The K-band Celestial Reference Frame (K-CRF) project has recently published high-resolution VLBI images of 731 AGN sources spanning up to 27 epochs, totalling an impressive 5078 images (de Witt et al. 2023). All these images have been derived exclusively from our Very Long Baseline Array (VLBA) observing sessions. Building on this achievement, we have further expanded our imaging efforts by completing imaging for an additional 5 VLBA sessions. As a result, we have now successfully captured images of 817 sources at up to 33 epochs, resulting in a grand total of 6095 images (available from our K-band imaging database). We are also excited to share that we are in the final stages of completing the imaging process for all VLBA sessions up to August 2023, which will result in a database of more than 16,000 images. 

A detailed analysis of the images has allowed us to determine several quantities that provide useful indicators of the quality of each image and the suitability of each source as a VLBI calibrator (e.g., for astronomical VLBI phase referencing observations at K-band) or a K-band Celestial Reference Frame source (de Witt et al., 2023). In addition, modelfitting has allowed us to determine, for each image, the angular size and radial extent of the brightest and second brightest component and the position angle between them as well as estimates of the overall extent and direction of the source structure. While VLBI images of CRF sources show that, in general, they appear more compact at K-band (24 GHz) than at X-band (8.4 GHz), they can still exhibit measurable extended emission at K-band. To this end, we started a project which aims to apply structure corrections directly to the data during the astrometric analysis process using updated models of the source structure obtained from the imaging and modelfitting (de Witt et al., 2023). This endeavour is only possible because of readily available VLBI images from our dedicated K-band CRF observing campaigns to map and monitor the source structure. Producing a CRF with structure corrections applied to all sessions would be a worldwide first!

Within the framework of our K-band Imaging and Source Structure VLBI Program, our primary objective is to maintain a dynamic and up-to-date database of high-resolution, multi-epoch K-band images. This initiative is driven by the need to consistently monitor the evolutionary patterns of source strength and morphology. By doing so, we continuously assess the suitability of each source for its role as a VLBI calibrator and K-band reference source. However, our aspirations extend beyond merely expanding the historical record of observations and tracking source structural changes. Our broader goal is to delve into a comprehensive exploration of the impact of source structure on the K-CRF and implementing a sophisticated model to correct for the effect of source structure in our astrometric and geodetic analyses. We regard this initiative as a critical step towards refining the accuracy of the K-CRF.