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Light Pollution Reduction Filters

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Posted by Daniel Johnson on May 29, 2003 03:07:54 UTC

My experience comes with other brans of light-pollution-reduction (LPR) filters, such as Lumicon and IDAS. However, I'll share a few generalities.
Only a few types of objects gain much from use of LPR filters. The best enhancement comes from those objects that put most of their light at a few narrow wavelengths, such as emission nebulae and some reflection nebulae. The filters let through those wavelengths and reject others, improving the signal-to-noise ratio. They help a lot with the Orion nebula, Omega nebula, and other such bright nebulosities. They make planets and stars look fake because the colors are unrealistic. Besides, planets are bright enough that city lights harm their view very little. (The nebulae just mentioned remain realistic in color, since they have only those specific colors that are transmitted anyway.) Other faint fuzzy objects such as galaxies gain only slightly. They emit light at all wavelengths, and so they are dimmed almost as much as the city lights.
I have enjoyed my IDAS LPR filter as well as my high-contrast Lumicon filter. The IDAS filter does a better job than most of maintaining realistic colors on stars. The high-contrast filter dims the city lights the most. The low-contrast Lumicon filter doesn't make as much difference as I would like to my eye. However, if your scope has a small aperture (say, 6 inches or less), you may find that high-contrast filters dim everything too much, and that a low-contrast filter is best for you. There is so much variation (scope size, specific wavelengths emitted by the lights in your neighborhood and city, and your own tastes) that it's hard to know which filter will suit you best.

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