The method employed consists of measuring the stellar wavefront slope angle differences over two or more small pupils, separated by some distance. This method, being a differential one, is inherently insensitive to telescope tracking errors. This feature is extremely important in the case where relatively small telescopes are used in exposed outdoor conditions, thus subject to wind buffeting.
The primary telescope optics consist of an all- spherical- surface design, modified- Maksutov system, having a primary mirror, secondary mirror, and a thick meniscus corrector lens assembly, mounted close to the secondary. These optics form a conventional Cassegian focal plane image of the star through the central perforation in the primary mirror. The NSM Hartman screen attached at the front of the telescope tube, contains four (4) sub apertures, in an equally spaced array. Each aperture is 10 cm in diameter. The screen can easily be removed from the front of the telescope tube if necessary, for example, to replace it with one having different- sized apertures. The ancillary optics located in the backplane area form a relayed image of the pupil and hence the sub apertures at a convenient location on the backplane optical bench at this reimaged pupil plane. A precision adjustable optical wedge assembly is located at the pupil image plane. The wedges diverge the respective pupil beams to form four separate images at the camera.
A seeing measurement is computed from a set of 100 images taken at approximately three frames per second (presently being upgraded to ~200 frames per second). The positions of the four star images are measured in each frame of the set and the differential motion between frames computed. The atmospheric condition required to produce this level of movement is interpolated from a pre- computed simulation of the system and the atmosphere. The result is the Freid parameter, Ro, for the sample period.