The UVOT MIC image intensifier

The detector is a Microchannel plate (MCP) Intensified Charge coupled device (CCD) or MIC Fordham et al. . Each photon incident on the S20 multi-alkali photocathode can release an electron which is amplified a million-fold using a three stage MCP. The cloud of electrons excites photons in a P46 fast-phosphor screen which are fed through a fibre taper to a CCD operated in frame transfer mode. The exposed area corresponds to 256x256 CCD pixels, but after readout the photon splash is centroided to 8 times higher resolution, providing an effective image that is 2048x2048 pixels square. The nature of the centroiding process is such that the effective size each of the 8x8 sub-pixels on the sky is not exactly the same, leaving a modulo-8 (MOD-8) pattern in the untreated image which can be corrected for in data processing. However information loss that occurs when more than one photon splash is registered on a CCD pixel within the same CCD readout interval (coincidence loss) can cause some pattern to remain after correction for bright sources.

As noted above, the finite time over which each exposure is integrated on the CCD, the frame time, results in coincidence losses if the photon arrival rate is high enough. Statistically, there is a chance that multiple photons arrive within one CCD frame with spatially overlapping pulse profiles, in which case only one arrival will be recorded. This means that fewer source photons are detected than are incident on the detector, resulting in a non-linear response with source brightness. Making use of the statistical nature of the effect, the coincidence loss can be corrected, and an expression for point sources has proved very effective in UVOT photometry, e.g., Poole et al. , Breeveld et al. . Extremely bright sources, above the brightness limit for coincidence loss correction, suffer a further loss due to interference from events registered in neighbouring CCD pixels.

The background due to dark current in the detector is very low; instead the sky background is the limiting factor for faint sources. The sky background in the grisms is comparable to that in the UVOT white (clear) filter since both grisms transmit the :math:`2800-6800~AA` optical band, as well as shorter wavelengths in the case of the UV grism.