The DGM Optics Galaxy Contrast Enhancement (GCE) Filter
MSRP: $80
Broadband "Light Pollution"
filters have often been the subject of some discussion or even heated debate
among amateur astronomers when it comes to their effectiveness. This is particularly the case for their
use on continuum emitters like galaxies.
Some amateurs report no enhancement at all, while others report mild
contrast increases on at least some kinds of galaxies in certain aperture and
power ranges. I tend to be in the
later camp, although most of the time, I tend not to use filters at all on
galaxies. With this in mind, I
have recently had an extended look at a new and perhaps better
"galaxy" filter: the
DGM Optics GCE filter.
What exactly *is* a
"broadband" filter?
Well, broad-band "Light-pollution
Reduction (LPR) filters are designed to 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 or natural sources which contribute to
skyglow, while passing as much of the other wavelengths as possible. The "standard" design tends
to have a broad primary passband located somewhere between about 4500 to 5300
Angstroms, as well as a wide area of low transmission, usually somewhere
between about 5100 Angstroms in the yellow to around 6400 Angstroms in the red
part of the spectrum. A few pick
up in the far red wavelengths (mainly for H-alpha coverage), but usually, the
eye's sensitivity is rather poor there, so generally, far red coverage isn't a
huge factor for performance. The
exact passband shapes vary widely, with some having multiple passbands,
smoothly sloping mild passbands, or sharper narrower passbands that are halfway
between the "standard" LPR design and the true narrowband
"nebula" filters.
For things like emission nebulae, much of their light still gets
through, resulting in somewhat more contrast than without a filter. However, for objects which emit light
over a broad range of wavelengths ("continuum emitters"), some of the
light from these objects is being thrown out in an attempt to help exclude some
of the sources of skyglow. This
can result in a dimming of the target object, which is why some amateurs do not
like using these broadband filters.
Still, in some cases, a subtle increase in contrast can be seen with at
least some galaxies at some magnifications with some filters. Whether it is enough to justify their
cost is a valid question, as the amount of boost in the contrast reported is
often rather small.
PRODUCT CHARACTERISTICS:
The
DGM Optics GCE filter is definitely in the broadband filter
classification. The reviewed unit
was a simple 1.25-inch filter which threaded well into all of my 1.25 inch
barrel eyepieces and my Lumicon Multi-filter Selector with no trouble. The filter showed no obvious surface defects, and appears
to be properly protected (it is a "first surface" hard oxide thin
film filter). For comparison, I
used my Lumicon Deep-sky filter, a sort of "gold standard" when it
comes to broad-band LPR filters. I
did hold both filters up to my simple visual spectroscope and immediately
noticed some differences between the two filters. The Deep-sky filter has a very sharp primary passband with
steep filter skirts and nearly zero transmission from about 5400 Angstroms to
around 6200 Angstroms, as well as from 4400 down to about 4100 Angstroms. This design completely kills off most
of the common light pollution emission lines with the exception of some of the
weaker secondary lines of High-pressure Sodium lighting. The main broad yellowish-orange
emission band of HP Sodium is fairly well dealt with however, resulting in a
noticeably darker sky background when used visually.
By contrast, the GCE filter shows
a slightly broader primary passband than the Lumicon Deep-sky, with somewhat
more sloping filter "skirts" (edges). The primary Light-pollution "notch" of the GCE
filter is also slightly narrower than that of the Deep-sky. This, along with the wider primary
passband, makes the overall light throughput of the GCE a bit higher than in
the Deep-sky, although in the primary passbands, the transmission of both
filters was still very high. The
transmission towards the middle of the GCE's main Light-pollution
"notch" is fairly low, but is not quite zero, as just a little light
could be seen in the primary notch mainly on either side of the notch's center
wavelength. The transmission picks
up again in the deep reddish-orange part of the spectrum and remains fairly
high all the way to the eye's visual red wavelength cutoff around 7000
Angstroms or so. The sky
background in the GCE was darker than without a filter but not quite as dark as
was seen in the Deep-sky filter, especially under suburban light
pollution. When held up to the eye
to view the area around streetlights, HP Sodium lights were brighter through
the GCE filter than through the Deep-sky, with the ground beneath the lights
showing the bright orangish hue of the light easily. In the Deep-sky filter, the ground was noticeably darker
with only a faint bluish-green glow being seen on the pavement. Similarly, Mercury vapor lighting was
also much more attenuated by the Deep-sky filter than by the GCE. Thus, while both filters do at least
attempt to deal with the light pollution lines, the Lumicon Deep-sky filter is
considerably more aggressive and somewhat more effective at blocking their
direct effects.
FIELD TESTING:
I
tested both filters in several observational sessions running from late fall
2006 to early spring of 2007. I
used the filters on a number of galaxies and a few other selected deep-sky
objects using my NexStar 9.25 inch Schmidt-Cassegrain (59x, 78x), and my 100mm
f/6 refractor (15x, 22x, mainly for M31 and M33). The filters were placed in the Lumicon Multi-filter selector
to get instant comparisons between the filters as well as between filtered and
non-filtered use. I observed
approximately 40 of the more prominent galaxies and three galaxy clusters under
dark sky conditions (ZLM 6.5 to 6.8), as well as a number of these same objects
viewed from my front yard under what I would term "mild" light
pollution (ZLM 5.4 to 5.8).
Previous experiments had shown that neither broadband filter was very
effective on galaxies when used under brighter in-town skies, so no other
observations under worse light pollution conditions were performed.
The results of this survey were
somewhat mixed. Under dark skies,
the DGM Optics GCE filter seemed to provide a somewhat better view than the
Deep-sky filter for many of the galaxies surveyed. Howver, at least a good
portion of the time, the differences in performance were so small that is was
difficult to judge which filter was better or whether *any* filter provided
much of a gain over non-filter use.
On at least a few objects, the GCE did indeed provide a noticeable boost
in the contrast over non-filter use which, while not as much as is seen when
nebula filters are used on emission nebulae, was still just a bit startling
when I first encountered it. My
initial target was M102 (NGC 5866 for those of "Messier purists" out
there), a nice nearly edge-on SA0 galaxy with a high surface brightness (S.B.
11.9). There, the GCE filter was
the clear winner, providing a somewhat darker sky background while not dimming
the galaxy much over non-filter use. The Lumicon Deep-sky filter also provided a gain in
contrast over non-filtered use, but the galaxy seemed just a bit fainter than
in the GCE filter. A similar
result occurred with nearby NGC 5907 which was better in the GCE filter than in
the Deep-sky. M101 was lower in
the sky, but still was high enough to test the filters. For it, the GCE seemed to be providing
just a bit more brightness than the Deep-sky filter, and the view was slightly
superior to non-filtered use. The
patchy spiral structure was marginal, but was a bit easier to see with the GCE
than without a filter. This
impression was repeated for quite a number of the objects viewed. The GCE often seemed to perform just a
bit better than the Deep-sky filter under dark sky conditions, mostly in terms
of providing a slightly higher object surface brightness. When looking at groups of galaxies,
smaller less-prominent ones tended to get noticed more immediately when the GCE
filter was first inserted, although when it was removed, I then was still able
to see them. In total, the GCE
filter was deemed effective on about 60% of the galaxies which were surveyed
under dark sky conditions.
The difference between the two
filters varied from object to object.
On some objects, the Deep-sky filter was at least as good as the GCE if
not slightly better, most notably on NGC 253, where the dark detail seemed to
show up about as well with the Deep-sky as in the GCE. On M31, the GCE filter seemed to have
only a very slight edge over the Deep-sky when used in my 100mm f/6 refractor,
but with M33, the Deep-sky provided just a hair more contrast, especially when
I used the larger NexStar. Also,
in a number of other cases, I actually preferred the view *without* a filter,
so the improvement in the view of all galaxies was not universal. Indeed, I had a slight preference
for non-filter use for both NGC 7331 and Stephan's Quintet. On M104, I also liked non-filter use
for the galaxy, as the object really requires moderate to high power for the
best view. Still, overall, the GCE
tended to have the edge in performance over the Deep-sky, at least when the sky
was nice and dark. One characteristic
that I had noticed with the Deep-sky also occurred with the GCE, namely the
effect of increased magnification.
In a number of cases, a similar apparent boost in the quality of the
view was seen when the power was increased somewhat in a non-filtered
scope. On M51, usually I have
trouble seeing the discreet spiral arm structure easily unless I get the 9.25
inch SCT up to something over 80x, yet with the GCE filter, the spiral form
kind of jumped out at me at only 59x.
I still preferred the view at higher power without a filter, but the
mild improvement in the view at lower magnification could not be ignored. Similarly, M100 shows its spiral arms
better with the GCE than without a filter, although again, increasing the power
also helped bring out the structure.
When used under the mild light
pollution at my home in-town, both filters lost some of their effectiveness,
although in many cases, the view was still at least slightly better than
without a filter. The Lumicon
Deep-Sky filter was consistently somewhat better at providing a darker sky background
and a bit more contrast than the GCE under the increased skyglow, although
again, the improvement in detail was often rather marginal. This was probably most notable on NGC
891 (see simulation below). I used
the NexStar's Go-To ability to slew to the galaxy but was not sure I was seeing
it anywhere in the field. Once the
filters were in-line, the galaxy became noticeable, although it still wasn't
exactly standing out like a sore thumb.
The Deep-sky filter provided the better view, as I could see the main
dark lane across the nuclear bulge a bit easier than when using the GCE
filter. The filters also lost some
of their effectiveness on galaxies when used at moderate powers (beyond 7x per
inch of aperture). This, along
with the difficulty in getting and staying properly dark-adapted in-town, may
explain why some people have reported no improvement when using filters on some
galaxies.
I also tried the GCE filter on
the globular cluster M13 from my dark sky site, and while the sky background
was definitely darker, I liked the view without the filter better than with
it. When used on some diffuse
nebulae under dark skies, the GCE did help a little, but I found that the
Deep-sky filter helped the contrast just a bit more. From in-town, the difference in performance was a bit
greater, with the Deep-sky filter still more effective on nebulae, although
nowhere near as effective as a true narrowband nebula filter. Thus, as a mere "Light
Pollution" filter, the Lumicon Deep-sky is probably a somewhat better choice
than the GCE filter is.
CONCLUSIONS:
The
DGM Optics GCE filter is at least somewhat effective on a number of galaxies
when viewed under dark sky conditions, and in many cases, is a bit more
effective than the Lumicon Deep-sky filter. The improvement is often subtle, requiring low power and
some study to see. However, under
even mild skyglow conditions, a
standard LPR filter like the Lumicon Deep-sky may be somewhat more effective
than the GCE, although again, any improvement in the view will be slight at
best.
David Knisely
