Broadband "Light-pollution Reduction" (LPR) filters are designed to improve the visibility of deep sky objects by blocking out the mercury vapor, sodium vapor, and other such emission lines from man-made or natural sources which contribute to light pollution. In turn, these filters allow a broad range of more useful wavelengths to pass to the eye or imaging sensor. By selectively screening out of some of the background sky glow, the contrast is increased and allows deep sky objects to stand out against the background.
Broad-band filters don’t eliminate the effects of light pollution or make the objects brighter, but they can improve the visibility of many deep-sky objects. Emission nebulae are the most improved, but these filters can also give a slight contrast boost to reflection nebulae and a few of the larger and more diffuse galaxies. Large versions of these broadband filters that can fit over DSLR lenses can improve imaging of wide fields when sky glow is present.
Broadband filters can offer a noticeable boost in contrast and visibility of the fainter outer detail for objects like the Orion nebula (M42), the Lagoon nebula (M8), the Trifid nebula (M20), and others. The improvement isn’t as noticeable on star clusters or small galaxies.
Since some light is blocked by the filters, objects may look fainter from a dark sky site when using a broad-band filter than without one. Severe levels of light pollution can be too much for the broad-band filters to handle effectively, so observing from a dark site is preferred in these cases.
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