detector. The latter consists of a photocathode followed by three micro-channel plates as intensifier and by a tapered fiber-optic bundle connected to a fast scanning CCD. The usable area is made of 256 by 256 physical pixels but a centroiding process locates the events to 1/8 of a pixel thus mimicking a 2048 by 2048 device. The field of view is 17 arcmin by 17 arcmin with a centroided pixel size of 0.5 arcsec. At the end of the OM exposure, the cumulated image is downloaded to the ground. An engineering mode allows to transmit the whole image but the routine science mode necessitates predefined windowing and/or binning. The telescope was designed so that the limiting magnitude would be no less than 24 when working in unfiltered light. More details can be found in Fordham et al. 1992 and Mason et al. 1996.

XMM-OM holds a 6-filter, UV and optical, photometric system. Three bands of the system were designed to match the Johnson’s $U\!BV$ system (Johnson 1955; Bessel 1990). The rectangular profiles of the latter filters render the colour transformations between the XMM-OM and the Johnson systems quite complicated (see Royer & Manfroid 1996 for a discussion on the untransformability 