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Comments on “Suggested quasi- Cassegrain system for multi-beam observation of FAST"


Comment on “Suggested quasi- Cassegrain system for

Author: Prof. Gerard R. Lemaitre

  1. Aix Marseille Universite - AMU Laboratoire d'Astrophysique de Marseille - LAM/AMU 38 rue Fréderic Joliot-Curie, 13388 Marseille CEDEX 13, France Email :

I found the paper “Suggested quasi Cassegrain system for multi-beam observation of FAST” excellent and well presented. The proposed quasi-Cassegrain system (i.e., RC design or aplanatic design) would bring about a high gain in resolution-detection and allow the use of many detectors at the field of view, thus it shows diffraction-limited performance for frequencies as mentioned in the RAA paper.

The Arecibo radio telescope design includes a three-mirror system (M1 a spherical reflector, M2 a concave reflector on a 900-ton large movable platform) and a detector in drift-scan recording – as the authors briefly mention in italics on p.1 of RAA. This allows correction for only spherical aberration. M1-M2 forms a two-mirror Gregory design and M3 projects the focal surface backwards close to the M1 focus. The system provides a small field of view, is just “stigmatic”, and then suffers from coma aberration.

In the proposed quasi-Cassegrain system, FAST uses M2 mirror and focal surface that both would require a movable platform. If one assumes that M2 is rigidly connected to the Cassegrain focal surface (FS2), the platform should continuously control tilts, focusing, and x and y translation decenters of FS2.

However, the lateral displacements in translation (or decenters of FS2) of the platform should be minimized to small values if the platform design is arranged so that it rotates around the prime focus of the M1 300m aperture paraboloid which lies itself close to the axis of the 500m aperture. This would lead to a platform design much smaller and lighter in weight than that of Arecibo. This would be an unrivalled advantage for FAST with quasi-Cassegrain.