detectors The ACS MAMA detector is a STIS flight spare. Its detector system consists of a MgF2 entrance window indium sealed to an evacuated chamber holding a photocathode, a curved MCP, and an anode array. Photons entering the detector are converted to electrons at the photocathode and accelerated into the 12 µm pores of the C-plate (the MCP ( figure)). The microchannel plate is an imaging electron multiplier with a very high gain within a small volume. It consists of a close-packet array of fine glass tubes with secondary emission coefficicents greater than unity. Accelerated electrons entering these channels produce secondary electrons which in turn produce more electrons and so on. The secondary emission properties of the tubes creates a charge coud of 7×105 electrons at the plate exit (see figure). The anode array is placed in proximity focus to minimize dispersion of this cloud.
STIS FUV-MAMA becomes somewhat problematic when imaging very bright objects. The problem is, in essence, a change of the quantum efficiency and therefore of the photometric stability. There is a dependance on the local and global count rates (in counts/second) with local rate limits set by the MCP pore recharge time, itself a function of the strip current. Higher strip currents yeild shorter pore recharge times. If charge is being extracted from the pores through the multiplier effect faster than it can be replaced, then a gain loss per event results. Overall, STIS MAMAs show a 10% deviation from linear operation when the input rate is of 225 counts/pix/sec and 340 counts/pix/sec for the FUV and NUV MAMA respectively (Argabright et al. 1998).
Local aging effects are also expected for STIS FUV MAMA. There are several mechanisms which provide better protection for the ACS MAMA: the calibration door may be used to block illumination following safing events, the SBC filter wheels can be used as a shutter (every third position is opaque) when local rates are in excess, and when the SBC is not in use, the HRC folding mirror M3 prevents illumination from reaching the SBC (Cox eta l. 1998). In general, local input rates should not exceed 1.5×106 counts/sec for more than one second and the local input count rate needs to be kept below 500 counts/sec in areas of four or more pixels. If these set rates are exceeded, two protection mechanisms are available. The Bright Scene Detector (BSD) and the Software Global Monitor will shut off the high voltage. The Local Rate Check (LRC) monitor is based on a 0.3 second initial exposure taken before each exposure rather than a dark. A 1024×1024 pixel array is rebinned to convert a mosaic of 4×4 pixel arrays into single "super" pixels. If any of the new pixels recieves more than 225 counts, the filter wheel is moved to an opaque position and the observation proceeds as a pseudo-dark. The instrument Science Report ACS 98-03 (Cox et al. 1998) lists the imaging bright limits for point source and extended objects in all the available filters.
