Optimized unit telescopes for interferometric arrays

The resolution of a conventional telescope is determined by the spatial extent of the collecting surface, usually the primary mirror. Astronomical interferometers achieve increased fine detail by using unit telescopes spaced over large distances to increase the spatial extent. The required wavefront quality places very tight tolerances on the unit telescopes and they should be designed with the prime goal of meeting the wavefront specification. The unit telescope must be optimized for the role of a beam compressor rather than attempting to modify a conventional design. Two alternative designs that minimize the number of reflections in the telescope will be considered, a crucial feature in obtaining the lowest possible wavefront error and maximizing throughput. The first, a siderostat has fixed imaging optics and a large steerable flat mirror to enable sky tracking. The second, an "Alt-Alt" system consists of two intersecting altitude axes in a "gyroscopic type" structure. A small flat lies at the intersection of the altitude axes to direct the starlight at a constant height and direction out of the telescope. The benefits and limitations of each are shown along with the key design issues that determine the most appropriate unit telescope for implementation in an interferometric telescope.