XMM-Newton Users Handbook

3.5.2 OM detector

The OM detector is a compact image-intensified photon-counting instrument. Each of the (redundant) OM detectors consist of a micro-channel plate (MCP) intensified CCD (MICs) with $384\times288$ pixels, $256^2$ of which are usable for science observations. Each CCD pixel has a size of about $4''\times4''$ 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 two successive MCPs (Fig. 83). This leads to an amplification of the signal by a factor of ca. $10^5$. The detector achieves a large format through a centroiding technique, subsampling the $256^2$ CCD pixels into $8\times8$ pixels each, as described in the next section. Therefore, the final array has a format of $2048\times2048$ pixels, each about $0''.476\times0''.476$ on the sky. The field of view is therefore $1024''$ on a side, or $17'\times17'$. The light-sensitive surface is an S20 photocathode optimized for the UV and blue. This provides sensitivity from 180 nm (set by the detector window) to 600 nm.

The internal noise (dark noise) of the detectors is extremely low. However, in the UV the Galactic diffuse background amounts at about 7% of the rate seen in darkframes. It can generally be ignored by comparison with other, cosmic sources of background, although it may play a rôle for diffuse faint sources (e.g.: diffuse galactic light).

Figure 83: Sketch of the OM micro-channel plate intensified CCD (MIC) detector.