Astronomers require larger and larger radiation-gathering apertures for their telescopes to penetrate farther into the mysteries of the universe. Large telescopes collect more photons and thus see fainter objects. These large apertures also increase the angular resolution across the object, as we saw in Chapter 10. For practical reasons such as telescope mass and thermal inertia, astronomers who build ground-based telescopes now partition primary mirrors greater than about 8 m into smaller segments, with each segment aligned so that its surfaces fall on the desired concave surface of the larger mirror.
Large space- and ground-based telescopes are often divided into two stages. Stage 1 is the large collector, which forms an imperfect image in complex amplitude and phase. Before an image is detected, stage 2 of the optical system takes this imperfect image and removes wavefront aberrations using WFSC. This principle was discussed in Chapter 11. In Chapter 5 we learned that the number of reflections in the optical system needs to be small in order to minimize the power that is absorbed by the mirrors and that does not reach the focal plane. Consequently, rather than correcting the wavefront at an image of the telescope pupil, many telescopes correct the wavefront at the secondary mirror of the astronomical telescope before the primary focus. Correcting the wavefront at the secondary limits the FOV of the telescope that can be corrected. However, most astronomical applications today use a narrow FOV (limited by the expensive digital array detector size or by atmospheric turbulence), so wavefront control (WFC) at the secondary is sufficient.
Frequently, the optical system for large telescopes is divided into three parts: the large precision telescope radiation collector, a metrology and WFSC system (see Chapter 11), and the instrument that analyzes the radiation. The motivation behind this configuration is to reduce the cost of the telescope system. For ground-based telescopes, the two expensive subsystems are the dome and the very stiff massive mechanical structure required to hold the mirror, the secondary, and its supports.