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Re: (IAAC) Re: (NSAAC) light-pollution filters



Lew Gramer wrote:
> >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...
>
> Clear skies!
> Lew Gramer
>
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Lew,
The Lumicon O-III is a very narrowband filter.  In fact, it is known as a line
filter.  Orion does not sell a filter comparable to the O-III.
The H-Beta filter is useful on a relatively small number of deep-sky objects, most
notably the Horsehead, California, and Cocoon Nebulae, although the list is
growing.  An Orion UltraBlock or Lumicon UHC is the best choice overall if only one
LPR filter can be purchased.  Having said that, I probably use my O-III more than my
UltraBlock. <g>
David Knisely has been doing a survey on how effective the various LPR filters are
on DSO's.  An early version of his results can be found at
http://www.4w.com/pac/filters.htm.
It is worthwhile to do some personal empirical testing of the various filters to see
just how much enhancement, if any, occurs with various objects.
Some folks, including yours truly, also use LPR filters for planetary and double
star observing.
Dave Mitsky
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