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Hi Phil,
Congrats and well done mate. I assume the Astronomik filter is of the "UHC" variety -- ie a narrowband filter that permits both the doubly ionised Oxygen lines plus the H-Beta line. If so, it is a quite suitable filter for the purpose as most of the Horsehead Nebula (IC 434 & Barnard 33) is in the H-Beta line.
The smallest scope I've seen it in is 25cm and I've good reason to believe that in _perfect_ (and I really mean perfect) conditions 15cm will do the trick -- barely.
I always said I'd never buy a H-Beta filter because it has so few applications, but I took the plunge about 18mths ago. Let me tell you the improvemennt it makes on the Horsehead is huge.
In 31cm it goes from being a marginal threshold object to merely "difficult" using the H-Beta, and in 46cm, it is _bleedingly obvious_ direct vision -- no problemo with the filter. A UHC helps a lot, but the H-Beta is like flicking a light-switch on. My favorite view is with 20mm T2 Nagler at x111 with a 4mm exit-pupil and H-Beta.
Again, congrats and well done.
Best,,
Les D
Contributing Editor
AS&T
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