…continuedMars at Its All-Time Finest
How to Observe Mars
Opinions vary as to how large Mars's disk must appear to be studied effectively. For film photographers 10" has been a good rule, but visual observers can glimpse interesting details whenever the disk exceeds 6". CCD detectors are even better at this than the human eye.
But there is another consideration besides the planet’s size for observers living in the Northern Hemisphere. Mars begins May well south of the celestial equator at declination 20°, so it fails to rise more than 30° above the southern horizon (as seen from latitude 40° north). Seeing will suffer, and any crisp markings are likely to be smeared and softened by low-sky haze and turbulence in Earth’s atmosphere. By the end of August it will be getting a little higher each night, but not until mid-December does the planet cross the celestial equator heading north.
Good news for Southern Hemisphere readers: When Mars is nearest Earth in late August, it will pass almost directly overhead in your night sky!
Red or orange filters (W25 or W23A) penetrate Mars’s 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 atmosphere’s 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.)
Remember that Mars can always throw in something unexpected. The great dust storm that appeared right around the opposition date in 2001 had not been predicted to happen at all!