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Optical Seeing Measurements with an Optical Telescope on a Radio Antenna

Individual images of a video CCD camera with a 10-cm objective lens recorded for pointing performance tests of a prototype 12-m antenna for the ALMA project have been analyzed to evaluate both antenna tracking accuracy and optical seeing at the NRAO VLA site. Data of star image centroid motion have been compared with readouts of angle encoders and inclinomters on the azimuth axis in the yoke structure of the antenna. Under good tracking conditions, a power spectral density (PSD) of image motion during tracking over about 5 minutes had a Kolmogorov power index of −2/3. At frequencies > 4 Hz, the PSD showed a steeper decline due to a finite exposure time of the video camera (1/30 seconds) than −2/3, which suggests a wind speed of about 3 m s −1 at the level where the main turbulence occurs. At lower frequencies, a flattening of the PSD was observed with a turnover frequency of about 0.05 Hz, which in turn suggests an “outer scale” of about 60 m, a length of large-scale disturbances in Kolmogorov’s model. The image centroid fluctuations observed during all-sky pointing tests showed a dependence on air mass A as A 0.5 and were 0.5 to 0.9 arcseconds rms at the zenith. These measurements suggest that observations for 2–3 seconds can determine a star position with a typical error of 0.3 to 0.6 arcseconds.