Instead of a concave lens near the eye ... Kepler's advance created a new challenge, however. Because his telescope offered a much greater field of view, astronomers had plenty of room to improve ...

The central feature of the telescope is an exquisitely polished, concave primary mirror, 2.4 meters (7.9 feet) in diameter. The primary mirror sits beneath a smaller, secondary mirror that ...

One of the biggest limitations of Galilean telescopes, with their combination of a convex and concave lens, was their narrow field of view. Galileo used them to survey the lunar surface ...