Back to Volume
Paper: Placing the SIM All-Sky Astrometric Grid onto the ICRS
Volume: 194, Working on the Fringe: Optical and IR Interferometry from Ground and Space
Page: 134
Authors: Gaume, R. A.; Fey, A. L.; Boboltz, D. A.; Johnston, K. J.
Abstract: SIM anticipates using an all-sky astrometric grid for astrometric instrumental calibration and to perform wide-angle astrometry. To the extent that SIM is an astrometric mission, the all-sky astrometric grid is the cornerstone of mission operations. Among the reference objects, the SIM mission has plans to observe of order 100 extra-galactic sources to establish a quasi-inertial anchor for the all-sky astrometric grid. The International Celestial Reference Frame (ICRF) is a quasi-inertial reference frame based on the radio positions of 608 extra-galactic sources distributed over the entire sky. The ICRF is the realization of the International Celestial Reference System (ICRS) at radio wavelengths, and has been adopted by the International Astronomical Union (IAU) as the fundamental celestial reference frame, replacing the FK5 optical frame. The formal positional accuracy of the individual sources defining the ICRF is ~170 microarcseconds. In support of the SIM mission we have examined the 608 radio sources comprising the ICRF to determine suitability for observation with SIM, developing selection criteria based on radio flux density, source position history, and source radio structure, but without regard for optical brightness. Using these criteria we have compiled a preliminary list of ~62 sources roughly evenly distributed on the celestial sphere. Given the anticipated astrometric sensitivity of SIM together with source optical brightness, we estimate that total integration times of 4, 21, and 60 hours will be required for SIM to observe 30, 60, and 90 percent of the 62 source ICRF list. A significant increase in the positional accuracy of the ICRF must be made to adequately support the SIM mission. We outline a modest research effort to accomplish this goal.
Back to Volume