
22 2. Preliminaries: Definitions and Paraxial Optics
These two-mirror designs have the added feature that the system focal length is
easily changed simply by putting in a different secondary mirror, without
changing the physical length by a large factor. As an example, consider a
Cassegrain telescope with parameters Fi =3, p = 0.25, and m = 3. Using
Eqs.
(2.5.1) we find
A:
= 0.3125 and the normalized secondary mirror-focal
surface separation is 0.9375. If we choose to increase the telescope focal length
by a factor of three, hence making m = 9, while keeping j5 = 0.25, then
A:
= 0.125 and mk =
1.125.
The modified telescope is only 1.2 time