What does an adaptive primary do?

An adaptive primary is a concentrating mirror that redirects sunlight to a fixed focus—no matter where the sun appears in the sky. A mirror that accomplishes this task must adapt its curvature as the sun rises higher or sets lower in the sky. To a first approximation, the curvature of the mirror is toric, that is, the mirror is always approximately shaped like a small patch on the surface of a torus. The adaptation required is slight, the changes in curvature will be scarcely visible to a viewer looking sideways at the mirror.

In the animation above—rendered in POV-Ray using Mega POV—the zenith distance of the sun varies from 15°  to 75°, while the adaptive primary acquires a toric curvature by the thin-shell bending of an initially spherical mirror. The POV-Ray scene description file for the animation is here.

In addition to changing its curvature, an adaptive primary needs to follow the sun in two angular dimensions: turning to face the sun's azimuth while also tilting its normal to one half the sun's zenith distance. (At the most an adaptive primary only needs to be tilted 45° away from horizontal.)