This document provides a description of the algorithm and instructions for the correct usage of the RADEC2XY task, which is part of eSASS (the eROSITA Science Analysis Software System).
Task RADEC2XY calculates X and Y sky pixel coordinates (tangential parallel projection; pixel size: 0.05'') corresponding to the RA and DEC (J2000) event coordinates in an eROSITA event table, given an RA and DEC projection center provided on the command line. WCS header keywords describing the sky pixel coordinate system are updated or created if they don't yet exist. The X and Y sky pixel coordiates are for instance required for image binning with eSASS task evtool. Calibrated event files provided by the eSASS pipeline already contain X and Y sky pixel coordinates with respect to the field center of observation. These may be recomputed for a different projection center by rerunning task RADEC2XY as needed. Please note that the sky pixel coordinate system should only be used for event files covering a sky area of at most 20 deg in diameter as otherwise the assumption by some eSASS tasks that the sky pixels are equal in size will no be fulfilled with sufficient accuracy.