Imaging of more than 730 K-band celestial reference frame sources at up to 87 epochs

We present an overview of high-resolution images of more than 730 AGN sources from the K-band (24 GHz) celestial reference frame (CRF), which is based on observations made with the VLBA (supported through the USNO’s 50% timeshare allocation) from 24-hour sessions observed between 2015 and February 2023. Many of these sources are imaged for the first time at frequencies above X-band (8 GHz) with sub-mas resolution. The K-band images provide information about the source properties and for sources observed over multiple epochs they provide insights into the temporal behaviour of the sources. The use of K-band was motivated by the opportunity to access more compact intrinsic source morphology at that higher frequency as well as to leverage the factor of three improvement in interferometer resolution relative to the historically standard S/X-band (2.3/8.4 GHz) used for much reference frame and calibrator work. As most of the sources imaged here are in the K-band component of the 3rd International Celestial Reference Frame (ICRF-3), these images serve to characterize the objects used in that IAU standard. Lastly, we also compare images and astrometry from S (2.3 GHz), X (8.4 GHz), K (24 GHz) and Q-band (43 GHz) in order to study the astrophysical differences and determine the optimal frequency band for celestial reference frame observations.