0.003 seconds exposure, SBIG ST-8E CCD.
18" f/10.5 Ritchey-Chrétien telescope at prime focus.
Cassini is a large, heavily degraded crater on the north-eastern rim of Mare Imbrium. In the telescope, it appears as a large ring of hills surrounding 2 smaller craters. Once seen it is easily recognised.
In the image above, Cassini is close to the right hand edge. To the east (left) is a pair of large craters, Eudoxus and Aristoteles, while to the north (bottom) the Alpine Valley cuts through the lunar Alps. The isolated mountain to the east of Cassini is Piton.
