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.