This reminds me of the fun and informative article written by Barbara Wilson, "Magic Horse Head Eyepiece":
http://home.ix.netcom.com/~bwilson2/barbarasweb/MEyepiece.htmFor best contrast with visual observing according to Barbara, the key is to use an eyepiece with a H-beta filter that will produce an exit pupil of approximately 5 mm.
Eyepiece (mm) = exit pupil (mm) x f-ratio
Therefore, to get an exit pupil of 5 mm with a f/5 telescope, you will need a 25 mm eyepiece.
Exit pupil specifications for different filters (and their uses) are specified on Lumicon's website:
http://www.lumicon.com/store/pg/15-LUMICON-Nebula-Filters.aspxHere's a nice overview of the Horsehead Nebula region with observing hints:
http://www.nightsky.at/Photo/Neb/B33_WN.htmlI don't have a H-beta filter, but I do have a Baader UHC-S filter that band passes H-alpha, H-beta, and O-III, which I will try next time to observe at the Dark Site. Hopefully, that will also work with the formula above...