Monday, September 04, 2006

 

Heavens to Murgatroid! A Galaxy Filter?

Yeah, ol' Snagglepuss shore would be surprised that somebody’s making this claim. I know I am. What claim? D&G optics is about to begin selling an LPR (light pollution reduction) filter designed for viewing galaxies. What’s wrong with that? Well, heretofore, we’d always considered a galaxy filter akin to a “cloud filter;” that is, something theoretically not possible.

LPR filters work by using multicoated surfaces to reflect away wavelengths of light produced by earthly sources before these harmful photons can enter the field lens of an eyepiece. Layered coatings of different reflective materials fine-tune a filter for whatever range of wavelengths you want to accept/reject. This obviously works very well for nebulae. Just about every modern amateur, even those with access to dark sites, owns an OIII or a UHC.

While LPR filters work great for emission and planetary nebulae, however, they suck righteously when you try to use 'em to observe galaxies. With a UHC or OIII attached to your eyepiece, not only does the galaxy of your fancy not look better, it’s likely to disappear. Unfortunately, the wavelengths of man-made light sources, all those thousands of mercury vapor, sodium, and incandescent lights, fall in the same range of wavelengths as the light of the stars.

The bottom line is that light pollution filters are very effective at blocking man-made light, but that also inevitably means they dim or even extinguish starlight. That’s why they are no good for observing galaxies, star clusters, and reflection nebulae. All three of these types of object shine either by direct or reflected starlight. Too bad: no galaxy filter for you!

That's what your Old Uncle has always said, anyhow.

Not that I've been unchallenged in my assertion that "there are no galaxy filters." Some amateurs have insisted there most certainly is a galaxy filter—of a sort. They are talking about mild, broadband LPRs like the Lumicon Deep Sky and the Orion Skyglow. These filters, the story goes, darken an eyepiece field background enough to increase contrast with a galaxy without dimming the target object so much that any improvement is lost. So they say. I’ve tried this “trick” more than once, and my opinion has always been that any galaxy is always worse “with” than “without.” The only benefit is that the filter makes HII regions show up fairly well without completely cutting off the main galaxy. But, then, HII regions are nebulae, aren’t they?

Then, the other day as I was hanging out on the Yahoogroups, I noticed a couple of posts about a filter called the “GCE”—that, I soon discovered, was an acronym for “Galaxy Contrast Enhancement.”

What the—was this a joke?

At first I thought so, but then I learned that these filters are made by a very well-respected outfit, D&G optical, who are no doubt familiar to you as the long-time producers of very high quality achromatic refractors and classical cassegrains. A bit of looking around turned up a website:

http://www.omegafiltersebuyer.com/servlet/the-GCE-Filters/Categories

The company that owns this page, “Omega Astro Filters,” is a new one on me, but they say they are a well-known supplier of optical components to the aerospace industry. So what do I know? Just because I haven’t heard of them doesn’t mean they are not top drawer. If they haven’t been selling to amateurs, it’s not likely that I would have heard of ‘em.

Anyhoo, if you don’t want to navigate to this page, I’ll tell you what it says. First of all, the blurb on the GCE, which is accompanied by the image above, is headlined “Coming Soon!” How soon? Don’t ask me. Beneath the header, it says “September 1, 2006.” Alas, it still said that the last time I looked, just now, on September 4. ‘Twas the Labor Day weekend, of course, so I’ll keep checking back to see if it is updated.

Whatever. What do they say about the GCE? Not too much; this is all of it:

The Galaxy Contrast Enhancement filter aids in the visual observation of galaxies and Milky Way dust clouds and dark lanes. The GCE filter takes a different approach to enhancing galaxy observation by allowing high transmission through nearly the entire visible spectrum while rejecting only the harmful light pollution wavelengths. Because of those attributes it also is a very good general purpose LPR filter, unlike traditional wideband filters (that) exclude most of the red portion of the visible spectrum.
How does the GCE do this? Something that no one seems to have been able to do before? Doesn’t say, and don’t ask me, Bubba. I can think of a couple of scenarios, I guess, but that would be pure speculation. I will admit that in the 20 years since LPR filters started becoming really popular, there is no doubt that the art and science of optical coatings has come a long, long way.

A little digging on my part turned up some more info in the form of a press release from D&G, which included both some words of wisdom on how D&G and Omega fit together, and further details about the filter itself:

As of 8/20/06 DGM Optics began a partnership with Omega Optical of Brattleboro VT. This partnership involves my LPR/Nebula filters which will be manufactured, and sold through Omega Optical. In addition to the NPB (Narrow Pass Band) and VHT (Very High Throughput) nebula filters we have 2 new filters which will be available within the next 1-2 weeks. First is an OIII and the second, and most interesting, is the GCE (Galaxy Contrast Enhancement).
OK…but how the H-E double L does this thing work?

The GCE filter takes a unique approach to galaxy observation by using a "rejection band" design. A rejection band design essentially allows all of the visible spectrum to transmit while blocking only undesirable wavelengths. The resulting filter has the most total throughput of any LPR filter I'm aware of, and as such, has a very neutral impact on star brightness and color.
Maybe some of ya’ll know what that means or implies; I sure don’t.

So, what else? The news item from DGM goes on to say that the GCE has been tested, and that it has delivered good results on M31, making the dust lanes “somewhat” more visible, and that M33 is “improved.” The release also notes that the filter works best with the Population II rather than the Population I regions of galaxies. What that means to you and me, Elmer, is that the cores of galaxies are more improved than their arms.

Sigh. Wouldn't you know it? It’s the arms that are the problem in light pollution. I can haul the C11 into my urban backard and easily see lots of galaxy cores. The release further puts on the brakes by saying, “However because galaxies (sic) spectral profiles are generally broadband in composition. do not expect them to jump out the way nebula do with the NPB and VHT filters.”

Where does this leave me/us? Well, bubbas and bubbettes, back at square one, I reckon. I went from knowing there was no such thing as a galaxy filter, to hoping someone had made a breakthrough, to believing said breakthrough may be minor in nature. On the other hand, according to the informantion from D&G, the GCE works well as a plain old nebula filter. The price of the thing is quite reasonable, 80 US$ for the 1.25-inch and 160 US$ for the 2-inch, in line with what you’ll pay for Lumincon’s or TeleVue’s mainstream LPRs. Frankly, if this filter offers even a little improvement over the Skyglow/Deep Sky “trick,” I’d consider that money well spent. Hail yeah.

Do I expect much from the GCE? Can't say as how I do. Howsomeever…I’m always one to warn against burying the patient before he’s pronounced dead. I intend to take a wait and see on the GCE filter until I’ve got one in my hands or hear from someone I trust who's got one in her/his hands.

Yeah, I’ll admit it: I want there to be a galaxy filter!

Comments:
Hi Rod,

Dan McShane here, DGM Optics. I enjoyed your article and just thought I could answer to some of your wonderings about the GCE filter.

Oh, BTW D&G Optical is not involved in these filters.

Anyway I guess first I should describe the GCE`s spectral characteristics. So here`s the verbal discription; A rejection band is a filter that features high transmission over an extended range (visible in this case) with a specific area of rejection that blocks out undesirable wavelengths. They can be designed with the desired rejection band width, and can be centered according to the needs of the application. So these filters have greater total light throughput than widebands, and ultimately brighter images.

To put a numeric value on the total light throughput difference between the GCE and a typical wideband consider that, expressed in nanometers, the visible region of the spectrum is 400 nanometers wide. A typical wideband is 60-90 nanometers in bandwidth. Some widebands designs have side bands, also known as "skirts", which contribute to the total light transmitted. So if we allow say another 50 nanometers for the wideband we would have a total of about 150 nanometers of transmitted light for the visible region`s 400 nanometers that we humans can perceive.

In comparison the only area where the GCE DOES NOT transmit is less than the total thoughput (150 nanometers) of the typical wideband. So you get the idea; there`s alot more light getting through with a rejection band design vs. a wideband.

Now this simple comparison doesn`t take into account that human visual acuity goes to hell in a hand basket fairly quickly when it gets away from our peak in visual intensity, around 550 nanometers, or that how much and where a filter transmits or blocks has a significant influence, but it does give you a good idea of the difference in the amount of light that gets to your eye with a wideband vs. a rejection band filter.

So are these things really a true galaxy filter? Well, not in the way that a nebula filter works for nebula, since galaxy light is broadband and has no specific lines of emission. So the GCE will not make galaxies "popout" at you like nebula filters will for nebula. But the "trick" they seem to be the best yet for galaxies to date, and a noticable improvement over widebands. Plus as already noted they aren`t a "one trick pony" filter either.

Also I can report that as of this week we will have a small inventory of 1.25" GCEs with 2" filters to come in 2-3 weeks.

Dan McShane
DGM Optics
 
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