XMM-Newton Users Handbook

3.5.9.2 OM operating configurations

To ease the use of the OM and to maximise the scientific yield of observations in cases when no particular instrument configuration is requested, OM default configurations have been defined. There is one type of default configuration per OM observing mode, thus one for the imaging mode and one for the Image plus Fast mode (§ 3.5.9.1). Default windows have also been predefined for the V and UV grisms exposures. Moreover, two additional full frame modes are available which allow imaging observation of the whole OM field of view.

The possible configurations are as follows:

Figure 112: Setup of OM imaging mode default mode observations consisting of a sequence of 5 exposures. The science windows are indicated by solid lines, the detector windows by dashed lines. A 16 in-memory pixel margin around the science window is allocated to accommodate spacecraft drifts.

1. Science User Defined mode (Science User Def. in the RPS)

   Up to five windows (two of them in Fast mode) covering different areas of the detector can be defined (see Sect. 3.5.10.1).

2. Full-Frame imaging modes (Full Frame High/Low Resolution in the RPS)

   It is possible to obtain images of the whole OM field of view in full (high) or 2$\times$2 binned (low) resolution. These modes do not allow tracking (see § 3.5.4), and have higher instrumental overheads than the Modes using windows, either User Defined windows or Imaging Mode default. However, they allow homogeneous sampling of the whole OM field of view. They are therefore particularly well suited for survey studies or to obtain multiobject grism spectroscopy in a wide field. These modes have a single window, no other window can be defined (either in image or Fast mode) within the same exposure. Along an observation, Full-Frame imaging modes can be combined with modes using windows that require Field Acquisition (FAQ, see § 3.5.8), particularly Fast mode windows. In this case the Full-Frame exposures must be performed after all the exposures which need FAQ, because Full-Frame modes nullify the results of FAQ.

   The lack of tracking in Full Frame mode does not affect the quality of the image because, as experience has shown us, the drift of the XMM-Newton spacecraft is almost negligible (less than one arc sec per hour).

3. EPIC/RGS Imaging Mode (depending on the prime X-ray instrument)
   • A pre-defined setup of 5 consecutive exposures per filter, covering 92% of the total (17'$\times$17') OM FOV. Each exposure having 2 windows.

   • In each of the 5 exposures per filter a large fraction of the FOV will be covered by one large window , while the central 2'$\times$2' high resolution imaging mode window is observed in all the exposures. The continuous exposure of the central window, ensures continuous monitoring of the prime target (at the centre of the FOV; see Fig. 112).

   • The central window has high spatial resolution (0".5$\times$0".5) while the lateral windows have 2$\times$2 in-memory pixel binning (1”.0 pixels)

   • Warning: Since each pair of windows has a different frame time, the coincident loss correction can differ for bright sources. This can lead to systematic offsets in corrected count rates (and thus magnitude and fluxes) in each exposure. The User Defined mode with identical windows, always has the same frame time and is thus more stable for bright sources.

   Figure 113: Example of a field imaged with the default Image Mode configuration.

   

4. Fast Mode default (named EPIC/RGS Image Fast in the Remote Proposal System, RPS, depending on the prime X-ray instrument)
   • Image windows configuration as under 3 (i.e. pre-defined Image Mode windows).

   • An additional central window is operated in the OM Fast mode. Its size is 22$\times$23 in-memory pixels (10”.5$\times$10”.5). It is responsibility of the PI to carefully check the accuracy of the target coordinates, to avoid substantial flux losses.

   • The same warning as in 3 applies for bright sources. It is recommended to select the User Defined mode for variability study of bright sources.

5. Default grism configurations

   Two standard configurations are available for OM optical or UV grisms:

   • a predefined default window, which will contain the spectrum of the target (with its 0th order) at full detector resolution (Science User Defined mode in RPS)

   • a Full Frame Low Resolution exposure, which provides the spectra of all the targets in the OM FOV, with slightly degraded spatial (and hence spectral) resolution (Full Frame Low Resolution mode in RPS)

Examples of the Imaging and Fast default modes are shown in Figs. 112 and 113.

Apart from the configuration of the OM exposures, exposure times and filters have to be specified.

The described Default configurations for Imaging and Fast modes provide a very simple and easy to use set-up aimed at obtaining a good coverage of the main target located at the boresight and imaging at the same time the complete field of view (92% with 5 consecutive exposures).

However, due to the limitations imposed by the coincidence losses in the detector these Default configurations are not recommended in case of bright sources and in variability studies. As explained in section 3.5.5, the coincidence losses (and their correction) depend on the frametime. The different detector windows defined for the five exposures of the Default configuration may have very different frametimes leading, for high count rates, to very large and inhomogeneous corrections and errors. It is better in these cases to select a unique window using Science User Defined mode.

Observers should be aware that accurate target coordinates are important when using the fast mode, to ensure optimum centring of the source in the small fast mode window. Nevertheless, users should also be aware that occasional failure of the initial field acquisition (FAQ) exposure, and also, sometimes, spacecraft drift during long observations, can also lead to targets being notably displaced from the window centre, which can impact on the consistency of the photometry between exposure and even introduce spurious variability in the most extreme cases. These points are described in more detail in CAL-TN-0224.