A spectral examination of the most popular light pollution filters in amateur astronomy.
By Rob Brown
with significant contributions from Howard Banich and Chuck Dethloff, special thanks to Howard Banich and Joe Hedrick for lending filters.

In early February, 2002, the Rose City Astronomers engaged in a lengthy and interesting discussion on their e-mail list about the performance of LPR filters in various telescopes, with the discussion centered on performance reduction in fast scopes (F/4 or so.) The discussion revealed that there is a lot of speculation, confusion, and wisdom within our group. I created this webpage to answer common questions, and most importantly, to dispel myths before they propagate. Since I have ready access to good equipment, I thought it would be helpful to measure several LPR filters in collimated light and at F/4 to help people get a sense of what is really happening. If you are brand-new to filters, please click on FAQs first, before you look at the spectral plots.

FAQs about Filters

New! Lumicon Swan Filter
Sirius Optics Mars03 Filter
Filters Compared
Lumicon Oxygen III at F/4 and Collimated Light
Orion Ultrablock at F/4 and Collimated Light
Orion SkyGlow in Collimated Light
Lumicon Deep-Sky at F/4
Lumicon UHC at F/4 and Collimated Light
Lumicon H-Beta at F/4 and Collimated Light

Description of Experiment

FREQUENTLY ASKED QUESTIONS, with responses contributed from Howard, Chuck, and Joe.

1) What filter should I buy first?
The Orion Ultrablock, Lumicon UHC, and Oxygen III all give dramatic results on most nebulae, in light polluted skies and in dark skies. OIII is a very useful filter and performs most notably on the Veil Nebula. Chuck recommends OIII as a first choice. The UHC is a good complement to the OIII as each has its own merits. The performance of the Deep-Sky filter isn't as dramatic, but Joe uses it most frequently in his New Jersey skies. (note: I own only one filter, the Orion Ultrablock, and I'm beginning to suspect I'm missing out on something....)

2) What nebulae can I observe with a filter?
Emission nebulae of all types, including planetaries and diffuse nebulae. Reflection nebulae do not generally benefit from filters, but Chuck reports the Deep-Sky gives some improvement, especially in presence of moonlight. Here's a link showing you the performance of filters on 81 objects!   http://pages.sbcglobal.net/raycash/filters.htm (Your mileage may vary)

3) What won't work with my filter?
Generally, galaxies, star clusters, and anything non-nebulous don't improve as dramatically as emission nebulae. Use the Deep-Sky on these objects when the Moon is out and you may see improvement. It's always fun to experiment. The Deep-Sky works great on Jupiter!

4) Do I have to have a big scope to use a filter?
Absolutely not! (However, light gathering power always helps.) Chuck makes the point that filters in smaller scopes (under 6") don't respond as well. However, there are plenty of bright nebulae suitable for small scopes, even binoculars. Would you believe filtered naked eye?

5) What if I have a fast scope?
You've come to the right place. See spectral plots, above. Short answer: Filters work well down to at least F/4. We have experimental results to show it, and lots of experience in the field.

6) Do light pollution filters work in dark skies?
Wonderfully! Some might even say they work their best in dark skies.

7) What about that H-beta filter?
Ideal for the Horsehead and not too much else (California Neb, Barnard's Loop?). (I suspect there may be interesting applications we haven't thought about. Anyone care to share unusual stories?)
 

Description of Experiment
The spectral plots were obtained using a PhotoResearch Pritchard 1980B Spectrophotometer and a stabilized incandescent projected light source. The beam from the light source is substantially collimated. The light was collected into an integrating sphere prior to the spectrophotometer. For measurements at F/4, a lens assembly was placed in the beam, which focused the beam to a spot at about F/2, but the cone of light was truncated to F/4 by the integrating sphere. See figure, below:

Figure 1: Collimated Beam Measurement
 

Figure 2: Measurement in F/4 beam