Narrowband filters work by transmitting only a small fraction of available light to the eye, and in doing so block out wavelengths common to light polluting sources such as streetlights while still allowing wavelengths common to most nebula to pass through. The idea is to darken the background sky (especially useful in areas with some degree of light pollution) to increase the contrast between nebula clouds and surrounding empty areas.
After a little investigating, I decided to go with the Orion Ultrablock because, while not as highly rated as the Lumicon Ultrahigh Contrast, it gets almost as good marks and for significantly less (I had a gift certificate to Orion). Orion service proved as good as always, though I feel like they really bleed you on shipping prices for small objects, but I have always received my products rapidly and reliably from them.
Depending on how the filter is turned into the light, it appears purple or sea green or mirror like. I decided to test it out last night before the just passed full moon could arise. My test instruments were the 12 inch dobsonian, and two identical 25 mm Orion plossl eyepieces. This way, only the filter would be different, but by using two eyepieces I could quickly shift from one view to the other. Unfortunately, the conditions outside did not seem favorable. A bank of clouds began moving in just as I targeted my first nebula, M. 42. I only had about 10 minutes of observation time before I had to pack it up. So, while I'll discuss my initial impressions, this review can only be introductory in nature, and I'll try to post a more thorough follow-up later.
In the pamphlet included with the filter, it specifically says to try the Orion nebula the first time you use it. Also included is a wavelengths scan report, and mine claimed all wavelengths below 473 nm and above about 505 nm would be blocked. With this in mind, I targeted the Orion nebula first with the unfiltered eyepiece. In a word, amazing, this scope continues to impress me and it never fails on this nebula. The view revealed an expansive brightness beyond the central trapezium area including sharply defined "wings" and intricate cloud detail. And all this without sufficient dark adaptation for a regular viewing session. Based on this, my expectations for the filtered eyepiece were high. Switching over, my first thought involved the deep green coloration of everything in eyepiece. Once passed this initial surprise, I noticed that the areas of the nebula shown closely matched the unfiltered view. This surprised me, so I slipped over to the very bright multiple star in front of the nebula. This area also has nebulosity, and recently I had had difficulty distinguishing it from the bright star. With the filter, the star was dimmed and the surrounding nebula apparent. Once spotted, however, I had little difficulty in the unfiltered eyepiece for the same area.
I tried to jump over to M. 78 and M. 45 to continue my test, but the clouds had already moved in enough to send me packing. With this short comparison, I did not see as large a difference between the filtered and unfiltered views. However, I suspect that under better conditions with longer dark adaptation, and with a larger variety of subjects, I might have a different experience.
The filter does help one negative, which all narrowband filters may share, so I'm not sure it is specific to this particular one. In changing from the unfiltered to the filtered eyepiece, I noticed what seemed to be extra light in areas not previously seen. Moving around different patches of the sky and switching between filtered and unfiltered, I realize that what I was seeing was a reflection off of the backside of the filter. Typically when deep sky observing I set up walls of tarp to block extraneous light, but in this case I did not. Stray light was being reflected from my eyeglasses through the eyepiece and bouncing back off of the filter. This was distracting. I doublechecked this by holding both eyepieces up to a light, and in the filtered eyepiece I could easily see my reflection coming back at me through the eyepiece. In the unfiltered one, this is not a problem.
With this limited run, I cannot see the filter as a huge improvement to my observing tools though. With the new moon approaching in a few weeks, I should have a couple of nights to try this out again soon.
