The Real Story on Light Pollution Filters
By Shawn Grant
Light pollution filters are the biggest source of confusion for beginners as well as some veterans. What do they really do? Do they really work? Which to get? Can I use them at a dark site? This article will attempt to answer all of these questions and dispel many myths. First we will dispel a few myths on light pollution filters.
Myth number one: Light pollution filters decrease sky glow caused by artificial lights and enhance all deep sky objects. Yes they decrease sky glow but they also decrease the brightness of stars, star clusters, globular clusters and galaxies making views of them worse. Stars, galaxies and star clusters emit light from all wavelengths including the same wavelengths as street lights which the filter blocks. That is why light pollution filters diminish the view of such objects. Light from emission nebula and planetary nebula has a different wavelength of light then streetlights and the light of a nebula passes right through the filter. So light pollution filters true function is to block sky glow to add contrast making emission and planetary nebula more visible. For now on I will not use the term light pollution filter. They are nebula filters. Always call them that because that is what they enhance. Light pollution filter is a misleading name.
Myth number two: Nebula filters work only in light polluted skies. That is completely false. Nebula filters enhance nebulas no matter where you are, New York City or a thousand miles away form the nearest light source.
Myth number three: Only large telescopes can benefit from nebula filters. For many years Walter Scott Huston wrote a monthly deep sky column in Sky & Telescope. He died a few years ago and the column stopped. In one of those columns Walter wrote about people holding a nebula filter up to their eye to view nebulas like the California, Rosette and others. Without the filter no nebula was seen and with the filter a nebula was seen. If a nebula filter works for the eye it will work with a telescope of any size.
There are so many nebula filters which one should I buy? There are two major types. One called a broadband and the other a narrowband. The narrowband has a few sub types that I will go into later. The broadband (also called wideband) filter blocks the least amount of light and enhance nebulas the least. Some popular brands are Orion Sky Glow and Lumicon Deep Sky. Celestron and many other companies make them as well. Most people confuse a broadband filter as the filter to decease sky glow and enhance all deep sky objects. Basically a do all filter. In fact this is how all companies market their broadband filters. As with myth number one we know that this is false. The real use for broadband filters is to enhance some reflection nebulas, HII regions (nebula) in galaxies such as M101 and M33 and astrophotography of nebulas only. It doesn’t produce dramatic results but can help. You will not use a broadband filter much.
The other type is the narrowband. Some popular brands are the Orion Ultrablock and Lumicon UHC. These filters block out the most sky glow and enhance emission and planetary nebula the most. Narrowband filters do significantly enhance nebula and they work very well. There has been countless times I was observing in an area that the chart says there was a nebula and I couldn’t see one. I screwed on a narrowband nebula filter and the nebula appeared. Even bright nebula such as M42 and M8 show lots more detail with a narrowband filter then without. Planetary nebula shows more detail as well. Many times a narrowband will help in spotting a planetary nebula. Pass the filter between the eye and the eyepiece. You will see the planetary blink. Narrowband filters such as the Ultrablock or the Lumicon UHC do such a good job you can’t go without one. A narrowband filter is just as important of an accessory as an eyepiece, telrad and star charts. Get one!
There are two other narrowband filters that allows less light through then the Ultrablock or the UHC. These are both made by Lumicon and they are call the Oxygen III (O-III) and H-beta. The O-III does a better job enhancing planetary nebula and some emission nebula then the Ultrablock or UHC. Some emission nebula looks a tad worse then the UHC or Ultrablock. The O-III cannot be beat for planetary nebula. The H-beta has a limited use. It enhances only a few emission such as the California and IC 434 making the horsehead easier to see. It will also enhance some very faint emission nebula. Many people call the H-beta the horsehead filter because in many cases it is the only way to see the horsehead nebula.
In summery nebula filters are great if you understand what they do and how to use them. They are for enhancing nebulas only and not other deepsky objects no matter what ads might say. If you were only to get just one get the Orion Ultrablock or the Lumicon UHC. It doesn’t matter which they are both of equal quality and equal price. If you can afford two it would be a toss up between a broadband or the O-III. Personally I would get the O-III. If you can afford three get which one you didn’t choose for just two. For those of you who are serious deep sky observers and can afford it, get all four. I hope this answered a lot of questions and help you make wise purchases and know when to use a nebula filter and when not to.Back