XMM Users' Handbook

   
OM operating modes

The OM can operate in the two modes listed in Table 13. These allow either imaging or fast photometry or both simultaneously.

 

Table 13: The science data acquisition modes of OM

$\parbox{3cm}{{\bf Mode$^1$ }}$	$\parbox{11cm}{{\bf Special feature}}$
$\parbox{3cm}{Imaging}$	$\parbox{11cm}{2-D imaging with long exposure times ($>100$\space s)}$
$\parbox{3cm}{Fast}$	$\parbox{11cm}{High time resolution, providing 2-D time-sliced images over small areas.}$

Notes to Table 13:
1) Both modes can be used simultaneously, as long as total memory capacity and telemetry bandwidth are not exceeded.

• Imaging mode

  Imaging can be performed with different pixel sizes. The tradeoff is that the images with unbinned pixels take more memory and more telemetry, and therefore the windows must be small or the exposures long in order to allow the instrument memory to empty. A $2\times2$ pixel onboard binning is the default, resulting in pixels (1'')², but $1\times1$, $2\times2$ and $4\times4$ pixel binning is possible. Up to 5 imaging mode windows of up to $480\times480$ pixels can be assigned.

  One such image covers ca. the central quarter of the OM FOV, which again is about 1/4 of the EPIC FOV. To collect data over the entire OM FOV, more than one exposure must be obtained.

  The maximum allowed integration time for an OM imaging mode exposure is 5 ks. Since the OM in its imaging mode produces accumulated images, there is no timing information attached to individual incoming photons.

• Fast mode

  In its fast mode, the OM does not produce accumulated two-dimensional images, but instead produces event lists like the X-ray instruments. This mode is useful for monitoring rapidly variable sources, for example AGN or accreting binaries. Two small windows can be assigned, each of a maximum of 512 pixels only. The data are obtained in 50 ms time slices, each preserving the pixel information. No drift correction is performed, but the drift information is telemetered, so that the correction can be applied in the post-processing on the ground.

  The maximum allowed integration time for one fast mode window is 4.4 ks, and 2.2 ks each if two are used simultaneously.