Lyman-Alpha Photometer

Description

This experiment was designed to observe from within the geocorona weak external enhancements of celestial Lyman-alpha sky. The instrumentation consisted of a sky-scanning hydrogen Lyman-alpha photometer with a 3-A bandwidth. The instrument was equipped with (1) an insertable SRF2 scattered light testing filter, (2) a hydrogen-filled type 304 stainless steel cell resonance filter (which suppressed the strong Lyman-alpha airglow), (3) an oxygen bandwidth determining filter, (4) an EMR type 543-T photomultiplier with an NaCl cathode, (5) a rotating plane mirror optimized for reflection at 1216 A, and (6) a platinum-foil secondary standard inflight calibrator. The photometer was mounted external to the spacecraft above the Z-axis door, allowing a clear view of both horizons. The scanner plane was canted 20 deg from the spacecraft axis to avoid direct observation of the sun, and therefore the instrument scanned most of the celestial sphere exclusive of a cone of 20 deg half-angle around the solar and antisolar points. Zenith observations were made once per scan and were taken alternately with the resonance cell on and off. The automatic functions of the photometer, which had a field of view of 5 deg at half-maximum, were programmed to operate from a clock pulse generated by the rotating scanner mirror. The calibration signal was generated for 4 of every 32 turns of the scanner, which turned once every 40 s, and the SRF2 filter was inserted for 2 of every 16 turns. The photometer sensitivity was about 1 rayleigh during early operations but decayed to approximately 10 rayleighs after a few days. Count rate data were obtained during the period June 6 to 18, 1969, and were of excellent quality.