Light pollution reduction (LPR) filters can be described as "frills" for observers blessed with dark skies, but for those of us living in urban and suburban areas, they are almost as important as eyepieces—for viewing some objects, anyway. What is the story on LPR filters (Plate 39)? It is important to know what they will not do first. The beginner, seeing advertisements in the astronomy magazines for LPR or "deep sky" filters, naturally thinks his or her problems with bright skies are over. Screw one of these things onto an eyepiece, and all those wonderful galaxies, nebulas, and star clusters will pop right out. Would that were so. LPR filters can help, but only with some types of objects and only to an extent.
Understanding the capabilities and limitations of LPR filters requires an understanding of how they work. To the eye, one appears to be nothing more than a darkly tinted red or blue piece of glass, no different from any other filter used in astronomy or photography. In reality, LPR filters are made by a considerably different process. They consist of an optically flat piece of glass that has had multiple layers of reflective material deposited on one surface in a vacuum chamber. Each layer reflects a different set of wavelengths of light. Light enters the filter from the telescope's optical system (filters are normally screwed onto the field lens end of the eyepiece) and hits the filter. The "good" wavelengths pass right through and into the eyepiece. The "bad" wavelengths—especially those from mercury vapor and sodium streetlights—are reflected away. Manufacturers choose filter coatings based on the wavelengths they wish to admit and exclude. This should make it obvious that LPR filters don't make objects brighter. They improve contrast between deep sky objects and the background sky by suppressing the light pollution wavelengths that make the sky bright.
That all sounds good, and LPR filters do work, but they have some severe limitations, the most serious being that they do not work on all objects. Unfortunately, light emitted by stars falls into the same range of wavelengths as that from earthly light sources. This means LPR filters are nearly useless on star clusters. The light from the stars making up these objects is rejected along with earthly light pollution. Galaxies are also made of stars, so LPR filters do not help them, either. One manufacturer is now offering a supposed "galaxy filter," but nevertheless LPR filters are only effective on planetary and diffuse nebulas. Period.
Do you still want an LPR filter? Prepare to be confused by the large number of different types and brands available. A little study of the magazine ads, however, reveals that they come in three "flavors" that represent their passbands. Passband is a forbidding-sounding word, but the concept is simple. A light pollution filter's passband refers to the range of frequencies of light allowed to pass through it. Filters are available with wide (broadband), normal (medium), and narrow (line-filter) passbands. Each type is different and is suited to a different application. Most types of LPR filter are available in either 1.25- or 2-inch formats and are marketed by a number of companies in the United States and Europe, with Lumicon, TeleVue, Orion, Thousand Oaks, and Baader Planetarium leaders in the field.
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