XMM Users' Handbook

   
OM default 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 fast mode (§ 3.5.5). For each default configuration slightly different setups are offered, depending on the prime X-ray instrument, in order to align the OM science windows properly with the boresight of the prime instrument. If possible within the total time allocated for X-ray observations - and not overriden by GO input in XRPS- the OM will conduct the following set of default observations.

  

Figure 68: 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.

Imaging mode default

• Imaging with (default) $2\times2$ in-memory pixel binning (1.0'' pixels).

• A setup of 5 consecutive exposures per filter, covering 92% of the total ( $17'\times17'$) OM FOV.

• In each of the 5 exposures per filter a large fraction of the FOV will be covered by one large window, while a second, smaller imaging mode window ensures continuous monitoring of the prime target (at the centre of the FOV; see Fig. 68).

2.

Fast mode default

• Window configuration as under 1.

• An additional central window is operated in the OM fast mode. Its size is $22\times23$ in-memory pixels ( $11''\times11.5''$).

• Two imaging mode windows per exposure, covering the maximum possible FOV while not exceeding the hardware and software limits to operate at the same time a third window in the fast mode.

Within a default configuration observation, OM users can specify their choice of filter(s) and exposure times.

• Integration time for exposures

  To achieve the best possible efficiency in the use of the limited hardware capabilities, an integration time of 1 ks is recommended for each exposure. With a setup of 5 exposures per filter, this leads to 5 ks duration per default observation with one given filter. Note that longer integration times per exposure are allowed (§ 3.5.5). Shorter exposure times are limited by the OM telemetry allocation vs. required downlink time as described in § 3.5.9.2.

• Filter choice

  For maximum scientific yield (including the consideration of creating a useful archival database), the use of the following filters (in that order) is recommended:

  1.

  B

  2.

  UVW2

  3.

  U

  4.

  UVW1

  A different set of filters for observations in the OM default configuration may be chosen by the user based on scientific considerations (and a special scientific justification should be provided). However, the sequence in which the filters are used must comply with the instructions provided in § 3.5.6.1. The four filters listed above would be used in the order U, B, UVW1 and UVW2.

Example 1: With an exposure time of 1 ks, an observation using the imaging default mode (with 5 exposures) takes 5 ks (per filter). Within a total observing time of, e.g., 10 ks (which is determined by the feasibility of the planned X-ray observations), there would be time for only two filters. The recommended choice of filters would then be B and UVW2 (in that order).

Example 2: A 20 ks X-ray observation allows for four imaging default mode OM observations, preferably with the U, B, UVW1 and UVW2 filters.

Example 3: Users interested in U, B, V photometry would choose the filters in the order V, U and B. In order to reach similar limiting magnitudes within a total observing time of 20 ks, two default mode imaging observations with the V filter (requiring 10 ks) and one each with the U and B filter (in that order) can be chosen (cf. § 3.5.6.1). Alternatively, instead of choosing two 1 ks integrations in the V filter, 2 ks exposures may be defined.