Nebula filters

Urban Orion

The constellation of OrionOrion is not only the best known winter constellation, but it is also a great urban constellation. Thanks to its distinctive arrangement of rather bright stars, Orion shines through in heavily washed-out skies, even when few other constellations can recognized. It is also the home of the Great Orion Nebula (M42) big, bright, and easily visible even in washed-out skies. However, M42 is only the most famous of many interesting astronomical objects in Orion. And best of all for urban astronomers, quite a few of Orion's objects—like the constellation itself—can be enjoyed in light-polluted skies. Here is a run down...


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….


Orion UltraBlock Filter

 Orion UltraBlock filter, 2 inch version with 1.25 inch version seen behind itThe 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, 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…


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