XMM-Newton Users Handbook

4.6.1 Mosaic mode

Along the mission a number of scientific cases have been identified where the goal requires the observation of sky regions larger than the field of view of the EPIC cameras (galaxy clusters, supernova remnants, crowded fields, solar system objects). Since no raster, dithering or tracking modes were included in the original design of XMM-Newton, a target region larger than the field of view can only be achieved by a series of individual, independent observations, everyone having its own operational and instrument overheads (§ 4.5.1 and 4.5.2). For programs not requiring long integration times per pointing, these overheads may reduce significantly the observing efficiency, specially for EPIC-pn.

The Mosaic mode has been defined to keep a high observing efficiency when observing large fields for relatively short integration times. This is done, basically, suppressing the upload (for MOS) and calculation (for pn) of the EPIC offset tables⁹ at every pointing, except in the first one of a series of consecutive, adjacent pointings.

The detailed strategy is as follows: a number of nearby pointings are scheduled consecutively; at the first attitude of the sequence fixed offset tables are uploaded for MOS cameras and an offset table is specifically computed for pn with the filter wheel at the CLOSED position; once the EPIC filters selected by the PI are set, the exposures in all EPIC and RGS instruments are started and continued uninterruptedly until the end of the last pointing of the series.

This strategy reduces the overheads to the time required for re-pointing and for mapping the Star Tracker field of view. It should be noticed that the instruments are still collecting science data during this time; it is considered as overhead just because the accuracy of the attitude reconstitution during such periods is slightly worse than during stable pointing.

Since MOS cameras are already operated with fixed offset tables, the new mode has no noticeable effect on the quality of their data products. The offset table computed for pn through the filter CLOSED may result in a degraded spectral resolution for sources producing a non-negligible optical loading. Nevertheless, and except for sources extremely bright in the optical, the effect can be suppressed with the selection of the adequate filter. The only restriction to the EPIC filters is that they cannot be changed during the complete pointings series. Different EPIC cameras may use different filters, but they all must be set in Full Frame mode.

The total duration of any Mosaic observation ($t_{\rm obs}$) is limited to the visibility of the target region within a single orbit, as for any other observation. For $n_{p}$ pointings with a duration $t_{p}$ each

$$t_{\rm obs}\,=\,t_{\rm SETUP}+n_{p}\times t_{p}+(n_{p}-1)\times t_{s}$$

where $t_{\rm SETUP}$ is the time needed to configure the EPIC cameras at the beginning of the observation and $t_{s}$ is the total time requested for every re-pointing¹⁰. This later time is assumed constant in the formula because another constraint on the Mosaic mode is that the distance between consecutive pointings cannot exceed 1 degree so that the duration of the re-pointing slews is fixed. There is a practical lower limit to the distance between consecutive pointings driven by the accuracy of the slews and of the attitude reconstitution, the pixel size of the cameras and the PSF's of the telescopes; this lower limit is currently set to 12arcsec.

Additional constraints exist for the shortest and longest pointing duration. The former is set to 2ks in order to assure a minimum efficiency in the operations. On the other side, the efficiency is not significantly improved by the Mosaic mode for sufficiently long individual pointings (15ks).

The effective integration time at a given location in the target region depends on the individual pointing duration, but also on the offsets and on the vignetting of the telescopes. All these factors need to be carefully considered in the design of a Mosaic observation.

Figure 105: Simulated EPIC-pn exposure maps for a 5$\times$3 mosaic observation with pointing offsets of 1arcmin (top-left), 15arcmin (top-right) and 30arcmin (bottom). The colour bar at the top gives the effective exposure relative to the duration of individual pointings.

Three examples of Mosaic exposure maps are shown in Fig. 105. When the offsets are small compared to the scale length of the vignetting the result is a sort of blurred exposure map, with the effective exposure time in the core close to the total duration of the observation. If the offsets are similar to the two-folding distance of the vignetting factor, the resulting exposure map becomes relatively smooth on a spatial region larger than the field of view of the cameras. For larger offsets, the exposure map is a combination of snapshots at different attitudes.

In Mosaic mode observations it is possible to include OM exposures, but only during stable pointing periods avoiding re-pointing slews.

It has to be remarked that the Mosaic observing mode does not mean new operating modes of the instruments, except for the offset table used in the EPIC-pn exposures. Therefore, the descriptions of the instruments in § 3 are all applicable, including the recommendations, caveats, warnings, alerts and restrictions.

An exposure map simulator tool is available under the link:
http://xmm.esac.esa.int/external/xmm_science/AO9/epic_mosaic.shtml.