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The primary function of the ultraviolet spectrometer (UVS) is to measure the density of nitric oxide between the altitudes of 100 and 200 km in the terrestrial upper atmosphere by observing the (1,0) and (0,1) gamma bands. The UVS design is similar to instruments flown on the Solar Mesospheric Explorer (SME), Pioneer Venus, and several rocket flights. It consists of an Ebert-Fastie spectrometer, an off-axis telescope, and two Hamamatsu phototube detectors. The spectrometer has a focal length of 125 mm and uses a 3600 l/mm mechanically ruled plane grating which produces a dispersion of 1.8 nm/mm at the detectors. The phototubes each have fused silica windows and a cesium telluride photocathode. The telescope is an off-axis parabola with a 250 mm focal length and is used to image the spectrometer slit on the limb. The combination of the spectrometer and the detectors produces a spacing of 22 nm between the two channels and the exit slits are sized to give each detector a 3.7 nm bandpass. The grating in the spectrometer will be set to place the (1,0) gamma band ( 215 nm) on one detector and the (0,1) gamma band (237 nm) on the other detector. Both channels have a sensitivity of 450 counts/second/kiloRayleigh.

The UVS is mounted with its optical axis perpendicular to the spin axis of the S/C. Its telescope images the entrance slit of the spectrometer on the limb with the long axis of the slit parallel to the horizon. The image of the slit on the limb is 3.5 km high, which determines the fundamental altitude resolution of the instrument. The integration time of the is set to 27 milliseconds. To minimize requirements on the S/C, data are stored for the downward limb scan only. Allowing for some overscan, this produces 65 samples per spin from each channel. The storage operation is initiated by a signal derived from the horizon crossing indicator in the ADCS. The data are stored in a buffer which is emptied, time-tagged, and stored once per spin by the S/C microprocessor.

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