During cycle 15, the calibration program 11021 executed, taking data that will allow more accurate determination of the SIs and FGSs positions in the HST focal plane. This will improve the accuracy of absolute astrometry/pointing with HST. Guidestar Catalog 2 has reduced coordinate error in guidestars and targets by factors between 2 or more, to around 0.25 arcsecond. In order for this to translate into comparably accurate astrometry keywords in the science products, the positions of the FGSs and SIs in the HST FOV must be calibrated to a level well below this. Post-SM4, such calibrations' will increase in blind pointing accuracy, allowing for more efficient COS acquisitions.
This calibration, and future variants of it, determine and maintain V2,V3 positions and orientations of the SIs and FGSs to better than 50 mas and 0.04 to 0.004 degrees of roll.
Figure: A large astrometric field (open cluster M35) is observed in order to determine positions of the SIs and FGS relative to each other.