During reduction of the Ultra Deep Field in 2004, it became apparent that the distortion solution constructed in 2002 did not allow images taken at different orientations to be mapped into each other with simple rectilinear transformations. In an effort to examine the time dependence of the WFC distortion, I cross-compare the many observations that have been taken of the outer calibration field of 47 Tuc. I find that the linear skew terms have changed monotonically since ACS was installed in 2002. The size of the change is appreciable: a star at the corner of the detector in late 2006 will be about 0.3 pixel off from where the original 2002-based distortion solution says it would be. These skew errors cancel out when relating data sets taken at the same epoch and same orientation, but they must be accounted for when dealing with observations taken at different roll angles. A simple correction to the 2002 solution is provided.
Instrument Science Report ACS 2007-08
- Pub Date:
- July 2007
- Hubble Space Telescope;
- Space Telescope Science Institute;
- Distortion Solution;
- WFC Distortion