News and Views RAA 2020 Vol 20 page 1

Author: Ming Liang

National Optical Astronomy Observatory (NSF’s National Optical-Infrared Astronomy Research Laboratory) 950 N Cherry Ave. Tucson Arizona 85726, USA liang@noao.edu or liangming@gmail.com

The design concept of applying a quasi-Cassegrain system to improve the performance of FAST, the world’s largest radio telescope, discussed in this paper is very creative and innovative. The key point of this concept is to add a secondary and change the main reflector shape. This idea should be originated from “shape variable mirrors" suggested by Ding-qiang Su in 1986.  Since the shape of the reflecting area on the main reflector is actively changed to a hyperboloid (or paraboloid), the image quality is much better than that of keeping the main reflector as a fixed spherical surface. The design is also very practical and feasible. The required deformation of the main reflector of the quasi-Cassegrain system can take advantage of FAST existed surface active control capability. Since the dimension of the inserted secondary reflector is not too large, it can be assembled with the receiver feeds as an integrated package to exchange with the current primary feed cabin.

If such a Quasi-Cassegrain system can be realized on FAST, it will become another highlight of this project which has already been famous for its size. And this will attract the attention of the astronomical world again.