A High Precision Telescope Pointing System

A novel method of telescope pointing has been developed based on He-Ne laser interferometric measurements of mirror position. We have applied this technique to a Pfund-type telescope, which consists of a fixed parabolic mirror illuminated by a flat mirror which rotates in altitude and azimuth. If the parabolic mirror is stationary the angle of pointing depends only on the change of orientation of the flat mirror with respect to the parabolic mirror. The relative angles of mirrors with radius of 1 meter are measured to a precision of ~.05 arcsec. Fluctuations in the index of refraction of the atmosphere between the mirrors are the primary source of this limit; however, changes in pointing position involving only a gradient in the index of refraction perpendicular to the optic axis are largely compensated by this pointing technique. Conversion of interference fringe counts to precise angle of pointing involves solutions of equations of modest complexity, but is easily handled by a small computer.