(IAAC) Re: (NSAAC) light-pollution filters
>Has anybody had any experience with light-pollution filters? ... I'm
>wondering if they'd be useful for our light-polluted northern MA skies.
Bob, I use 3 filters regularly - a Lumicon UHC, Lumicon OIII and Lumicon DeepSky
in that order. The UHC roughly corresponds to Orion's UltraBlock, while the
DeepSky is similar to Orion's SkyGlow. Orion has no equivalent to the OIII, I
don't think - that is an extremely narrow-band filter. If I had to choose ONLY
one of these, I'd pick the one I use most frequently - the UHC.
This seems like a lot, but if I had a Hydrogen Beta filter, I'd probably use
that frequently too! Don't accept conventional wisdom on the appropriateness of
particular filters for particular objects: until you've actually viewed a given
object under any given conditions at any given magnification, you can never be
sure which of these "standard" filters may improve your view of that object and
which won't! This is ESPECIALLY true of emission-line objects - which includes
planetary nebulae, supernova remnants and HII regions, of course, but may also
include emission features in galaxies or in other compound-type objects.
Just to illustrate this, I and two friends were out observing planetary nebulae
and other objects with a 17" f/4.5 dob last Saturday night, in spite of a bright
waxing gibbous moon being in the sky all evening. We'd focused on the brightest
objects because of the moonlight, including planetary nebulae like the Cat's Eye
NGC 6543 in Draco and NGC 6210 in Hercules. We were interested to find that the
DeepSky actually brought out some of the internal structure in these objects,
like the bright knot on the West edge of the Cat's eye, better than either the
UHC, OIII or no filter at all did! I'd never noticed the DeepSky doing much over
no filter for either of these objects, so we had to attribute this effect to
moonlight - but why these details were MORE visible with the DeepSky than with a
UHC, must be one more example of the strange effects of contrast...
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