This week, I continue my three-part series on filters by giving specific recommendations about planetary observing through color filters. Mars lies at opposition as 2007 ends, and it's already on most observers' minds, so let's start with the Red Planet.
As I mentioned last week, choose the filter density that's correct for your telescope. For example, if I suggest a red filter for a certain feature, choose 1) a #21 (orange) filter if your scope's aperture is 4 inches or less; 2) a #23A (light red) filter if your scope's aperture falls between 4 and 8 inches; and 3) a #25A (red) filter if you use a scope larger than 8 inches.
Yellow filters darken blue and brown martian features and brighten desert areas. Red filters increase contrast between light and dark features more than yellow filters and bring out dust clouds. In particular, the #25A (red) filter gives maximum contrast to surface features. Also, it enhances fine surface details along with dust-cloud and polar-cap boundaries.
Green filters work well on Mars' polar caps. They darken red and blue features and enhance frost patches and surface fog. Both frost and fog appear at sunrise and sunset, but fog dissipates after about an hour; frost can last all day.
For martian clouds, grab your blue filters. Look for atmospheric clouds, discrete white clouds, limb hazes, equatorial cloud bands, and polar cloud hoods. If you have no filters and are wondering which one you should buy first, choose a #80A (blue). This filter enhances all clouds, ground frost and fog, and the polar ice caps.
Jupiter, which currently sits in Scorpius, just north of Antares, responds well to many filters. Blue filters bring out the contrast within the bright zones and sharpen brighter cloud features. Green and blue filters darken the planet's belts. Yellow filters darken blue jovian features such as festoons, which appear from time to time within the equatorial region. Red filters will brighten and enhance white spots and ovals seen in the South Temperate Belts and Zone.
Saturn's colors are less bright, and the planet has less overall contrast than Jupiter. I'd describe Saturn's brighter zones as off-white, slate-gray, or yellowish at times. Occasionally, Saturn's belts exhibit bluish-gray, brown, or red colors. You can enhance these features by using yellow, orange, or red filters. Bright patches sometimes appear on the ringed planet. Use a green filter at such times. And the best filters for the rings are #56 (light-green) and #80A (blue).
Next week, I'll tackle every other kind of eyepiece filter. Deep-sky observers, that blog's for you.