MSRP:
$75 (1.25”); $90 (2”)
by
David Knisely
There
has been considerable controversy concerning the effectiveness of
various broad-band “Light Pollution Reduction” filters
ever since the first decent ones appeared in the early 1980's. Some
observers contend that these filters offer mild but definite
improvement in the views of various deep-sky objects, while others
say that the broad-bands offer nearly no improvement at all and are
not really worth the money. I am somewhat in the former camp, as I
have seen some benefit from the broad-bands, so when I started
upgrading to 2-inch format eyepieces, I decided I needed a new
filter. I purchased the Orion SKYGLOW LPR filter in hopes that it
would perform at least as well as my old 1.25” Lumicon Deep-sky
filter. Overall, the Skyglow has shown itself to be a proper
broad-band unit with characteristics similar to some of the other
broad-band filters.
What
exactly *is* a "broad-band" filter?
Broad-band
"Light-pollution Reduction” (LPR) filters are designed to
help improve the visibility of a variety of deep-sky objects by
blocking out the common Mercury vapor, Sodium, and some other
emission lines from man-made and natural (airglow) sources which
contribute to skyglow, while passing as much of the other wavelengths
as possible. Common LPR designs tends to have a broad primary
passband located somewhere between about 4500 to 5300 Angstroms, as
well as a broad “notch” of low transmission, usually
somewhere between about 5300 Angstroms in the yellow to around 6300
Angstroms in the red part of the spectrum. The shapes of the
passbands vary somewhat, with some designs having multiple passbands,
smoothly sloping mild passbands, or sharper narrower passbands that
are halfway between the "standard" LPR design and the true
narrow-band "nebula" filters. With emission nebulae, most
of the light from their narrow spectral lines (Oxygen III and
Hydrogen-beta) still gets through, resulting in somewhat more
contrast than without a filter. However, for objects which emit light
over a broader range of wavelengths
("continuum
emitters"), some of the light can get excluded in an attempt to
help reduce the effects of some of light-pollution sources. This can
sometimes result in a slight dimming of some objects, which is one
reason a few amateurs may feel that these broad-band filters aren't
worth much visually. Still, the sky background is generally darkened
and in some cases, a subtle increase in contrast has been reported
with at least some objects like clusters and some galaxies. Also,
astrophotographers often use broad-band filters to help reduce the
effects of light pollution on wider-field images of the sky.
The
Orion SKYGLOW:
The filter comes in two standard models for 2” and 1.25”
barrel eyepieces. The review unit was a 2” model about 5 cm
(2”) across and 1 cm thick including the threads. The filter
easily threaded fully on to both of my 2” eyepieces. However,
it would only go on about half a turn on my Williams star diagonal's
threaded 2”-1.25” adapter and somewhat less than half a
turn on the front of the diagonal itself and on the front of my Tele
Vue star diagonal. This was a little disappointing, although even at
half a turn, the filter was securely mounted and in no danger of
coming off. Shaking the filter gently resulted in a faint rattle
caused by a slightly loose retaining ring. It took only a gentle
touch with an improvised spanner to tighten the ring and eliminate
the rattle, although it wasn't exactly terribly loose to begin with.
The filter came in a small round 2” filter case with a small
foam edge insert to hold the threaded portion in-place and prevent
the filter from sliding around in its case (a nice touch). Tilting
the filter produced the usual varying color hues common to most
interference filters. The reflected color of light off the filter's
surfaces was more neutral than the pale gold reflections off my old
Lumicon Deep-sky filter.
Specifications:
One nice thing about the included materials with the Orion SKYGLOW
filter was the spectrophotometer tracing of the filter's transmission
curve from about 4000 angstroms (400 nm) to 7000 angstroms (700 nm).
The Skyglow's primary passband begins around 4550 angstroms, rising
rapidly to a transmission of over 97% in the 4750 to 5100 angstrom
range (over 85% transmission from 4700 to 5210 angstroms as well).
The transmission passband fell rapidly with increasing wavelength to
near-zero transmission around 5500 angstroms (almost identical to the
same wavelength as the Lumicon Deep-sky filter's yellow cutoff). The
Skyglow's primary passband had a Full-Width at Half-Maximum of 640
angstroms, which is quite comparable to the Lumicon Deep-sky's FWHM
figure of 715 angstroms. The Deep-sky's high transmission portion of
its curve was a little bumpy, generally between 83% and 94% maximum
transmission with irregularity in transmission towards the blue wing
(managed an 85% or greater transmission from 4800 to 5250 angstroms).
The Skyglow, like the Lumicon Deep-sky, also has a red secondary
passband that begins around 6000 angstroms and rises to 50% at around
6340 angstroms and then to 97% at the H-alpha line (6563 angstroms).
The Lumicon's red passband starts around 6200 angstroms and rises in
a similar manner to 50% at 6450 angstroms and up to 97% at H-alpha.
Thus, the Deep-sky excludes a little more red light than the Orion
Skyglow does, although with the lack of visual sensitivity at those
wavelengths, the difference in performance to the eye is likely to be
small. Here are the basic figures, although they show more the
similarities of the two filters rather than significant differences:
FWHM:
SKYGLOW: 640 angstroms Deep-Sky: 715 angstroms
FW@85%:
SKYGLOW: 510 angstroms, Deep-Sky: 460 angstroms
Max.
Primary Passband Transmission:
SKYGLOW: 98.5%, Deep-sky: 94%
Transmission
at H-alpha:
SKYGLOW: 97%, Deep-sky: 97%
Observations:
I tested the Orion SKYGLOW filter in my hand-held spectroscope, in a
camera pointed at surface night lighting, and visually in my 100mm
f/6 refractor and 9.25 inch f/10 SCT at fairly low powers. In the
spectroscope, the primary passband of the Skyglow looked slightly
brighter and more uniform than my old 1986 vintage Deep-sky's did.
Both filters did fairly well against the major Mercury Vapor lines
from local lighting, although my interior Mercury fluorescent lights
did get through both filters with a band in the blue-green part of
the spectrum. Indeed, the violet “G” emission line at
4358 angstroms got nicely blocked by the Skyglow, but was still
faintly present in the Lumicon Deep-sky. In camera images of a
street lit by a Mercury vapor light, the Skyglow reduced the light
nicely, with the glare reduced to a pale glow on the street below.
The Skyglow also gave a more color-neutral and pleasing view with
slightly higher light throughput than with the Deep-sky filter. The
Deep-Sky also attenuated the Mercury glow on the ground as well, but
gave it a somewhat more violet cast.
On
High-pressure Sodium, both filters were a little less effective than
with the Mercury vapor lighting, but they still provided very
noticeable rejection of the wide orangish glow that gives HP Sodium
lighting its characteristic color. The Skyglow had a slightly wider
and stronger red secondary passband, so its rejection of the entire
primary HP Sodium vapor band was just a tad less aggressive than that
of the Lumicon Deep-sky, although the Skyglow still cut deeply into
it. In camera images of the scenes lit by HP Sodium street lights,
the Skyglow noticeably cut down on that yellowish-orange glow over
non-filtered use, although the ground had a more magenta hue than
that seen with the Deep-sky filter. Indeed, the impression was that
the overall image was a tad brighter in the Skyglow than in the
Deep-sky filter, and that impression continued when visual
observations of objects using the filters were conducted.
Visually
with the telescopes, the Orion SKYGLOW performed much like my old
Lumicon Deep-sky, providing a somewhat darker sky background and a
slight to moderate gain in contrast on various deep-sky objects.
Star images were fairly sharp across the field, although they did
show the usual faint red halo when slightly defocused (caused by the
multiple passbands). The broad-band filters performed somewhat
better under dark sky or mild light pollution conditions than they
did under moderate light pollution. On emission nebulae, the gain
was somewhat more than was seen with continuum objects, although I
still liked the use of a true narrow-band filter for nebulae more
than the Skyglow. M57 under moonlight was easier to notice at only
24x in my 100mm f/6 when the filters were used, as was M27. In
comparison, I did like the view of both nebulae just a tad more in
the Skyglow filter than I did with the Lumicon Deep-sky filter. The
results with M42 were quite similar with again a very slight nod to
the Skyglow. Indeed, with competing moonlight, I saw a few more
stars in the small cluster Dolidze 5 when using the Skyglow than I
saw with the Deep-sky filter. Without a moon, I was particularly
impressed with seeing M33 jump out a bit with hints of one of its
arms in my 100mm refractor when the Skyglow was used from my
driveway, despite the in-town conditions (ZLM 5.6). Indeed, the two
main dust lanes of M31 were noticeably enhanced when I used the
Skyglow in my 9.25 inch SCT, although they were visible without the
filter as well. All in all, the Skyglow filter was doing about what
I expected a decent broad-band filter to do.
Summary
Remarks:
Overall, I did like the Orion SKYGLOW broad-band LPR filter, as it
not only nicely filled my need for a 2” model, but was as good
or maybe just a hair better than my 1.25” Lumicon Deep-sky
broad-band. I would have to give the SKYGLOW the following letter
grades:
Construction:
B (loose retaining ring, slight thread miss-match).
Optical
Quality:
A (no visible defects in the filter and uniform coatings).
Performance:
B+ (as good as or maybe slightly better than the Lumicon Deep-sky).
Cost/Benefit
Ratio:
B (well, that goes for most broad-band filters).
Overall
Rating:
B+
Conclusion:
the Orion
SKYGLOW LPR
filter is a reasonably effective broad-band filter for helping
enhance the views of some deep-sky objects. It offers performance
similar to that of other decent broad-band filters like the Lumicon
Deep-sky, and if one knows what to expect, can be beneficial to
amateurs who may be looking for just a little more contrast.
David
Knisely
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