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The DGM Optics Galaxy Contrast Enhancement (GCE) Filter
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.
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.
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.
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.
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