Sky & Telescope

observer’s log Beyond the Dawes Limit: Observing Saturn’s Ring Divisions In theory it can’t be done, yet under optimal seeing conditions you might discern delicate ring structures through surprisingly small telescopes. By Thomas Dobbins and William Sheehan century ago a heated de- bate was raging about the network of canals crisscross- ing the globe of Mars, first Areported by the Italian astronomer Gio- vanni Schiaparelli in 1877. Percival Low- ell, the most fervent and eloquent advo- cate that the canals were real and not some sort of optical illusion, described them as “little gossamer filaments only . but threads to draw your mind across the millions of miles of intervening void.” One of the arguments advanced by skeptics was that the canals — at least in the form of the exceedingly fine lines described by Lowell — were far too narrow to be resolved by his telescope. Some cited the oft-quoted Dawes limit, which states that the resolving power of a telescope in arcseconds can be calculated by dividing 4.56 by the telescope’s aperture expressed in inches. This empirical for- mula was derived in the 1860s by a keen- eyed British amateur, William Rutter Dawes, following an extensive series of observations of double stars of similar brightness through a variety of modest but optically excellent refractors. 1 Lowell and his allies countered that 0.03 arcsecond — ⁄14 the Dawes limit — This spectacular false-color image of Saturn’s this was a case of comparing apples and even when handicapped by moderate at- ring system was created by A. Tayfun Oner oranges. Granted, the Dawes limit might mospheric turbulence. using images snapped by Voyager 2 on Au- be valid for point sources of light seen So it turns out that had there actually gust 23, 1981. Several of the lesser-known against a dark background (like double been canals on Mars, they could have dark ring divisions seen here are within reach stars), but the eye’s ability to resolve lin- been resolved through surprisingly mod- of ordinary amateur telescopes. ear objects against a bright background est telescopes. And there are very real is far greater. Lowell conducted experi- “little gossamer filaments” to be seen The Cassini Division ments on the threshold visibility of tele- elsewhere in the solar system — the divi- Discovered by Gian Domenico Cassini in graph wires with the naked eye and de- sions in the rings of Saturn. Like the 1675 with an unwieldy nonachromatic termined that they could be glimpsed Martian canals, the reality of many of refractor of about 2.5 inches aperture when their apparent diameter was only these elusive features was the subject of and a focal length of 20 feet that magni- 1 0.7 arcsecond — ⁄50 the Dawes limit. The controversy for many years. Right now, fied 90×, the most prominent division in Harvard astronomer William H. Picker- with Saturn riding high in the sky and the rings is 4,700 kilometers wide. At the ing found that he could detect a human the rings tilted open as much as 24°, mean opposition distance of Saturn, it hair through an 11-inch telescope at a backyard astronomers are challenged to subtends a span of only 0.7 arcsecond, so distance of nearly a quarter mile, when coax the utmost in resolution from their Cassini managed to achieve a 2.6-fold its apparent width was reduced to only telescopes. improvement over the Dawes limit. Sky & Telescope November 2000 117 observer’s log observer’s Left: This 1837 drawing of the rings of Saturn by Johann Franz Encke depicts the broad, dusky feature in the middle of ring A that he saw through Berlin Observatory’s 9.6-inch Fraunhofer refractor, the telescope used to discover the planet Neptune nine years later. Note the closely spaced divisions that he recorded near the inner edge of ring B. From Mathematische Abhandlungen der Königlichen Akademie der Wis- senschaften zu Berlin, 1838. Right: James Keeler’s drawing of Saturn through the 36-inch Lick refractor shows the hairline void (arrowed) that now bears the name Encke Division. Exhibited at the Chicago World’s Fair in 1893, today this famous rendering hangs in the foyer of Allegheny Observatory in Pittsburgh, where Keeler served as director from 1891 to 1898. Cassini’s instrument was one of the was verified by Pioneer 11 in 1979. Then of Berlin Observatory, repeatedly saw a monster “aerial” telescopes for which the the Voyager spacecraft revealed the pres- broad, low-contrast marking in the mid- 17th century was notorious. Despite its ence of narrow ringlets composed of 10- dle of ring A through the observatory’s small aperture, its very long focal length meter-diameter boulders of dirty ice re- 9.6-inch Fraunhofer refractor, and he no doubt made the telescope difficult to volving within the Cassini Division. These was even able to measure the position use for protracted observations. Never- difficult and varied observations are well of the feature with a filar micrometer. theless, its optical quality must have been worth attempting to repeat in the weeks The inner boundary was located about far better than is generally assumed. before and after opposition when the one-third of the distance from the outer Nebraska amateur David Knisely has shadow of the globe is prominent. edge of the Cassini Division to the outer found that it is difficult to surpass Cassi- edge of ring A. ni’s feat, even with modern telescopes. Confusing Nomenclature Dawes was one of the first observers to Curious about the minimum aperture Known as ring A in modern parlance, confirm the feature reported by Encke. required to discern the Cassini Division, the ring exterior to the Cassini Division Observing with a 9-inch Newtonian re- on a night of steady seeing Knisely ex- is 14,600 kilometers wide and has a steel flector owned by his friend William Las- perimented with a series of off-axis aper- gray hue reminiscent of the bluing on a sell on a hazy but exquisitely steady night ture stops fitted over the tube of his 10- rifle barrel. As early as the middle of the in September 1843, Dawes found “the inch Newtonian reflector. Holding the 18th century, the Scottish optician James outline of the planet was very hard and magnification constant at 141×, he grad- Short thought he saw ring A divided by sharply defined with power 450,” afford- ually reduced the aperture in 10-milli- several dark lines, but no other observer ing “by far the finest view of Saturn that meter increments from 80 mm to 50 could confirm his suspicion until well I was ever favored with.” The dark divi- mm. With a 60-mm (2.4-inch) aperture, into the following century. In 1823 the sion was “pretty obvious.” He estimated he could detect the Cassini Division only Belgian astronomer Lambert Adolphe its width as one-third that of the Cassini in the ansae (tips) of the rings, but with Quetelet was able to see the outer ring Division and its location as “rather out- the aperture reduced to 50 mm he could “divided in two” through a 10-inch re- side the middle” of ring A. no longer make it out with certainty. fractor at Paris Observatory. Two years Observing from Malta with a 24-inch In 1851 William Stephen Jacob, while later, the British amateur Henry Kater’s reflector nine years later, Lassell also saw observing in India, reported that the 6-inch Newtonian reflector at a magnifi- ring A much as Encke had depicted it: Cassini Division appeared slate-colored cation of 280× revealed three “dark divi- “No division, properly so called ...but through Madras Observatory’s 6.2-inch sions, extremely close, one stronger than there was an evident shade in the middle refractor at 365×, and that he was able to the rest dividing the ring about equally,” of its breadth and occupying about one- trace the jet-black shadow cast across it flanked by narrower divisions on either third of it.” At the close of the 19th cen- by Saturn’s globe. A half century later the side, one of which was close to the outer tury the renowned planetary observer French astronomer Camille Flammarion edge of the ring. Kater never again suc- Eugène M. Antoniadi characterized the found that during moments of very ceeded in catching more than uncertain division as “a zone of a rarefication of steady seeing his 10.2-inch refractor glimpses of these delicate markings and particles” and took pains to depict it as a showed the Cassini Division as dark gray, concluded that they were probably not diffuse shading. This appearance is with- causing him to speculate that “there is permanent features. in the grasp of a first-rate 5-inch tele- probably some matter in it.” This surmise In 1837 Johann Franz Encke, director scope on a very steady night and should 118 November 2000 Sky & Telescope Keeler Gap Ri Encke Division ng A “Encke M inimum” Cassini Division R ing B Ri ng C Maxwell Gap The principal components of Saturn’s ring system are shown in this illustration based on Voy- Advertisement ager images. The narrow gap in ring A that was described by Keeler has been named for Encke, and the broad, diffuse shading that Encke actually saw is now informally referred to as the “Encke Minimum.” Note the four ringlets within the Cassini Division, each measuring about 500 kilometers across. 1 prove fairly easy through a decent 8-inch only ⁄50 that of the Cassini Division.

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