Pointing, Tracking, Guiding, and Dome Seeing at the AAT

Abstract

The Anglo-Australian Telescope (AAT), though based on the design for the Kitt Peak and Cerro Tololo 4 metre telescopes, differs in many significant respects. E.g. its declination axis is offset from the polar axis and its polar axis structure is much stiffer. Also the computer was, from the outset, more closely integrated with the telescope drive and control than was previous practice. The sound structural design, good gearing, and accurate mirror supports, together with thorough computer calibration, allowed the AAT to set new standards in pointing by the time it was fully operational in 1975. Good pointing saves time in acquisition, greatly aids day time observing, and leads to good blind tracking. Currently, calibrations covering the whole observable sky give rms radial residuals very near 2 arc sec. Blind tracking errors are generally within ± 1 arc sec for 1 hour or more and natural frequency oscillations less than 0.1 arc sec peak to peak. Better tracking is, of course, available using the offset autoguiders, which employ image dissectors and function on stars to about 15m. Intensified SEC Videcon television cameras are used for acquisition and for guiding with light reflected from spectrograph slit jaws.

Initially, the AAT seeing suffered from poor matching of inside and outside air temperatures because the intended active cooling of dome air was cut from the installation to save funds, leaving an inadequate capacity for forced ventilation. Correlation of observed seeing with dome temperature excess indicated degradation of about 1/2 arc sec/°C. In 1980 the forced ventilation was upgraded to change the dome air in less than 4 minutes, with optional flow direction, upwards (exhausting out the observing aperture) or downwards. Measurements of seeing within the dome, but with the dome open, now show degradations typically 0.4 and 0.6 arc sec diameter with upwards and downwards ventilation respectively. Stellar seeing measurements, including routine observations in which the ventilation direction was alternated nightly, have also indicated superiority of the upwards flow. Taking advantage of this, filters can now be installed on the air supply to keep the optics cleaner.