Offset pointing calibrators for large radio telescopes

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 (sigma (alpha) cos delta approximate to sigma (delta) approximate to 0."5) positions from the NRAO Very Large Array Sky Survey (NVSS) uniformly covering the sky north of J2000 delta = -40 degrees. The NVSS images, restored with a theta = 45" FWHM Gaussian beam, were also convolved to larger beam sizes theta = 90", 180", 360", 540", and 720". The catalog lists the maximum beam size theta (m) for which each calibration source remains unconfused and a single Gaussian fit yields an rms position error sigma less than or equal to theta /100. For all delta > -40 degrees, the average angular distance to the nearest calibrator is only [phi] approximate to 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 [phi](-1) approximate to 30.