Test Objects

A number of lunar features have become test sites for a telescope's resolution. Probably the most famous features in this regard are the tiny craterlets on the floor of the crater Plato (shown later in this chapter in Figure 10.21), the rilles near the

Figure 10.1a. The major lunar seas. Key (clockwise from top): Nu = Mare Nubium; Hu = Mare Humorum; In = Mare Insularum; OP = Oceanus Procellarum; Im = Mare Imbrium; SM = Sinus Medii; Fr = Mare Frigoris; Va = Mare Vaporum; Se = Mare Serenitatis; Tr = Mare Tranquillitatis; Cr = Mare Crisium; Fe = Mare Fecunditatis; Ne = Mare Nectaris; Au = Mare Australis. Original image captured by Jamie Cooper, using an Orion Optics SPX Newtonian (250 mm f/6.3) and Canon 300D.

Figure 10.1a. The major lunar seas. Key (clockwise from top): Nu = Mare Nubium; Hu = Mare Humorum; In = Mare Insularum; OP = Oceanus Procellarum; Im = Mare Imbrium; SM = Sinus Medii; Fr = Mare Frigoris; Va = Mare Vaporum; Se = Mare Serenitatis; Tr = Mare Tranquillitatis; Cr = Mare Crisium; Fe = Mare Fecunditatis; Ne = Mare Nectaris; Au = Mare Australis. Original image captured by Jamie Cooper, using an Orion Optics SPX Newtonian (250 mm f/6.3) and Canon 300D.

Figure 10.1b. The author's favorite webcam imaging targets referred to in the text. Craters and geological features are indicated in white and Apollo landing sites (11, 12, 14 - 17) in black. Apollo 13 did not land on the moon due to the onboard emergency. In keeping with the visual telescopic view, all images have south at the top. Key (clockwise from top): M = Moretus; CL = Clavius; T = Tycho; SR = Schiller; W = Wargentin; SC = Schickard; PI = Pitatus; M-C = Mercator/Campanus and the Hippalus rille region; SW = Straight Wall; G = Gassendi; ORI = Mare Orientale behind the limb and invisible; C = Copernicus; AR = Aristarchus; P = Plato; A = Alpine Valley; PO = Posidonius; TR = Triesnecker; L = Langrenus; PV = Petavius; TH = Theophilus and Cyrillus; F = Fracastorius; P/A/A = Ptolemaus, Alphonsus & Arzachel crater chain. Original image captured by Jamie Cooper, using an Orion Optics SPX Newtonian (250 mm f/6.3) and Canon 300D.

Figure 10.1b. The author's favorite webcam imaging targets referred to in the text. Craters and geological features are indicated in white and Apollo landing sites (11, 12, 14 - 17) in black. Apollo 13 did not land on the moon due to the onboard emergency. In keeping with the visual telescopic view, all images have south at the top. Key (clockwise from top): M = Moretus; CL = Clavius; T = Tycho; SR = Schiller; W = Wargentin; SC = Schickard; PI = Pitatus; M-C = Mercator/Campanus and the Hippalus rille region; SW = Straight Wall; G = Gassendi; ORI = Mare Orientale behind the limb and invisible; C = Copernicus; AR = Aristarchus; P = Plato; A = Alpine Valley; PO = Posidonius; TR = Triesnecker; L = Langrenus; PV = Petavius; TH = Theophilus and Cyrillus; F = Fracastorius; P/A/A = Ptolemaus, Alphonsus & Arzachel crater chain. Original image captured by Jamie Cooper, using an Orion Optics SPX Newtonian (250 mm f/6.3) and Canon 300D.

crater Triesnecker (Figure 10.2), and the Alpine Valley rille (Figure 10.3). All of these targets are excellent ones for testing that new instrument and, unlike planetary features, they are (literally) cast in stone and available every month. The lunar crater Plato is well known to all lunar observers and is sited at 9° west and 52° north, squeezed between the narrow Mare Frigoris and the northern edge of the Mare Imbrium. Plato can appear almost circular when a southerly libration tilts it nearly 7° toward the center of the Moon's disc. But it appears highly elliptical when northerly librations tilt it 7° toward the north of the disc. Under good seeing conditions and a suitable illumination even a new observer will be able to glimpse half a dozen tiny craterlets on the smooth, dark floor of Plato. The largest of these craterlets is the central one, which has a diameter of three kilometers. The next three largest floor craterlets (two of which are side-by-side) are between two and three kilometers in diameter, but the rest are much smaller. Just how many crater-lets are on the floor of Plato and, specifically, how many can be imaged from Earth? This is a very good question and one that does not have a definite answer. Using a Celestron 11 at a focal ratio of 31, and an AtiK webcam, the U.K.'s Damian Peach has resolved 22 Plato craterlets, the smallest ones being barely more than 0.5 kilometers across. This translates into an angular resolution of about 0.3 arc-seconds, or slightly below the theoretical resolution of a 28-cm (11-inch) aperture. But this by no means represents the limit of Earth-based resolution. Bruno Daversin, using a 60-cm f/16 Cassegrain at the Ludiver facility near Cherbourg in northern France (l'Observatoire Planetarium du Cap de la Hague, www.ludiver.com) has recorded 100 tiny craterlets on Plato's floor, some only 0.3 kilometers across. At these tiny

Figure 10.2. Triesnecker and its rille system imaged on September 5, 2004, with a ToUcam Pro webcam and a 250mm f/6.3 newtonian at f/38. The field spans 110 kilometers. Image: M. Mobberley.
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