I guess the other aspect of all this is the discussion of filters and conditions. My filter passes Ha, Hb and OII, so it's fairly broad - possibly making the detection of the horsehead more difficult compared to more specialised H-beta ONLY filters.
And even among Hb filters some are broader than others, with different peak transmission data. Am I correct in thinking that Lumicon is the go-to these days?
I've seen the horsehead through my 16" using the above-mentioned UHC filter, and in the same night, completely failed to see it through a 40" telescope (unfiltered) most likely because the sky had moved (as it does) and the horsehead was swimming in horizon muck.
I can imagine that it may well be in reach of a 12" instrument under good conditions , especially near meridian-crossing, with a quality dedicated H-beta filter.
Thanks Don. Now I have gear-lust again
Markus