A new command called pol2noise has been added to the Starlink SMURF package. It allows the error values stored in a POL2 vector catalogue to be checked for consistency. For instance, to check the errors on the Q column, do:
% smurf
% pol2noise mycat.FIT Q
replacing myfit.FIT with the name of your vector catalogue (“Q” can be replaced by U, I or PI). It will display three pictures on the current KAPPA graphics device. So for instance, to use an xwindow graphics device, you can either use KAPPA:gdset to set xwindows as the default graphics device prior to running the above commands:
% kappa
% gdset xw
or you can specify the device directly on the pol2noise command line:
% pol2noise mycat.FIT Q device=xw
This will produce an X window showing something like this:
The left hand picture shows the noise level in the background regions determined from the local variation between the Q values in the catalogue. The middle picture shows the noise level recorded in the DQ column of the catalogue. The right hand picture is a scatter plot of the values in the other two pictures. The best fit line is shown in red (the slope and intercept of this line,together with the RMS residual, is displayed on the terminal screen when pol2noise ends). The upper limit of the data included in the scatter plot is shown as a red contour in the two images.
For more information on the available options and how pol2noise determines the background regions and the displayed noise levels, do:
% pol2noise --help