Maíz Apellániz, Jesús; Cox, Colin
Instrument Science Report ACS 2008-02, 18 pages
We have used ACS/SBC observations of two UV astrometric fields derived from ACS/HRC data to create a new geometric distortion solution for the Solar Blind Channel. The new solution consists of three components: (a)a filter-dependent linear part that takes into account the existence of three epoch ranges in the SBC alignment;(b) the remaining 24 higher-order components of a 4th degree polynomial; and (c) a fine-correction look-uptable. Some of the previous solutions introduced possible errors of several pixels when producing mosaics dueto an incorrect orientation. That issue has been solved and the accuracy due to the solution itself (excludingrotation) has been improved by an additional factor of 1.5-2.0. As a result, it is now possible to measurepositions of medium to high S/N stars with a relative astrometric accuracy of 3-4 mas.