You move the tester to bring the light close to the block and near the
center of curvature. Then, slide the block up or down so the light returned by the main mirror is at the height of the viewing aperture.
It has a
center of curvature, and as long as the centers of curvature of the secondary elements are in line with that of the primary we are in good shape.
The test is quantitative since the fringe spacing corresponds to 1/2 wave on a mirror's surface when the interferometer is located at the
center of curvature. If we are testing a telescope by autocollimated light with the help of an optical flat, the fringe spacing equals 1/2 wave on the wavefront.