During the past several months, we have discussed Galileo's most important discoveries made with telescopes of his own manufacture.
Although lenses had been known in the western world about 300 years before, their main use was for the making of spectacles. The earliest telescope patent was filed in 1608 by, among others, Hans Lippershey of the Netherlands.
Galileo made his own version of this device in the summer of 1609. It consisted of a convex lens at the front of the telescope and a concave lens where the eye was placed. It magnified objects about 3 times.
Not much could be seen with these instruments so Galileo produced better versions in the next few months.
When Galileo turned his telescope to the skies in the fall of 1609, he used a telescope which magnified objects about 20 times. This much more powerful instrument helped him be the first to see Jupiter's moons and other astronomical wonders.
The telescope was about 1 meter long and only the central part of the convex lens was used so images were not very sharp and bright. A small field of view of about one quarter of a degree made it difficult to find objects and keep them in the field of view.
It is therefore not surprising that Galileo did not see all of Jupiter's large moons on January 8, 1610. Check out the February 2009 Starry Night Times to learn more.