Pointing Calibration of a High-resolution Millimeter-wave Antenna by Star Observations

Book excerpt: An extensive pointing calibration program for the drive system and mount of the AFCRL 29-ft millimeter-wave antenna is described. The optical pointing accuracy was determined by measuring the positioning errors of the optical axis of the antenna. Since all sources of optical pointing errors produce equivalent RF errors, all RF errors except those produced by feed system and reflector asymmetry are determined. A brief description of the antenna together with the integrated IBM 1800 data aquisition and control system, tracking software, and an accurate time-synchronization method are presented. The star tracking program, which includes a method of star selection, precession correction, and refraction correction, is described. The analysis and derivation of the various pointing error parameters used to determine the best-fitting functions that characterize the positional errors are presented. Next, the star data are reduced by fitting the nightly error measurements to the best-fitting error function from which the values of the error parameters that take into account the servo, collimation, azimuth plane tilt, elevation axis skewness, and residual refraction are determined. Finally, the results of the star data analysis and statistical data are presented. The rms residual optical pointingg error, after pointin calibration, averaged about 3 percent of the antenna beamwidth, that is, 7.5 arc seconds in both azimuth and elevation. (Author).