Array of low-energy X-ray imaging sensors
The Array of Low Energy X-ray Imaging Sensors (ALEXIS) featured curved mirrors whose multilayer coatings reflect and focus low-energy X-rays or extreme ultraviolet light the way optical telescopes focus visible light. The launch of ALEXIS was provided by the United States Air Force Space Test Program on a Pegasus Booster on April 25, 1993. The spacing of the molybdenum (Mo) and silicon (Si) layers on each telescope's mirror is the primary determinant of the telescope's photon energy response function. ALEXIS operated for 12 yr.
OSO-3
The Third Orbiting Solar Observatory, OSO 3, carried a hard X-ray experiment (7.7 to 210 keV) and an MIT gamma-ray instrument (>50 MeV), besides a complement of solar physics instruments. The third Orbiting Solar Observatory (OSO 3) was launched on March 8, 1967, into a nearly circular orbit of mean altitude 550 km, inclined at 33° to the equatorial plane, deactivated on June 28, 1968, followed by reentry on April 4, 1982. Its XRT consisted of a continuously spinning wheel (1.7 s period) in which the hard X-ray experiment was mounted with a radial view. The XRT assembly was a single thin NaI(Tl) scintillation crystal plus phototube enclosed in a howitzer-shaped CsI(Tl) anti-coincidence shield. The energy resolution was 45% at 30 keV. The instrument operated from 7.7 to 210 keV with 6 channels. OSO-3 obtained extensive observations of solar flares, the diffuse component of cosmic X-rays, and the observation of a single flare episode from Scorpius X-1, the first observation of an extrasolar X-ray source by an observatory satellite. Among the extrasolar X-ray sources OSO 3 observed were UV Ceti, YZ Canis Minoris, EV Lacertae and AD Leonis, yielding upper soft X-ray detection limits on flares from these sources.[7]
ESRO 2B (Iris)
Iris was mainly intended to study X-ray and particle emissions from the Sun, however, it is credited with some extra-solar observations. ESRO 2B (Iris) was the first successful ESRO satellite launch. Iris was launched on May 17, 1968, had an elliptical orbit with (initially) apogee 1086 km, perigee 326 km, and inclination 97.2°, with an orbital period of 98.9 minutes. The satellite carried seven instruments to detect high energy cosmic rays, determine the total flux of solar X-rays, and measure trapped radiation, Van Allen belt protons and cosmic ray protons. Of special significance for X-ray astronomy were two X-ray instruments: one designed to detect wavelengths 1-20 Å (0.1-2 nm) (consisting of proportional counters with varying window thickness) and one designed to detect wavelengths 44-60 Å (4.4-6.0 nm) (consisting of proportional counters with thin Mylar windows).[8]
Wavelength dispersive X-ray spectroscopy (WDS) is a method used to count the number of X-rays of a specific wavelength diffracted by a crystal. WDS only counts X-rays of a single wavelength or wavelength band. In order to interpret the data, the expected elemental wavelength peak locations need to be known. For the ESRO-2B WDS X-ray instruments, calculations of the expected solar spectrum had to be performed and were compared to peaks detected by rocket measurements.[9]
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