Although he built about 100 telescopes, Galileo considered just 10 of them good enough to use. At their best, they could magnify objects about 30 times.
Galileo’s telescope had several major problems. First, his telescope had a small field of view, which means you could only see a small amount of the sky through it at one time. As Galileo’s telescope magnified an object, it would concentrate on a smaller and smaller area. For instance, after magnifying the moon about 20 times, a viewer could only see about a quarter of the Moon’s surface.
Second, the telescope suffered from spherical aberration. Light rays that pass through the center of a spherical lens focus on a different point than light rays that pass through the edges of a spherical lens. This makes the image fuzzy. Galileo tried to solve this by putting a large lens in a telescope, but covering part of it up with thick paper so the light would pass through only the central part of the curved lens. This made the image clearer, though less bright.