The Orion UltraBlock is a narrowband filter intended to improve the contrast on emission nebulas. It does this by letting through the hydrogen-beta and ionized oxygen light frequencies common to emission nebulas and blocking other light frequencies, including most common sources of light pollution. The UltraBlock filter gets good reviews on Cloudy Nights [2], but there are often comments on the forums that while it works well in dark skies, it doesn’t perform so well with heavy light pollution.

 I’ve found the UltraBlock to be very effective in my light polluted skies. It helps me both see more details in objects that are already visible and pull out objects that otherwise I couldn’t see at all. It works particularly well with planetary nebulas. Orion’s UltraBlock gets two thumbs up from me as a light pollution fighter. Let me explain why…

The Orion UltraBlock comes in both 2” and 1.25” sizes (I have both). It lets through 90-95% of the light in a roughly 20nm wide band that includes the hydrogen-beta emission line (485.6nm) and doubly ionized oxygen (O III) emissions lines (495.9 and 500.7nm). It has a relatively sharp cut-off that pretty much blocks all light frequencies below 475nm and above 525nm. In particular, it blocks:

• mercury vapor lights (404.7, 435.8, 546.1, 577, and 579.1nm)

• sodium lights (570, 583, 600, and 617nm)

• skyglow (atomic oxygen) (557.7nm and 630nm)

 Unfortunately, high-pressure sodium (HPS) lights have many emission lines spread across the spectrum, including one line very close to the O-III emission doublet. So an UltraBlock filter won’t help as much if you have lots of HPS lights in your area.

Note that transmission is high but not 100% for the desired frequencies. So in an absolute sense objects observed with an UltraBlock are always slightly fainter. However, for the kinds of objects you can observe in light polluted areas the contrast improvement more than makes up for the light loss. 

Performance on brighter nebulas

On brighter nebulas, the UltraBlock filter really enhances the contrast, helping to bring out a lot more detail. Here are a few specific examples of what the UltraBlock brings out when observing from my light polluted skies:

• M27

  On moonless nights with good transparency, my 10 inch Dob will show the apple-core shape of M27, and the brighter diagonal lane within the apple-core. But due to the light pollution, that’s about all I can see. With the UltraBlock, that diagonal lane is very clear and I can see hints of structure in the brighter knots in the opposing corners as well as a mottled appearance throughout the brighter areas. On nights of outstanding transparency, using the UltraBlock I have even glimpsed the "wings” on either side M27. Without the UltraBlock, I simply can’t see these features.

• M57 

  The UltraBlock really makes this nebula jump out crisply. With the UltraBlock I have been able to see the fraying of the outer edge of the annulus at the western end, and sometimes also the eastern end. These features are invisible without the UltraBlock.

• M42

  The Orion Nebula appears quite spectacular using the UltraBlock filter. The filter makes very clear all kinds of complex structure to south of Trapezium that is not visible (or barely visible) without the filter. The UltraBlock also shows filaments of nebulosity across the “mouth” in the nebula, as well as nebula filaments curving from either side of the mouth, especially with the slightest use of averted vision or telescope jiggle. In my washed-out skies, I can’t see these things without the UltraBlock.

Performance on fainter nebulas

I have also found the UltraBlock filter to be very effective in letting me see fainter nebulas that otherwise I can barely see or that I simply can’t see at all.

• M97 (magnitude 9.9)

  This nebula always sits in the big dome of light pollution that spoils my northern sky. Because of this, M97 is always rather hard to see, even knowing exactly where it is. When I can see M97, I only manage to see it using averted vision. The UltraBlock filter really makes M97 very distinct and clearly visible even without using averted vision.

• M76 (magnitude 10.1)

  Like M97, M76 also sits within my northern light dome, and is even harder to see than M97. In fact, I usually can’t find M76 without using the UltraBlock. But it shows up clearly using the UltraBlock. Having found it using the UltraBlock, I can then usually also see M76 without the filter by using averted vision and jiggling the scope. With the UltraBlock filter M76’s rectangular shape is pretty distinct; without the UltraBlock M76 looks rather amorphous and the rectangular shape is not at all clear.

• NGC 7048 (magnitude 11.3)

  NGC 7048 is a real “challenge object” for my 10 inch Dob under light polluted skies. But using the UltraBlock and a 15mm Meade SWA eyepiece on a clear fall night last year, I was able to see it. I had to use averted vision, but NGC 7048 was clearly visible as a faint little soap-bubble. After making careful note of where NGC 7048 was in the eyepiece star field, I took out the UltraBlock and tried to see it unfiltered. No matter how hard and long I looked, or which eyepieces I tried, I simply couldn’t see this object at all without the UltraBlock. This is the best example I have of the UltraBlock filter making the invisible, visible.

Some closing remarks

Unfortunately, narrowband filters like the UltraBlock are not designed to work on objects that emit a continuous spectrum such as galaxies or star clusters (although you can use it to enhance HII regions in spiral galaxies like M33 or M101, but you’ll need really dark skies to see those). The UltraBlock filter also gives everything a somewhat ghoulish green tint. I don’t mind it at all, but some friends don’t like it.

Finally, the UltraBlock filter works by reflecting, not absorbing, the light it blocks. This means that any stray light entering through the eyepiece will be reflected back out the eyepiece and into your eye. It takes only a little stray light to completely wash out your view, so be sure to shield the eyepiece from any stray light when observing with the UltraBlock.

In summary, the UltraBlock works as advertised, increasing the contrast on emission nebulas to make faint ones more easily visible and reveal additional detail in brighter ones. While it isn’t a magic solution, I’ve found that it works quite well in light polluted skies, helping you see things you won’t see otherwise.