Galileo Galilei was the first astronomer to use a telescope to study the heavens. Galileo made a number of observations that finally helped convince people that the Sun-centered solar system model (the heliocentric model), as proposed by Copernicus, was correct. What is a Galilean telescope?
The original design Galileo Galilei came up with in 1609 is commonly called a Galilean telescope. Galileo’s telescope with which Jupiter's moons could be observed was made with a plano convex objective (the lens toward the object) with a focal length of about 30 to 40 inches, and a plano-concave eyepiece with a focal length of about 2 inches. The eyepiece was in a small tube that could be adjusted for focusing. The objective lens was stopped down to an aperture of 0.5 to 1 inch. , and the field of view was about 15 arc-minutes (about 15 inches in 100 yards). The instrument's magnification was 15-20. The glass was full of little bubbles and had a greenish tinge (caused by the iron content of the glass); the shape of the lenses was reasonable good near their centers but poor near the periphery (hence the restricted aperture); the polish was rather poor. The limiting factor of this type of instrument was its small field of view--about 15 arc-minutes--which meant that only a quarter of the full Moon could be accommodated in the field. Over the next several decades, lens-grinding and polishing techniques improved gradually, as a specialized craft of telescope makers slowly developed. But although Galileo’s telescope, and variations of it, were made with higher magnifications, they were practically useless because of the small field of vision.
How does a Galilean telescope work?
The Galilean telescope was innovative in that he was the first to expand the range of magnification of the new spyglasses beyond 3X, using his particular set of lenses. In Sidereus Nuncius, Galileo described how these two lenses served to magnify an object.
Optical diagram of Galilean telescope y - Distant object ; y’ - Real image from objective ; y’’ - Magnified virtual image from eyepiece ; D - Entrance pupil diameter ; d - Virtual exit pupil diameter ; L1 – Objective lens ;L2 - Eyepiece lens e - Virtual exit pupil - Telescope equals Parallel rays of light from a distant object (y) would be brought to a focus in the focal plane of the objective lens (F' L1 / y’). The (diverging) eyepiece (L2) lens intercepts these rays and renders them parallel once more. Non-parallel rays of light from the object traveling at an angleα1 to the optical axis travel at a larger angle (α2 > α1) after they passed through the eyepiece. This leads to an increase in the apparent angular size and is responsible for the perceived magnification. The final image (y’’) is a virtual image, located at infinity and is the same way up as the object. The reason for the surprise and the main points of the geometry of the Galilean telescope are shown in this diagram.
The eye and the negative eyepiece are to the left. The positive objective is on the right. Whether you like it or not, the eyepiece forms an image of the objective lens and this is shown as the oval shape between the two lenses. The significance of this image is that it serves as the exit pupil for the telescope, which means that every ray of light leaving the eyepiece appears to come from it. In the case of the Galilean telescope, the eye must necessarily be to the left of the eyepiece and some distance from the exit pupil. This leads to a tunnel vision effect which is very...
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