Two-mirror heliostats

Optical diagram of a two-mirror, or telescopic, heliostat. All profiles are parabolic and confocal. The primary mirror is gimbaled and bendable to accommodate the changing angles of the sun. The secondary mirror is fixed.

Definition: In solar engineering, a two-mirror heliostat, or telescopic heliostat, is a heliostat composed of two off-axis, parabolic mirrors, arranged in the configuration of a Mersenne telescope.

The larger primary mirror of a two-mirror heliostat is gimbaled and thin-shell bendable to accommodate the apparent movement of the sun. The small secondary mirror, which redirects concentrated sunlight toward the target, is rigid and fixed. The advantage of two-mirror heliostats over conventional one-mirror heliostats is that they can be packed closely together without incurring blocking losses—even when aiming at a target of low angular elevation. The optics and tracking motions of two-mirror heliostats are identical over the entire field.

The telescopic heliostat as a Mersenne telescope

Optical diagram of a Mersenne telescope. This is Mersenne's illustration of 1636 reoriented to be observing the zenith. Image quoted from Fred Watson, "Stargazer: the life and times of the telescope."

I learned recently that the afocal, parabola/parabola Cassegrain telescope is more properly termed a Mersenne telescope. Marin Mersenne's 1636 design for a two-mirror, reflecting telescope is the basis of the telescopic heliostat. In a telescopic heliostat, both mirrors of the Mersenne telescope (the objective, or primary; and the eyepiece, or secondary) are off-axis parabolas. The objective of a telescopic heliostat also operates with varying off-axis angles of incidence and in consequence must be able to bend to adjust its profile.