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

--- Lew Gramer <dedalus@latrade.com> wrote:
>...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...
Hi Lew,
The DeepSky filter is a complement to the UHC and O-III filters, in the
sense that it samples the opposite parts of the visible-light spectrum.
The UHC and O-III are more or less narrowly centered on the O-III pair
of lines in the green part of the spectrum. But since that is also the
(very general) neighborhood where mercury vapor lights pollute, the
DeepSky filter blocks those out. It is basically a magenta filter,
which has TWO passbands, one in the red and one in the blue; while it
blocks out the yellow-green in between [For this reason, it also gives
very nice views of Mars and Jupiter].
So the Deepsky filter works on galaxies by passing the blue light of OB
associations in galactic arms, and the red H-II regions. It relatively
weakens the yellowish main-sequence stars of galactic disks. As far as
your experience with planetary nebulae is concerned, most conventional
planetaries emit most of their visible light in the O-III and N-II
bands, in the green; but not all. A few, so-called 'low-excitation'
nebulae, emit heavily at the red end of the spectrum, and are thus
enhanced by the use of the Deepsky filter. Off the top of my head, I
recall that NGC 40 in Camelopardalus is one such; Campbell's Hydrogen
Star in Cygnus is another. I've got a list I've slowly compiled
somewhere... . 
These 'low-excitation' nebulae appear red in color photographs, rather
than the more usual blue-green; but I would not therefore assume the
converse, that all red planetaries qualify as 'low excitation' nebbies.
It is these 'low excitation' nebulae whose appearance is enhanced by
the Lumicon H-Beta 'Horsehead Nebula' filter.
Dark skies,
Thomas O'Hara, PhD
San Diego, California, USA
"Blessed are they who expect no gratitude;
For They shall not be disappointed."
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