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



Thomas- thank you for the excellent explanation. I acquired my DSF back
in 1992 when I started observing and expected a silver bullet solution.
Naturally I was initially disappointed and it took me some time to learn
the subtilty of this excellent filter. Although Lumicon touts it for
photography mainly it does as you say. Notwithstanding Lumicon's caveat
I find it useful for galaxies and now I know why. What is interesting is
that in a very dark sky-- in the ABSENCE of light pollution, it provides
(I think) even greater contrast gains. This summer in a very dark sky I
was looking at M33 in Triangulum with my 10" f/6 with a 35mm Panoptic
with and without the filter. Without the filter granulation and star
clouds were visble but no structure. With the filter the characteristic
spiral form visible on photographs was pretty clear. By that I mean, the
relatively inexperience guy I was observing with said "wow-- it looks
sort of like a backward 'S'" This is not a case of averted imagination,
it was a pretty easy observation at the time, and the filter provided
the breakthrough contrast. PJT
Thomas O'Hara, Ph.D. wrote:
> 
> --- 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,
> 
> Tom
> 
> 
> =====
> Thomas O'Hara, PhD
> San Diego, California, USA
> thomasohara@yahoo.com
> 
> "Blessed are they who expect no gratitude;
> For They shall not be disappointed."
> 
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Philip J. Tramdack
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Roscoe L. West Library
The College of New Jersey
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