| This thesis describes the complete development of four ultraviolet detectors for use at grazing incidence angles in a solar science instrument. The instrument is one of twelve on board the Solar Heliospheric Observatory (SOHO), a satellite to be launched in November 1995 with a minimum design lifetime of two years. The accurate measurement of abundances and small scale motions in the sun's atmosphere, requires that the detectors combine high speed and resolution with the low power and mass compatible with space flight. These detectors represent a novel use of microchannel plates (MCPs), in the detection of extreme ultraviolet at grazing angles of incidence and at high count rates. Furthermore, this is the first time a spiral anode will have been used on a flight instrument. In order to get the maximum science out of the mission, it is crucial that the instrument be fully calibrated. For this reason emphasis is placed, in the thesis, on dividing the detector into its constituent parts and on understanding the various processes involved, from the arrival of the photon to the output spectrum. Computer modelling as well as experimentation is used in the investigation. Starting with an introduction to SOHO in the context of Solar Physics, a description of the instrument reveals the requirements of the detectors. As one of the key parts of the detector, MCPs are described and results are presented from experiments investigating their behaviour. The thesis then describes how the SPAN readout is made and used, and how it is configured to suit the MCP set up. Finally, the flight design, building and testing is described, followed by results from the flight calibration test and predictions of problems and successes in the detectors' mission. In summary, the detectors meet the requirements of resolution ([less-than]50 [mu]m FWHM), speed ([greater-than]105 random), background count rate ([less-than]0.2 cs-1 cm-2) and integral non-linearity ([less-than]0.03[percent]). They fulfil the expected quantum efficiency (3-12[percent]). However, limitations from using 8-bit digitisation with the SPAN anode, leads to a high level of differential non-linearity (27[percent] r.m.s.). There are also problems with count rate dependence and long term gain depression in the MCPs. The sources of the problems are analysed and modelled where practicable, so that they can be minimised for flight operations. |
