Far ultraviolet imaging from the IMAGE spacecraft. 3. Spectral imaging of Lyman-alpha and OI 135.6 nm

Two FUV Spectral imaging instruments, the Spectrographic Imager (SI) and th e Geocorona Photometer (GEO) provide IMAGE with simultaneous global maps of the hydrogen (121.8 nm) and oxygen 135.6 nm components of the terrestrial aurora and with observations of the three dimensional distribution of neutr al hydrogen in the magnetosphere (121.6 nm). The SI is a novel instrument t ype, in which spectral separation and imaging functions are independent of each other. In this instrument, two-dimensional images are produced on two detectors, and the images are spectrally filtered by a spectrograph part of the instrument. One of the two detectors images the Doppler-shifted Lyman- alpha while rejecting the geocoronal 'cold' Ly-alpha, and another detector images the OI 135.6 nm emission. The spectrograph is an all-reflective Wads worth configuration in which a grill arrangement is used to block most of t he cold, un-Doppler-shifted geocoronal emission at 121.567 nm. The SI calib ration established that the upper limit of transmission at cold geocoronal Ly-alpha is less than 2%. The measured light collecting efficiency was 0.01 and 0.008 cm(2) at 121.8 and at 135.6 nm, respectively. This is consistent with the size of the input aperture, the optical transmission, and the pho tocathode efficiency. The expected sensitivity is 1.8x10(-2) and 1.3x10(-2) counts per Rayleigh per pixel for each 5 s viewing exposure per satellite revolution (120 s). The measured spatial resolution is better than the 128x 128 pixel matrix over the 15 degrees x15 degrees field of view in both wave length channels. The SI detectors are photon counting devices using the cro ss delay line principle. In each detector a triple stack microchannel plate (MCP) amplifies the photo-electronic charge which is then deposited on a s pecially configured anode array. The position of the photon event is measur ed by digitizing the time delay between the pulses detected at each end of the anode structures. This scheme is intrinsically faster than systems that use charge division and it has a further advantage that it saturates more gradually at high count rates. The geocoronal Ly-alpha is measured by a thr ee-channel photometer system (GEO) which is a separate instrument. Each pho tometer has a built in MgF2 lens to restrict the field of view to one degre e and a ceramic electron multiplier with a KBr photocathode. One of the tub es is pointing radially outward perpendicular to the axis of satellite rota tion. The optic of the other two subtend 60 degrees with the rotation axis. These instruments take data continuously at 3 samples per second and rely on the combination of satellite rotation and orbital motion to scan the hyd rogen cloud surrounding the earth. The detective efficiencies (effective qu antum efficiency including windows) of the three tubes at Ly-alpha are betw een 6 and 10%.