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Matt,
Congrats on your first view of the Horsehead!
With your 16" & under Bortel 3/4 skies, the Horsehead should be visible without needing a filter. The Horsehead's reputation as being a difficult object is way over rated as being able to see it is far more a measure of the quality of transparency and the lack of people being able to recognise good transparency vs everything else. That it can be seen in a 4" aperture speaks volumes to this being the case.
From my Club's Bortle 4 site & my 17.5" dob, I have had nights when seeing the dark pillar was very easy with no filter & with amazing detail with a filter. I also had nights when it took all my experience & using filters to make out a featureless blob. And i also had nights when it remained totally invisible no matter what I did. Same site, same scope, different qualities of transparency.
What is surprising about this dark pillar is how large it really is. Far larger than the dark pillars just below Trumpler 14 in Eta Carina, but of course far less stark.
Being able to see the Flame neb is not good enough to determine if transparency is good enough - it is way too bright by comparison. Instead I use the Horsehead itself to tell me exactly how good transparency is. I use the Flame as a landmark, sure, but not much else. And I use the Horsehead's visibility from then on to sort out my final set of targets exactly because I now know just how good transparency really is. Not much use trying to spy out angle's-breath galaxy arms when conditions are not up to it. And it isn't just the Horsehead that I use to determine transparency, but a swathe of celestial features both telescopic & naked eye. No matter how dark the sky may be, if transparency isn't up to scratch you will only be setting yourself up for a night of frustration if you don't recognise this most fundamental aspect of amateur astronomy. And no bit of kit will tell you this either. Your own eyes are best.
The take from all of this is to encourage you to come to recognise transparency conditions. Work out which nights are best for the really faint stuff and nights for objects that are better suited to less than perfect. Chase down the Horsehead more times & learn what the sky is telling you by then looking away from the eyepiece & up at the sky itself and what you can now see of naked eye DSO's. You will be surprised at the differences once you recognise good over everything else.
Alex.
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