Untitled Document

29 August 2003

Using Filters to View Mars

by Daniel M. Troiani
Excerpted from "Mars at Its All-Time Finest", Sky & Telescope

Filters, which are very useful for visual observations of Mars, are now considered almost imperative for digital imaging. Whether you are working visually, with film, or with a CCD, a basic tricolor set of filters is highly recommended. Commonly identified by their Wratten (W) designations, they can be purchased from many telescope dealers and camera stores.

The color filter you use determines how far down through Mars's atmosphere you see. Red light reveals the surface best; blue light usually shows only the clouds and hazes. Even though Mars was just 6" across, Donald C. Parker of Coral Gables, Florida, captured the most prominent surface markings as well as equatorial cloud bands in these amazing CCD images with his 16-inch f/6 Newtonian reflector in December, 1998. Celestial south is up.

Red or orange filters (W25 or W23A) penetrate MarsÕs atmosphere rather handily, exposing such features as the polar caps. They increase the contrast of dark surface markings, and they are best for spotting dust storms. If a patch is bright in red and dim in blue, it's dust.

Green (W58) and blue-green (W64) filters bring out surface fogs, frost patches, and polar-cap extensions.

Blue (W38A or W80A) and violet (W47) filters, because of the Martian atmosphereÕs opacity to short wavelengths, are best at highlighting water-vapor clouds and polar hoods. Only in the early stages of the 2003 apparition is there expected to be much cloud activity, however.

Use the filter that provides the highest contrast for the type of feature you are trying to study. Observers with small (3- to 6-inch) telescopes will find that a yellow filter (W15) provides a brighter image and may perform better than a deep-red one. Those doing CCD imaging are forewarned to employ filters that reject infrared light. (The filters currently being sold for this purpose usually do; older photographic filters may not.)