Get Ready for Mars!
Planetary observers only get good views of Mars about every 26 months. Mars appears very large for observing every 15 to 17 years when it passes closest to the Earth during opposition and when it is also nearest the Sun at perihelion. August of 2003 is one of the best chances in our lifetime to see Mars so clearly, if the Martian weather cooperates.
In late August Mars will reach magnitude -2.9 and will dominate our southern sky with its fiery red coloration. On the night of August 26-27, the red planet will be closer to Earth than at any time in some 60,000 years. But planetary observers know that Mars can be a telescopic challenge despite its remarkable proximity at this time. The great Martian dust storm of 2001 appeared near opposition and robbed planetary observers of clear views. Also Mars will reach an angular extent of 25.1", but that's barely more than half the apparent diameter of Jupiter.
The use of color filters can be very useful for visual observations of Mars. Filters are considered almost imperative for digital imaging today. Whether you are working visually, with film, or with a CCD, a basic tricolor set of filters is highly recommended. Color filters are identified by their Wratten (W) designation and can be purchased from most telescope dealers and camera stores.
Planetary observers use the filter that provides the highest contrast for the type of feature they are trying to detect. Observers using small telescopes (3 to 4 inches) will find that a yellow filter (W15) provides a brighter image and may perform better than a deep-red one. Red or orange filters (W25 or W23A) penetrate Mars's atmosphere very well, exposing such features as the polar caps. Red filters also increase the contrast of dark surface markings and they can be used for detecting Martian dust storms. If an observed feature appears bright in red and dim in blue, it's probably dust. The green (W58) and blue-green (W64) filters bring out frost patches and polar cap extensions. The blue (W38A or W80A) and violet (W47) filters are best at detecting water-vapor clouds and the polar hoods of Mars.
With experience the patient planetary observer will detect clouds around Mars's bold features and limbs. Some discrete clouds are known to recur around familiar Martian sites, such as Libya, Chryse, and Hellas. One impressive example is known as the "Syrtis Blue Cloud," which circulates around the Libya basin and across Syrtis Major, changing its color to a distinct blue. Try to detect this cloud when Syrtis Major is near the planet's limb. Observing clouds through a yellow filter can cause the dark Martian features to appear green. Using higher magnifications (300 to 600) can help one detect limb brightening or limb arcs that are caused by scattered light high in the Martian atmosphere from dust and ice particles. This effect can be observed on both east and west limbs of the red planet. Using blue-green, blue, or violet filters can help Mars observers detect limb arcs.
Don't let the summer and fall of 2003 pass you by without observing the little red planet. Mars will be at its best and it could deliver some very impressive views. With experience at the eyepiece, the more subtle Martian features can be detected. The challenges of planetary observing can be as addicting as chasing those faint fuzzies of deep sky observing.
The following drawings by Paul Morow show how various features will appear in a modest-sized telescope.
Published in the August 2003 issue of the NightTimes