XMM-Newton Science Analysis System: User Guide

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5.13 Heliocentric velocity correction

Due to the motions of the Earth around the Sun and the spacecraft around the Earth, the observed wavelength of a line emitted by a celestial object is affected by the Doppler effect induced by the spacecraft velocity component in the direction of the target. Spectral lines measured from a high-resolution spectroscopy instrument, such as RGS, must then be corrected for the projected component of the satellite's velocity. The task rgsproc can be instructed with the switch withheliocorr to apply this correction and report the value in the VHELIOCOR keyword in the header of the spectrum. Starting in SAS v14.0, this correction is applied by default.

European Space Agency - XMM-Newton Science Operations Centre