Using an Active Primary Surface to Correct for low-order Manufacturing Errors in Secondary Mirrors of Large Reflector Antennas

摘要

In the fabrication of high-performance, low-cost secondary reflectors for radio telescopes, it is a significant challenge to avoid introduction of low-order surface errors such as astigmatism or coma. This arises primarily because low-order surface errors are easily induced by support structure placement or simple thermal variations in the manufacturing process. It is, of course, possible to bring these errors to within the required tolerance, but if an active primary reflector is present, it may be possible to relax the requirements on the secondary and perhaps lower its cost. In this paper, we take the Large Millimeter-wave Telescope (LMT/GTM) as an example system. We model the effects of correcting a deformed sub-reflector by using the existing segmented active primary. The sub-reflector deformation patterns employed are low-order (e.g., astigmatism or coma), but are allowed significant excursions from the nominal surface figure. For each case, we demonstrate the best theoretical performance, using the active primary to correct for the errors. Additionally, to determine whether such an approach would be practical, we also demonstrate the likely performance improvement that could be achieved using brief measurements on an astronomical source. In this approach, we introduce varying amounts of known low-order deformation patterns into the active primary and seek the combination that results in the maximum signal. Finally, we compare this result to the theoretical maximum and make recommendations on the practical utility of the approach.