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 384288 pixels, 256256 of which are usable for science observations. Each CCD pixel has a size of about 4''4'' on the sky. Photons coming from the 45-mirror enter the detector and hit the photo-cathode, from where electrons are pre-amplified by two successive MCPs (Fig. 99). This leads to an amplification of the signal by a factor of ca. 10. The detector achieves a large format through a centroiding technique, subsampling the 256256 CCD pixels into 88 pixels each, as described in the next section. Therefore, the final array has a format of 20482048 pixels, each about 0''.4760''.476 on the sky. The field of view is therefore 1024'' on a side, or 17'17'. The light-sensitive surface is an S20 photocathode optimised 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 detector is extremely low. However, in the UV the Galactic diffuse background amounts to 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 role for diffuse faint sources (e.g.: diffuse galactic light).