If the rotating mirror R were exactly at the principal focus, the moving image of the slit would remain upon the distant plane mirror M ( equal in diameter to lens L ) as long as the axis of the pencil of light remained on the lens, this being true regardless of the RM distance.
32.
Measuring the point there the laser spot hits the microscope when the spinning mirror is turned off-then measuring it again when the mirror is spinning allows you to figure out how much the angle the mirror turned through during the trip from the first reflection off to the plane mirror and back to the spinning mirror again.
33.
A "'heliostat "'( from " helios ", the Greek word for " sun ", and " stat ", as in stationary ) is a device that includes a mirror, usually a plane mirror, which turns so as to keep reflecting sunlight toward a predetermined target, compensating for the sun's apparent motions in the sky.
