Jupiter has been called "the amateur's planet," because it offers a wealth of opportunities for amateur observers to make contributions to planetary astronomy. All it takes are determination and the effective use of equipment you may already have.

Equipment Matters

For planetary observations, what telescope is best? The answer is simple: the one best capable of giving sharp, high-contrast views. First and foremost, this means large aperture. Right up there too is top-notch optical quality. Next comes telescope type; the best planetary scopes have traditionally been apochromatic or long-focus achromatic refractors and long-focal-length Newtonian reflectors. Telescopes with contrast-robbing large secondary mirrors, such as Schmidt-Cassegrains or Maksutov-Cassegrains, have been considered less desirable, but their central obstructions can be more than made up for by large aperture (assuming the optical quality is high), and many of these scopes have produced impressive results in recent years.

But no observer should put off observing Jupiter for lack of the perfect telescope. The truth is, the "best" telescope is one that you use rather than an ideal one that you don't use. Regardless of telescope type, the optics should be perfectly collimated. A well-made 5-inch refractor or 6-inch reflector on a sturdy tracking mount is really about the minimum for serious Jupiter observing. Larger instruments will allow scrutiny of fine detail and subtle low-contrast markings.

Although Jupiter is big and bright, it doesn't tolerate high magnification well — the image tends to go soft quickly. Consequently, you will rarely use more than 40x per inch of aperture. I find that my 8-inch is limited to about 200x on nights of steady seeing. As with the telescope itself, the eyepiece too must deliver sharp, high-contrast views. Many serious planetary observers prefer high-quality Plössl or orthoscopic eyepieces to complex models designed for ultrawide-field views.

Color filters that screw into eyepiece barrels can improve the contrast of certain Jovian features and assist in identifying them. As a general rule, choose a filter with a color opposite that of the feature you want to observe. For example, the Great Red Spot (GRS) and reddish brown belts are best seen with blue filters such as Wratten 82A (light blue), 80A (medium blue), or 38A (blue). Red filters such as Wratten 21 (orange-red), 23 (light red), and 25 (red) can be used to enhance bluish features, such as the projections and festoons often found on the southern edge of the North Equatorial Belt. I like to use yellow filters such as Wratten 12 (medium yellow) and 8 (light yellow) to enhance the contrast of the polar regions. The Wratten 8 filter is especially effective as a general-purpose contrast enhancer.

Experimentation is the best way to discover which filter works best with a given Jovian feature. For example, I've found yellow filters especially effective for viewing the low-contrast south temperate ovals. Depending on the viewing conditions, observing without a filter sometimes proves to be the best strategy.

The Observations

Of course, even the best telescope fitted with the proper filter is still at the mercy of the churning atmosphere above us. The Association of Lunar and Planetary Observers (ALPO) uses a scale of 0 to 10 to describe seeing conditions, with 0 being the worst and 10 the best. Unless the seeing is better than 5, you will most likely have to wait for another time to do high-power observing.

Jupiter is thrilling to view in just about any telescope. Even a small department-store refractor will reveal several cloud belts and its four brightest moons. Jupiter is also one of the most dynamic telescopic sights — you never get the same view twice. This is partly the result of its rapid rotation — gas-giant planets like Jupiter exhibit differential rotation; that is, they rotate more rapidly at the equator than they do at the poles. Jupiter's observable "surface" has two general systems of rotation that differ by approximately 5 minutes: System I (9 hours 50.5 minutes) and System II (9 hours 55.7 minutes). Most of the planet falls under the System II rotation rate, while System I rotation applies to the Equatorial Zone.

If you want to seriously study Jupiter, you should observe it as often as possible; the more time you spend at the eyepiece, the more adept you will become at seeing the planet's most subtle features.