Mars

One of the most difficult of astronomical objects to observe with any clarity, the planet Mars (fourth from the Sun) is in our near solar system neighborhood; yet, due to its very small diameter and indistinct surface features -- sometimes blurred by Martian haze and storms -- the 'warlike' red planet offers great visual challenges.

Watching the heavenly dance of Mars across the fixed stars is a fascinating study in celestial mechanics. At opposition (nearest the earth), Mars is a brilliant ochre "star" about 50% brighter than Jupiter. If at perihelion (closest to the Sun) during its opposition, when we are separated by a "mere" 35 million miles, Mars' disk measures about 24 arcseconds in diameter. But at its greatest distance from Earth -- 63 million miles -- the tiny disk measures just over 3 arcseconds!

Like our familiar Moon, Mars displays phases, though the planet's disk is only partially dimmed at minimum illumination. The albedo (surface brightness) can be quite high, and the strong contrast effect of the brilliant orange disk set against the dark sky (confused by a halo of optical reflections inevitable in almost any scope) make the eye prone to an artifact known as irradiation: bright details appear larger and more distinct against a darker background.

Add a bit of atmospheric distortion and the chromatic aberration of refractor telescopes, and Mars' blurry and shimmering image seemed to be graced with 'canals', mistakenly spotted by Percival Lowell and many other 19th-century observers. In fact, there is a myriad of unresolved surface detail: craters and differences in color and reflectivity in the deserts and so-called seas, or marias. However, there are of course NO canals, as confirmed by satellite imaging! (The author once saw this optical illusion: as recounted here in his wife's article about the 2003 Mars opposition.)

Mars' most striking details, visible even with a 60-mm aperture instrument at a favorable opposition, are the polar caps. Bright white patches near the poles, the caps vary in dimension with the seasons, suggesting that they are ices (probably carbon dioxide) forming or sublimating. Great storms arise in the thin Martian atmosphere, with winds up to 300 miles per hour, blowing dry soil into obscuring clouds. At such times, details vanish, though by using blue filters, differences in the clouds' albedo can be discerned. Details of the terrain may be detected at close oppositions, including Solis Lacus (the 'eye of Mars') and the dark Syrtis Major. The telescopic view is a streamlined, smoothed distortion of the crater-pitted surface, with contrasts artificially enhanced by irradiation. Colored filters (particularly orange, green, and blue) help reduce the glare and enhance contrasting features.

Saturn

Deep gasps of delight are heard at every star party during summer and fall when the planet Saturn is visible. Newcomers to astronomy seem to be astonished that the best-known feature of Saturn, its glorious ring system, can be visible in scopes with apertures as small as 50-60mm; unfortunately, such instruments frequently feature bad optics, poor oculars, and shaky tripod mounts.

To see the seventh of our Solar System's planets in all its glory, a scope with an aperture of 7-10 inches may reveal more detail than formerly could be captured in earthbound photos, even if made by the great observatories: at least until recent developments in digital imaging made it possible to eliminate much of the wavery seeing effects. Photos of Saturn from years past did not look as good as telescopic eye-views.

Saturn -- at 75,000 miles diameter -- is second in size of our planets only to its brother gas-giant, Jupiter. The naked eye easily detects the non-stellar nature of Saturn's bright disk: the planet does not twinkle like the stars, since it occupies an angular diameter of 15 to 20 arcseconds (planetary disk) and over 40 seconds (rings.) Binoculars offer the same indistinct view of the ring system that puzzled Galileo: with less than 25-30X magnification, most viewers cannot distinguish the rings separate from the disk of the planet.

The major gap in the ring system, discovered by the 17th century French astronomer Gian Domenico Cassini, is often narrower in apparent angular diameter than the theoretical resolution permitted by the earth's turbulent atmosphere, but thanks to the high contrast of the dark gap against the bright white color of the rings, the Cassini division may be viewed during much of the tilt cycle of the 29-1/2 year orbit of Saturn. The author has seen it clearly in his 80-mm refractor, and has once detected it in a very good 60-mm telescope.

Gigantic 19th-century visual telescopes clarified Saturn's details, enabling astronomers like James Keeler at Lick Observatory to draw details that have only recently been confirmed by satellite imagery. Dr. Clyde Tombaugh, discoverer of Pluto, has often seen the elusive "spokes" in the rings (as did this program's author with the great 36-inch Lick refractor during one glorious September night after one of his wife's "Music of the Spheres" concerts in the dome of the historic scope.)

James Keeler's superb 1898 drawing of Saturn captures the narrow Keeler (also called 'Encke') division in the outer A-ring, which may be detected only during superb "seeing" conditions: the author once spotted this narrow dark band in a 10" Newtonian, using an "apodizing" screen to break up diffraction effects and enhance sharp detail. However, jittery seeing in turbulent air may cause a "doubling" effect that can fool observers, as the outer ring is displaced by atmospheric disturbances.

An ideal image of Mars is shown at left, done by the Hubble Space Telescope (credits: David Crisp & WFPC2 Science Team, Jet Propulsion Laboratory/California Institute of Technology, and NASA.) In contrast, a drawing (made in near-ideal steady seeing by the author, with a 10" f/5.6 Newtonian at about 400X magnification, near the 1988 close opposition of Mars), provided the original source for the simulated eyepiece view. We have had quite a few views of Mars with 8 to 10" scopes, at closest oppositions, that yielded this much detail, using about 400x if permitted by the telescope optics and atmosphere. Note dark Sabeus Sinus, and "Lowell's Band" adjacent to the polar cap.

To the right of Mars is an ideal image of Saturn (produced by the Voyager probe, credit: NASA). Finally at far right is a simulation, sourced by a drawing made by the author using a 10" f/5.6 Newtonian scope under good seeing. Note the subtle color differences. Saturn probably has about this much detail in a good 5 or 6 inch aperture telescope at about 200x, though not this much color. In our 11-inch SCT, however, the color contrast is striking.