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by Al Milano 01/21/13

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Possibly the best kept secret in nebula filters, the GCE filters from DGM Optics

by Al Milano

I live in a heavily light-polluted area, right in one of the five boroughs of New York City. It's unfortunate; But, like many other dedicated urban observers, I don't let it stop me from enjoying visual astronomy. In my case, it's DSO's; In fact, it's almost always been DSO's for me. Except of course, for that first year... With the first small refractor kit - Back when I was 12 years old: During my first year of observing, I went for planets and the moon. I was on my own in this hobby, and wasn’t aware of all the really cool things there were to look at. I must also mention that I lived under skies that were somewhere between Bortle 2 or 3. All surroundings were invisible. You needed a flashlight just to walk to a friend’s house up the road. You couldn’t see your feet touching the ground. I only lived in that rural area for a few years *sigh* I’d give anything to live under skies like that again… Since then, it’s been nothing but Bortle 7 skies for me.

Well, I'm 43 now – And, despite living under seriously light-polluted skies: I'm a confirmed chaser of faint fuzzies. It also seems that I appreciate and enjoy binocular astronomy more and more each year. I'm always on the lookout for that next instrument (usually Binos these days), or an accessory to help me fight light pollution. Generally, anything that will give me a bit of an advantage, or slight edge in helping me to locate and view fainter and fainter DSO's.

For a few brief years (while in my late 20's) I was fortunate enough to own and use some fine optical instruments: A TeleVue Pronto (70mm refractor), along with Televue and Takahashi EP's; Fujinon 7x50's & 16x70's - Both with proprietary screw-on Fujinon EP nebula filters, as well as an 8" Dobsonian with lots of nebula filters... I took a short "hiatus" from astronomy for a while (while pursuing other hobbies), and ended up selling all of my high-quality equipment.

Today, my equipment consists of the following: Bushnell 8x30's, Barska 15x70's, Garrett Signature Series 10x50's, Celestron 80mm Spotting scope (20-60x), Bushnell 5" Truss Tube Dob, Apertura AD8 8" Tweaker's Dream Dob, Standard Bushnell & Apertura EP's, Tripods, and lots of Nebula filters...

My latest accessory purchase, was a pair of unique "nebula" filters from a company called DGM Optics. Not only are these filters priced lower than many other companies' offerings - But, according to an excellent review I read, they perform better than filters from the other well-known companies! I don’t recall the online location, where I read about the filters from DGM. The point is, I learned about them; and it was enough to get me interested in them.

I have no affiliation with DGM Optics, by the way (aside from being a satisfied customer).

I ordered a pair of their GCE filters for use on a couple of pairs of binoculars, etc., to help me in spotting DSO's.
GCE stands for Galaxy Contrast Enhancement! Neat, huh? Of course, I was initially very intrigued by them.

Well, it's true - I've seen it for myself now; using a pair of them on my Garrett Signature Series 10x50 binoculars (and also on EP's, with my 8" Dobsonian). They really do enhance the view of galaxies…And a lot more!

Here are the filters screwed onto the threaded eyepiece barrels of my 10x50's - This is basically their permanent location now:

As far as I know - These are the only filters currently in production, that are actually indicated for the image enhancement of galaxies! I think this is great! And although (as one might expect) the difference isn't jaw-dropping - So far, I have seen a noticeable improvement when viewing several of the brighter galaxies: M31, M51, M101. And I'm really looking forward to using them on many more (smaller & fainter) galaxies!

These filters are great for use on small aperture instruments - especially Binoculars. One of the reasons for this, is that they allow a generous amount of light through (while blocking light pollution, of course). You can see the details, from the Transmission Plot:

At a quick glance, they might be considered close to a 'Broadband' type of light pollution filter - As opposed to say, a UHC-type (for example). I suppose the only popular filters that they could be compared to, would be the 'Deep Sky' types. However the GCE's actually do improve the view on galaxies in my heavily light polluted backyard. And, I’ve never heard or read about any skyglow-type, or other LPR filters being able to accomplish this.

I've never taken the time to accurately determine what the NELM is at my observing site. But, I would say it's about magnitude 4.5 (maybe 5.0, on a really good night!). Using the GCE filters on my Garrett 10x50's, mounted on a good Manfrotto tripod set, I've been able to see (and confirm) the following objects: The Rosette nebula, The Christmas Tree Cluster, The Cone nebula, The Eskimo Nebula (NGC 2392), IC 405 & 410, and Pk219+31.1. There are a few more faint nebula that I thought I may have seen, using averted vision - But, cannot confirm 100%, so I haven't listed them.

This was just during the first few weeks since their arrival.

The GCE's give all objects under observation an interesting hue of magenta. Sort of a pinkish/purple, that I happen to find pleasing. It doesn't seem to be as restrictive, or distracting as other nebula filters that I've used. Anyone who has an assortment of nebula filters in their collection knows what I'm referring to.

In addition to my 10x50's, they perform very well when carefully taped to the eyepieces of my inexpensive Barska 15x70's. I also use one with my 8" Dobsonian when scanning/searching for DSO's, using wide-field (30 & 25mm) 1.25" eyepieces. They work very well with my higher-power EP's also (10 & 9mm).

I was going over my Astronomy notes recently...And realized, that there are some interesting technical details about these nebula filters that I can share. This is info copied directly from DGM Optics, hence the quotations: My comments are italicized.

"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, which exclude most of the red portion of the visible spectrum. - I like this very much, because until now, it seemed that you couldn't "have your cake & eat it too" - If you used a broadband filter, from other manufacturers, some of the desired wavelengths were getting blocked as well (especially the desired red).

"Specifications
The GCE filter design stresses maximum optical throughput utilizing state-of-the-art optical thin-film designs and materials. This filter is a “first surface” hard oxide thin-film and is much more durable and long lived than laminated “soft film” designs used by several famous makers." - I'm also very happy about this, no one wants a light pollution filter with a surface that will deteriorate over time.


"They are also much less prone to the internal reflectance problems that plague laminated filters. Laminated filters can produce a very undesirable “doubling” of stellar images often with one of the doubles being red. Contrary to popular belief, this image doubling is not due to the red sideband component that many nebula filter designs produce, but in many cases is due to laminated glass not being exactly parallel to the protective cover plate after lamination. Many laminated filters display this annoying characteristic." - After learning about this, I decided that I’ll be sticking with filters from DGM Optics for a while…

"This filter averages around 1% Transmission (optical density of 2) through the rejection region from 540 to 590nm, and average transmission greater than 90% Transmission in the passbands, and greater than 75% at the 656.3nm H-Alpha line. The design yields a filter with a very symmetrical shape and high optical throughput, for maximum enhancement of nebula with a minimum of loss of stars in the field of view.
The substrate is quartz and has measured total wavefront values of .25 waves. - I find this fact awesome. These filters don't use plain glass as their substrate; it's Quartz!
The combination of first surface optical thin-film technology and a polished, flat substrate enables this filter to be used with high magnification, a real plus for small planetary nebula." - And, finally, this last part - about being "a real plus for small planetary nebula" – Not only have I discovered this to be true; but It just so happens, that I have become very interested in hunting planetary nebula recently:  So, I was also happy to read this.

If there is any area that has room for improvement, it would be with the (1.25") metal filter housing. The small notches on the outer diameter of the filter housing: These serve to provide a better grip, or traction for a user's fingertips (while screwing the filters on or off of EP's). It would be nice, if they could be made to be less likely to slip between fingertips. My other nebula filters seem to be easier to hold on to, by allowing fingertips to gain a firmer grip. The actual filter cells on these GCE's could be considered "low-profile" when compared to the Zhumell brand filters, for instance.

As for me: One of the joys of using this GCE filter - Is that I simply leave a pair of them screwed into the threaded EP barrels of my Garrett signature series 10x50's - And, I just leave them attached all the time. I don't find my views of Asterisms, or Open Clusters darkened very much at all. Yet, they improve the views of all DSO's at same time. They constantly help me to detect nebulas (of all kinds), which I simply could not/would not see without the filters in place. I've actually tested this quite a few times. The best way I can describe it, is: It reminds me of the views seen through night vision instruments. Where a viewer sees lots of things that they couldn’t see before. Except, in this case it might be called “nebula vision”. Because with the GCE filters attached to the binos, I can see nebulae in places where they simply were not visible before.

It would be great, if someone who owns other name-brand "Broadband", “Skyglow”, or LPR filters (or DeepSky types) could also do some tests, verses the GCE filter...

The smallest (good-quality) binoculars I currently have, are my Garrett 10x50's. But, I believe these filters would also perform well on smaller Binos (if one were willing to experiment) - Which would be great when scanning for/observing even larger, diffuse DSO's. In fact, I believe I just gave myself an idea for a future project...

In conclusion, I just wanted to share my experiences. I happen to be a huge fan of binocular Astronomy (and nebula filters!). And, hope that fellow Astronomers, who have similar interests, will find these GCE's to be a useful addition to their equipment.

Clear skies!
-Al Milano

( http://urbanastronomy.blogspot.com/ )

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