Offset Pointing Calibrators for Large Radio Telescopes

Astronomy and Astrophysics – Astronomy

Scientific paper

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Catalogs, Reference Systems, Techniques: Miscellaneous, Telescopes

Scientific paper

We present a catalog of pointing calibrators suitable for offset pointing and for determining the pointing constants of large radio telescopes. It contains 3399 strong, compact, and unconfused radio sources with accurate (σαcosδ~σδ~0.5") positions from the NRAO Very Large Array Sky Survey (NVSS) uniformly covering the sky north of J2000 δ=-40deg. The NVSS images, restored with a θ=45'' FWHM Gaussian beam, were also convolved to larger beam sizes θ=90'', 180", 360", 540", and 720". The catalog lists the maximum beam size θm for which each calibration source remains unconfused and a single Gaussian fit yields an rms position error σ<=θm/100. For all δ>-40deg, the average angular distance to the nearest calibrator is only <φ>~0.03 rad, so offset pointing from these calibrators may reduce slowly varying pointing errors (caused by incorrect values for the traditional pointing constants, gravitational deformations, differential thermal expansion, refraction, etc.) by factors up to <φ>-1~30.

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