• Filters make nebulae brighter. This is incorrect. Filters subtract light, making objects dimmer. In the case of filters designed for light pollution suppression, wavelengths from certain types of artificial lights are blocked, but other wavelengths (particular the nebula emission lines) are allowed through, thus improving contrast. This allows longer astrophoto exposures to be made before the sky background begins to wash out the image desired. Visually, the contrast increase may be perceived to be an increase in brightness.

• Filters make nebulae easier to see. This is true only if properly used. If used in an urban environment, there are often sources of light (including the sky itself) which prevent full dark adaptation of the eye (about 30 minutes in full darkness is normally required).

• Filters work on all objects. This is not true of all light pollution suppression filters to the same degree. In general, broadband sources (galaxies, clusters, stars, or reflections of stars) will not experience as great a contrast gain as emission line objects. Filters designed for visual use or to select specific narrow emission lines (e.g. OIII) may suppress broad parts of the spectrum, decreasing the light from broadband sources even more than filters designed for balanced color photography.

• Filters will block all light pollution. This is not true. Many light sources such as car headlights are still broadband sources and will not be effectively blocked.

• Filters increase exposure times. While light pollution suppression filters will slightly increase the exposure times of emission line objects in their bandpass and affect broadband sources somewhat more, they do not require increasing the length of an exposure. However, because they increase the contrast of emission line objects compared to the sky background, it is usually desirable to increase exposure times to make better use of the imaging media's dynamic range.

• For photographic use, some color shift will occur because the filters do not block light in infinitely narrow bands. This affects the ratio of light in the three color bands being sampled, causing a shift in the perceived colors. The degree to which this occurs may depend not only on the filter being used, but also the type of color film used. Visually, color shifts are usually inconsequential since most extended objects will be below the eye's threshold for color detection.

• Narrowband filters are tuned to specific wavelengths by carefully controlling the thickness of layers of dielectric material on glass. Because the effective thickness changes when the filter is tilted, the wavelength tuning will also change. In practice, this means that wide-angle views will show a perceptible color shift between the center and edges of a photo.

Who needs a light pollution suppression filter?

This section of a panorama of the Milky Way is a composite of photographs taken over one evening in September 1999 from the site of the Orange County Astronomers, located near Mt. Palomar. The bright glow at the bottom is from San Diego city lights. No filter was used for this photograph.