High-resolution cross delay line detectors for the GALEX mission

The GALEX instrument consists of a 50 cm normal incidence mirror telescope in combination with a grism, and a dichroic beamsplitter system projecting images onto two detectors simultaneously. The objective of this instrument is to provide sensitive high resolution imaging (3 to 5 arc ") of galaxies in two bandpasses (1350 - 1800 Angstrom and 1800 - 3000 Angstrom), with the option of modest (lambda/6 lambda 100 - 200) resolution spectroscopy. We are currently developing the microchannel plate, delay line, sealed tube detectors for the Galaxy Evolution Explorer(1) (GALEX) mission to be launched in 2001. The specifications for the two detector systems for GALEX require a large 65mm circular format, with high spatial resolution (<30 mu m FWHM, similar to 2200 x 2200 resolution elements) and good linearity (+/-50 mu m), <1 event cm(-2) sec(-1) background and counting rates of 1.25 x 10(5) events sec(-1). One detector covers the 1350 Angstrom to 1800 Angstrom region with an opaque CsI photocathode, and the other uses a semitransparent CsTe photocathode covering 1800 Angstrom to 3000 Angstrom. We are implementing these detectors using a microchannel plate Z stack to amplify the signals detected by the photocathodes, and a cross delay line anode to provide the photon event positions. Also part of this system are high speed event processing electronics, low and high voltage power supplies, and a command and house-keeping interface. We are currently testing unsealed flight like detectors to verify the component performance prior to integration and processing of the sealed tube assemblies. The microchannel plates (10 mu m & 12.5 mu m pores, 80:1 L/D) have a 75mm format and provide gain greater than or equal to 2 x 10(7) with pulse height distributions as low as <30% and low background (<0.4 event cm(-2)sec(-1)). Cathode tests have achieved 50% QDE at 1152 Angstrom for CsI and 18% at 1800 Angstrom for CsTe. The cross delay line readouts have achieved = 30 mu m FWHM resolution at 2 x 10(7) with good image linearity (<+/-50 mu m) and the flat field images are dominated by the microchannel plate multifiber boundary fixed pattern noise.