The Galilean Telescope

Very soon, spectacle makers and scientists up and down Europe, learning of Lippershey's invention, were making similar instruments. Notable among the scientistswas Galileo Galilei, the great Italian physicist and astronomer, who fitted a plano-convex and a plano-concave spectacle lens into opposite ends of a lead tube, making a telescope that magnified three times.

They [the objects] appeared three times nearer and nine times larger in surface than to the naked eye," wrote Galileo. He experimented further and improved this erecting telescope as well as was possible with simple lenses, carrying the magnification up to 30 or more. This was about the limit of its usefulness, however, on account of the great reduction in the size of its field of view.

The invention of spectacles, which in the course of time led to the telescope,
was due perhaps to one Signor Salvino Armato, according to an inscription on
his tomb: "Here lies Salvino Armato d'Armati of Florence, inventor of spectacles. May God forgive his sins. The year 1317."

Ordinarily, rays from a distant object AB would, after refraction through the objective lens 0, meet to form an inverted image ba in the focal plane, but by interposing the concave eye lens E in front of that plane the rays are caused to become divergent, as though they had proceeded from the points A'B', where a virtual image of the object is formed. This image is erect and magnified; the amount of magnification is the ratio of angle c' to angle c.

As the eye pupil can hardly embrace all of the rays emerging from lens E, only part of the actual field shown can be utilized. Also, the exit pupil (explained in Chapter IX) is located inside the instrument. The field of view thus depends on the size of the eye pupil, and on the diameter of the objective lens. The Galilean telescope is found today in the form of opera and field glasses, but employing quite moderate magnification: 2 to 3 power in the opera glass, and 3 to 6 power in the field glass.

Keplerian Telescope