The Cassegrainian Telescope

Sieur Cassegrain, a Frenchman, in 1672 designed a second compound reflector, differing from Gregory's in that it employed a convex secondary, to be of hyperboloidal figure, placed inside of the focus of the paraboloidal primary (Fig. 7).

The image formed at f in this case is inverted. The amplification from the second reflection is, as in the previous case, in the ratio of fs to Fs, and while the Gregorian is seen to be capable of higher magnification, all that is necessary can be had from the Cassegrainian, and it has the advantage of being a much more compact instrument.

Although little was heard of this telescope for the next two centuries, it is worth observing that it survived the Gregorian, and is still widely used in observatories. The principal reason for its early lack of popularity was no doubt due to the difficulty of giving a hyperboloidal figure to the secondary mirror. This difficulty can be avoided by leaving the secondary with a spherical figure and undercorrecting the primary.5 It is also possible to leave the primary spherical, and to make all the correction at the center of the secondary, which will then have an oblate spheroidal form.

Newtonian telescope