Hi Andy,
It looks like your optimisation is paying dividends. My limit at H alpha for verifying faint Be stars was around Vmag 10 with the LHIRES 2400 and C11, though these were much bluer stars of course.
Steve,
I think mag13 should be doable with your setup too. I’ve not tested the limit of my ALPY in 600 form but for example doing the same back of envelope calculation from the other end with my modified ALPY 200, I can get to mag 17, where sky background becomes an issue. The resolution of the ALPY 600 is ~4x higher and your aperture area is ~1/3 of mine giving a 12x factor overall or ~2-3 mags
Cheers
Robin