光学ポインティング測定システムによるALMA 12-mプロトタイプアンテナ指向精度評価

Abstract

The Optical Pointing Telescope (OPT) of 10-cm has been built to make performance evaluations of a proto-type 12-m radio telescope for the Atacama Compact Array project under actual operating conditions. The OPT system consists of an objective lens with a 2X extender, a peltier cooled CCD video camera, and an image acquisition & control computer. The plate scales were found to be 1.16 arcsec pixel(exp -1). A red filter (R64) enabled to make daytime observations of stars brighter than m(sub v) less than 3.5 magnitude with a typical S/N larger than 20 by image integration for 1 - 3 seconds, which results in an error smaller than 0.1 arcsecond. The antenna testing was done at the VLA site of the National Radio Astronomy Observatory (NRAO) in Socorro, New Mexico, US, where star image centroid motions sampled at 30 Hz at zenith were found to be typically 1.0 arcsec rms in March 2004. Stellar image motions for 300 seconds showed a Kolmogorov - 2/3 turbulence relationship. Positional determination error decreased with N(exp -0.2), where N is the number of frames of consecutive exposures. These results suggest that individual measurements with integration time of a few seconds give an error of star position determinations of 0.4 - 0.6 arcseconds rms. The whole sky absolute pointing error under no wind and during night was measured to be 1.17 arcseconds rms. Pointing hysteresis errors (bi-directional repeatability) were found to be +/- 0.2 - 0.4 arcseconds. Continuous tracking measurements of Polaris during day and night have revealed a large pointing drift due to thermal distortion of the yoke structure. Retrospective thermal corrections were applied to tracking data of several stars for two hours, with a preliminary thermal deformation model of the yoke, and have found the tracking accuracy improved to be 0.1 - 0.3 arcseconds rms for a 15-munite period. The OPT was also successfully used to demonstrate fast switching motion capability of the antenna that enables it to move back and forth between two positions of 1.5 degrees apart within 1.5 seconds and settle to within three arcsecond pointing error. A careful data analysis has revealed, however, that the antenna mount during the fast switching motion displays dumped oscillation at the dumped natural frequency of about 5 Hz with a decay time of about one second.