Telescope

We want to form the images of distant objects such as planets. For this purpose, we use an optical instrument called telescope.

There are two kind of telescope:
The refracting telescope which use the combination of lenses, and the reflecting telescope that uses a curved mirror and a lens. Inthe two systems, we use the technique that the first image is the object for the second optical element.

We study here the refracting telescope.



If the object is placed at left infinity, its image will be located exactly at the right focal point fo of the first converging lens.

We place the second converging lens of fcal point fe to have approsimately fe = fo.
Therefore:
Tne angular magnification m is the ratio of the angular size seen with the telescope to the angular size without telescope. (with the unaided eye).
m = θ/θo = (ho/fe)/(ho/fo) = fo/fe

&heta;o is the angle subtended by the distant object at the objective, and θ is the angle subtended by the final image The image is virtual, so m is negative.

m = - fo/fe