The choice of a mode or configuration for OM should be driven by a compromise between the time resolution needed and the spatial coverage required for a given pointing. Time discontinuities are due to the overheads (see XRPS User's manual for a detailed description) needed to download the data and configure the instrument for the next exposure.
The full frame modes cover the whole FOV, but they have large overheads. If time resolution is needed, then windowed modes provide almost continuous coverage. The maximum time resolution is achieved in FAST mode (up to 0.5 sec). The default image configuration, or IMAGE+FAST, allows the user to cover 92% of the FOV in five successive exposures, obtaining in each of them a small window (less than 22 arcmin) centred on the pointing attitude and optionally a FAST mode window (10''.510''.5) centred at the same coordinates. If one needs time coverage on a larger area, one can make several short ( sec) Science User Defined Mode exposures (image window up to 7'.67'.6, if binned). However, for crowded fields, exposures may be lost, and longer exposure times are encouraged.
Field spectroscopy is better achieved using full frame low resolution, while if one wants to monitor spectral variations of the target, then there is a pre-defined window configuration for each of the grisms which allows obtaining only its spectrum.
Examples 1: Imaging
Example 1a: Standard Imaging Mode
Since an EPIC/RGS image mode exposure consists of five subexposures, a defined exposure time of, e.g., 1 ks gives a total exposure time of 5 ks for each 1-ks EPIC/RGS image mode exposure. The overhead consists of 1.3 ks (overhead times are given with numbers in italic font) at the beginning of the observation (including the star tracker acquisition time) and 1.6 ks for each exposure. If, for example, two EPIC/RGS image mode are defined, then the total observing time needed is: 1.3 ks + 2(1.6 ks + 5 1.0 ks) = 14.5 ks. The overhead times are added automatically when entering the data in the RPS tool.
Thus, an approved 10-ks observation allows the definition of only one EPIC/RGS image mode exposure: 1.3 ks + 1.6 ks + 5 1.0 ks = 7.9 ks. Conversely, only one filter can be used with this mode. The remaining 2.1 ks can not be used in this mode, because the overhead of 1.6 ks is only 500 sec shorter than the minimum allowed exposure time of 800 sec.
In order to use the full time, one can either use Science User Defined Mode (see below) or increase the defined exposure time by 2.1 ks/5=0.42 ks, thus with one EPIC/RGS image mode exposure with an exposure time of 1.42 ks, we get: 1.3 ks + 1.6 ks + 51.42 ks = 10.0 ks.
Example 1b: Science User Defined Mode
Alternatively, with 10 ks approved observation time, two OM exposures with 4 ks each can be defined in Science User Def mode. The overhead is 0.3 ks per exposure, and one can either cover the central 5'5' at high resolution, or 7'.67'.6 at low resolution (22 binning). The total required observing time will be: 1.3 ks+2(0.3 ks + 4.0 ks) = 9.9 ks. With the shortest sensible exposure time of 1 ks, the maximum number of exposures that can be defined is six: 1.3 ks+6(0.3 ks + 1.0 ks) = 9.1 ks, but note that significantly more overhead time is needed, and such a setup only makes sense if multiple filters are needed. For monitoring of expected variability, please see Example 3. Note, that for best efficiency, the remaining time (in the last case: 900 sec) can be added to one of the exposures, e.g., the last of the six exposures can be defined as 1.9 ks.
It is recommended to increase the exposure time of the exposure that uses the filter with the lowest throughput (see Fig. 101).
Example 1c: Full Frame Low Resolution
If Full Frame Low Resolution Mode is used, in 10 ks we can obtain two exposures of 1.6 ks each, in two different filters: 1.0 ks + [2(2.9 ks + 1.6 ks)] = 10 ks (the FF Low Res. exposure overhead is 2.9 ks). The overhead of the first exposure of the observation for this mode being 1.0 ks.
Example 1d: Multi-filter for longer Observations
A 28 ks X-ray observation allows four EPIC/RGS Image mode OM exposures:
1.3 ks + 4[1.6 ks+(51.0 ks)] = 27.7 ks.
Preferred choice of filters is: U, UVW1, UVM2, UVW2 (in that order). Due to the different throughput of the different filters (see Fig. 101), it is recommended to define longer exposure times for those filters with low throughput. In the above case, if the U filter is not really needed, one might prefer three exposures with ks, ks, and ks, yielding:
1.3 ks +
1.6 ks+(51.0 ks)
1.6 ks+(51.5 ks)
1.6 ks+(51.9 ks) = 28.1 ks.
If only a single filter is to be used, it is better to define only one exposure with the maximum exposure time of 5 ks: 1.3 ks + 1.6 ks+(55.0 ks) = 27.9 ks.
Example 2: Monitoring a variable point source
One can either use EPIC/RGS image fast mode or Science User Def mode. In the latter mode, define one image window plus one additional fast window, both centred on the source coordinates. Choose maximum exposure times (4400 sec) to minimise overhead. The calculation of exposure times is the same as in examples 1a and 1d. Depending on the expected time scales of variability, the time bin size for the fast window can be adjusted to values between 0.5 and 20s. For studies of variability it is recommended to use the same filter for all observations. While the use of UV filters is encouraged for imaging, it depends on the scientific aims whether strictly simultaneous X-ray/optical or X-ray/UV monitoring is of higher interest.