A small proportion of defects comprising bad or “cool” columns or pixels is inevitable in CCD detectors. In a dispersive instrument like the RGS, these detector defects, usually one pixel wide, correspond to fixed narrow wavelength intervals of about 10 mÅ per pixel (the RGS line-spread function has a FWHM of about 7 pixels). These wavelength defects sometimes coincide with features in the source spectrum that are important for the scientific success of an observation (see Tables 13 and 14).
One consequence of the pointing stability of XMM-Newton is that point-source photons of a given energy always end up in the same pixel of the detector. If an important spectral feature happens to fall on a defect, up to 20-50% of line flux may be lost depending on the filtering scheme employed. In addition, small defects can be difficult to distinguish from true, weak absorption lines. Further, there are the larger 70-100 mÅ gaps between the RGS CCDs.
When these circumstances arise, it is possible to change the spacecraft pointing in order to move the defects away so that more important parts of the spectrum are not affected.
The RGS Multipointing mode (MPM) allows the user to make five different pointings, with offsets in the dispersion direction of 0, 15 and 30 arcsec, corresponding to 0, 35 and 69 mÅ. In this way, the maximum flux loss in a spectral line is reduced significantly, and it becomes easier to distinguish weak absorption lines from defects. The dispersion step size is limited by the need to keep X-ray sources safely within the EPIC small windows.
For RGS, it is not necessary to start a new observation when slewing to the offset position. The instrument can be left on and useful observation time gained. On the other hand, EPIC pn offset maps must be generated at each pointing leading to a significant loss of EPIC pn exposure time.
A number of considerations must be taken into account in order to decide if the MPM could be applied with profit for a given observation.
European Space Agency - XMM-Newton Science Operations Centre