XMM Users' Handbook

   
OM detector

The OM is operated as a photon-counting instrument. Each of the (redundant) OM detectors has a format of $2048\times2048$ pixels, each $0.5''\times0.5''$ on the sky. The field of view is therefore 1024''on a side, or $17'\times17'$. The light-sensitive surface is an S20 photocathode optimised for the UV and blue. This provides sensitivity from 160 nm (set by the detector window) to 600 nm.

The resolution of the detector is wavelength-dependent, from 15 $\mu$m in the red to ca. 32 $\mu$m at about 250 nm, before improving again. With the filters described below, this corresponds to <1'' to ca. 1.8'', and with the magnifier (§ 3.5.6.3), it corresponds to the range 0.25''-0.45''.

The internal noise (dark noise) of the detectors is extremely low, and can generally be ignored by comparison with other, cosmic sources of background.

The OM detectors consist of a micro-channel plate (MCP) intensified CCD (MICs) with $384\times288$ pixels, 256² of which are useable for science observations. Each CCD pixel has a size of $4''\times4''$ on the sky. Photons coming from the $45^\circ$-mirror enter the detector and hit the photo-cathode, from where electrons are pre-amplified by three successive MCPs (Fig. 67). This leads to an amplification of the signal by a factor of ca. 10⁵. The detector achieves a large format through a centroiding technique, subsampling the 256² CCD pixels into $8\times8$pixels each, as described in the following.

  

Figure 67: Sketch of the OM micro-channel plate intensified CCD (MIC) detector.