best focus position is. I am aware of the difficulties H-Alpha filters create with DSLRs (Bayer matrix issues) but is there a recommended method/technique for autofocusing a DSLR with an H-Alpha filter?
Thanks for your help.