The implementation of an astrometric correction in catcorr is a temporary measure designed to support the bulk reprocessing of data during 2012/3 to make the 3XMM catalogue. It is expected that in due course this correction will be applied via an update to the calibration system.
A recent analysis by Mike Watson and Andy Read (University of Leicester) has identified a small positional shift which is a function of off-axis angle. The effect is of the order of 0.65 arc-seconds at off-axis angles of 15 arc-minutes, near the edge of the field, in the sense that the real position is closer to the axis than that originally measured by elmdetect. If the task parameter astcorr is set to yes (its default value is no), this correction is applied to the RA, DEC values in the OBSMLI file. The empirical formula is:
yoff = c(0) + c(1) * r + c(2) * r**2 + c(3) * r**3sysWhere r is the off-axis angle from the pointing position in arc-minutes, and yoff is then the offset in arcseconds, which is subtracted from the original off-axis angle. The current coefficients of the cubic are:
real :: c(0:3) = (/+0.0158736, -0.036454, +0.00942358, -0.000277756 /)
