Alignment methods for Cassegrain and RC telescope with wide-field of view

Ritchey-Chretien type or Cassegrain type has been widely used for earth-observing space camera. As most earth-observing cameras are required to scan the wide area in a single path, they generally have a wider field of view, compared to the ground-based telescope. However, the alignment of Cassegrain or RC telescope with a wide-field of view is not easy. One reason is that it has a central hole in the primary mirror so that it is difficult to find an optical axis. Another reason is that it can introduce much off-axis aberration such as coma and astigmatism, when it is aligned at onaxis with zero-coma condition. In this paper, we calculate the alignment accuracy using a conventional method for a RC type telescope of which diameter is 300 mm and field of view is 2.08 degrees. We suggest that the most effective alignment method for wide field of view system is a computer-aided alignment. With this method, it was-found that the variation of rms wavefront error of the telescope over the entire field of view was less than 10%.