Figure 10.3. The Alpine Valley and its rille imaged by Damian Peach on March 1st 2004 with a Celestron 11 SCT and ATiK 1HS webcam.
angles, so close to the telescope/signal-to-noise limits of the equipment, stacking as many images as possible is essential, even on such a high-contrast feature as the Moon. Tiny features only emerge when noise levels are low and noise decreases with the square root of the number of images stacked. On nights of poor seeing, especially near full Moon, even Plato's central craterlet may be hard to spot. However, under good seeing even a 25-cm telescope can reveal a dozen or more craterlets to the visual observer.
The rilles close to the small crater Triesnecker have long been a test of both optics and atmospheric stability for the lunar observer. Back in June 1981, Sky & Telescope featured a remarkable photograph of the Triesnecker rilles (page 512), taken by U.S. amateurs Thomas Pope and Thomas Osypowski in 1964. Using a 31.8-cm (12.5-inch) reflector at f/60 they had resolved craterlets only two or three kilometers across in the Triesnecker region. The one-second exposure was one of the most remarkable of its era and yet these days it could be equalled with a 100mm telescope and a webcam!
Triesnecker itself is only 25 kilometers in diameter and the widest of the Triesnecker rilles are only one arc-second (less than two kilometers) across. In addition, there is an assortment of tiny craterlets all around the region, varying from three to less than 0.5 kilometers in diameter and these provide an excellent series of resolution test objects. Once again, Bruno Daversin has achieved the best image of the region (at least the best I have seen) with the 60-cm f/16 Cassegrain of the Ludiver Observatory. Incidentally, the crew of Apollo 10 took a breathtaking and historic photograph of the Triesnecker crater and rilles, from a distance of 140 kilometers, in 1969. Comparison of the region with Daversin's photograph can be most instructive.
Unlike Plato's craterlets and the Triesnecker rilles, the Alpine Valley rille is a single, highly elusive feature that only the keenest eyed amateurs had recorded visually before the webcam era. Indeed, even as recently as the 1940s, some observers doubted its existence: it is very elusive unless seeing conditions are excellent. The Alpine Valley is situated at 49°N, 3°E and is more properly called Vallis Alpes. Almost 180 kilometers in length it cuts through the Montes Alpes that separate the Mare Frigoris from the Mare Imbrium. The tiny sinuous rille on the valley floor can be glimpsed in a 15-cm telescope, although I have rarely seen it in any instrument. It barely exceeds 1.5 kilometers in width (approx 0.8 arc-seconds) along its length and runs at a 45° slope to the north-south line, so can never be as favorably illuminated as the Triesnecker rilles. In terms of a resolution target it is, perhaps, the ultimate that the aspiring lunar imager can strive for.
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