UltraBlock and Small Scopes

Orion UltraBlock filter, 1.25 inchIf you peruse posts on Cloudy Nights and other forums on the web, you’ll find claims that narrowband filters don’t work well with small scopes, that you need at least 8 inches (or 10 inches, or 12 inches…) of aperture to get any benefit from them. I’ve never agreed, having used the UltraBlock to advantage with my 4.25 inch (108mm) Newtonian to see through local light pollution.

4.25 inch (108mm) Newtonian reflectionRecently, I put this hypothesis to the test with my 4.25 inch (108mm) Newtonian on a night with particularly bad haze and light pollution. It was a night on which 3rd magnitude stars were hard to see. I found that planetary nebulas which were invisible in the 4.25 inch showed up relatively clearly using the UltraBlock. So as far as I can tell, you can use the UltraBlock filter to help you see through light pollution just as effectively with small apertures as with large. Here are the details….

Washington DC recently had one of its typical summer nights with high humidity and hazy skies. The kind of night during which transparency is poor (2/5) and urban skyglow is at its worst. Add to this 95% percent illumination from a nearly full moon, and it made for a very poor night in which Albireo (β Cygni, magnitude 3.0) was barely visible to the naked eye. It seemed like the perfect night to test what the UltraBlock could do using a smaller scope (I have already written about how the UltraBlock works under washed-out skies using my 10 inch (250mm) Dobsonian).


M57 by NOAO/AURA/NSF My first target was the Ring Nebula, M57. Only 30 degrees from zenith, it was out of the worst part of the haze. M57 was easily visible in my 10 inch Dob without the UltraBlock, but it wasn’t crisp and the central hole, while visible, didn’t standout out. In the 4.25 inch reflector without using the UltraBlock filter, at first I couldn’t even see M57. After going back and forth between the 10 inch and 4.25 inch, I was eventually able to spot M57 with the 4.25 inch using a 15mm eyepiece (67x) and averted vision. Then I popped in the UltraBlock and took another look. M57 was now relatively easy to see using direct vision, with a sharp outer edge. However, the central hole wasn’t visible—at best I might have glimpsed the hole using averted vision. (Photo credit: NOAO/AURA/NSF)

I then shifted to a 9mm eyepiece (112x). Without the UltraBlock I simply could not see M57 with the 4.25 inch, no matter how carefully I looked. Even confirming with the 10 inch that I was looking in precisely the right place, I couldn’t see it. Next, I again popped in the UltraBlock. Now M57 was distinctly visible with direct vision in the 4.25 inch and I could also see the central hole using averted vision (but the hole wasn’t clear using direct vision).


M27 by Bill Schoening/NOAO/AURA/NSFFor the next test, I turned both telescopes to the Dumbbell Nebula, M27. M27 was located about 45-50 degrees above my eastern horizon and unfortunately right in one of my biggest light pollution domes. As before, I first found M27 in the 10 inch; it was only faintly visible. Turning the 4.25 inch on M27 using a 15mm eyepiece, I couldn’t see any hint of the Dumbbell Nebula. I triple and quadruple checked against the 10 inch to make sure I was looking in the right place, I jiggled the scope, I gave it my best try with averted vision, and still not even a glimpse of M27. Then I screwed in the UltraBlock and right way I saw M27. It was visible in direct vision, although the shape was rather indistinct. The edges were fuzzy and unclear, and I couldn’t really see the classic apple-core shape. But nevertheless, there it was. (Photo credit: Bill Schoening/NOAO/AURA/NSF)


On a night when light pollution was at its worst, with lots of haze and even a nearly full moon, the UltraBlock enabled me to see M27 and M57 using my little 4.25 inch Newtonian. Neither of these nebulas was visible without the UltraBlock. So don’t be dissuaded by the naysayers on the web: even with small telescopes, the UltraBlock helps quite a bit in urban light polluted skies. It doesn’t perform miracles—both M27 and M57 looked rather ratty and nowhere near their true glory. But the UltraBlock made the difference between seeing them and not seeing them.

Now I would be curious to see how the UltraBlock works with a small refractor (say 80-90mm) in washed-out skies. I don’t have a small refractor, so I am eager to hear from anyone who can try such a combination.