Combination of 20, 0.02 second exposures.
SBIG ST8XE CCD. 20" f/6.8 Dall-Kirkham cassegrain telescope at prime focus.
Theatetus Rille is located between the lunar Apennines and the lunar Alps, where Mare Imbrium breaks through the mountains and into Mare Serenitatis. It is not an easy object to observe, and requires the Sun to be at the right altitude to cast good shadows to render it visible, These conditions occur close to first quarter and just before 3rd quarter. The rille is often mistaken for Hadley Rille of Apollo 15 fame, however this rille is located some distance to the south and in a more mountaineous region. In the image above Hadley Rille is located near the top of the image, hidden in the shadows of the mountains.