Of an object at infinity, the objective supplies a real, upside-down image the size of which is directly proportional to the focal length; the eyepiece, which works as a magnifying glass, supplies a virtual image, still upside down with respect to the object, and its focal length must be short to ensure sufficient enlargement.
As stated in the previous article, since the distance between the objective and the object cannot be appreciably miscellaneousd, focusing is done by varying the distance between the objective and the eyepiece.
In the telescopic system, this distance is adjusted in such a way that the virtual image forms at infinity; thus the focus of the eyepiece coincides with the focus of the objective and the distance between the objective and the eyepiece is equal to the sum of the focal distances.
The telescope on display is a Keplerian refracting, or astronomical, telescope with two drawtubes which when open is 89.5 cm in length and when closed 69.5 cm.
The objective tube is made of wood, it is 64.7 cm in length with a diameter of 8.5 cm; it is equipped with a burnished brass objective holder. The objective lens has a 6.6 cm focal length and a diameter of 7.1 cm. ...continues