The Array of Low Energy X-ray Imaging Sensors (ALEXIS) satellite was built by the Los Alamos National Laboratory for approximately $17 million and was launched on April 15, 1993 on a Pegasus rocket from Edwards Air Force Base in California. The satellite contained an ultra-soft X-ray or EUV monitor experiment that consisted of six compact normal-incidence telescopes operating in narrow bands centered on 66, 71, and 95 eV. The satellite also contained a VHF broadband ionospheric survey experiment called BLACKBEARD.
ALEXIS scans half the sky with its three paired sets of EUV telescopes, although it cannot locate any events with high resolution. Ground-based optical astronomers can look for visual counterparts to the EUV transients seen by ALEXIS by comparing observations made at two different times. Large telescopes, with their small fields of view, cannot quickly scan a large enough piece of the sky to effectively observe transients seen by ALEXIS, but amateur equipment is well suited to the task. All participants in the ALEXIS project may comb the ALEXIS data for the coordinates of a likely current transient, then train their telescopes and observe the area.
There are six EUV telescopes which are arranged in three co-aligned pairs which cover three overlapping 33° fields-of-view. At each rotation of the satellite, ALEXIS is to monitor the entire anti-solar hemisphere. Each telescope consists of a spherical mirror with a Mo-Si layered synthetic micro-structure (LSM) or Multilayer coating, a curved profile micro-channel plate detector located at the telescope's prime focus, a UV background-rejecting filter, electron rejecting magnets at the telescope aperture, and image processing readout electronics. The geometric collecting area of each telescope is about 25 cm2, with spherical aberration limiting resolution to about 0.25°s. Analysis of the pre-flight X-ray throughput calibration data indicates that the peak on-axis effective collecting area for each telescope's response function ranges from 0.25 to 0.50 cm2. The peak area-solid angle product response function of each telescope ranges from 0.04 to 0.015 cm2-sr.
The spacing of the molybdenum and silicon layers on each telescope's mirror is the primary determinant of the telescope's photon energy response function. The ALEXIS multilayer mirrors also employ a "wavetrap" feature to significantly reduce the mirror's reflectance for He II 304 Angstrom geocoronal radiation which can be a significant background source for space borne EUV telescopes. These mirrors, produced by Ovonyx, Inc., are highly curved yet have been shown to have very uniform multilayer coatings and hence have very uniform EUV reflecting properties over their entire surfaces.
ALEXIS weighs 100 pounds, uses 45 watts, and produces 10 kilobits/second of data. Position and time of arrival are recorded for each detected photon. ALEXIS will always be in a survey-monitor mode, with no individual source pointings. It is suited for simultaneous observations with ground-based observers who prefer to observe sources at opposition. Coordinated observations need not be arranged before the fact, because most sources in the anti-Sun hemisphere will be observed and archived. ALEXIS is tracked from a single ground station in Los Alamos. Between ground station passes, data are stored in an on-board solid state memory of 78 Megabytes. ALEXIS, with its wide fields-of-view and well-defined wavelength bands, complements the scanners on NASA's Extreme Ultraviolet Explorer (EUVE) and the ROSAT EUV Wide Field Camera (WFC), which are sensitive, narrow field-of-view, broad-band survey experiments. ALEXIS's results will also highly complement the data from EUVE's spectroscopy instrument.
ALEXIS's scientific goals are to:
- Map the diffuse background in three emission line bands with the highest angular resolution to date,
- Perform a narrow-band survey of point sources,
- Search for transient phenomena in the ultra-soft X-ray band, and
- Provide synoptic monitoring of variable ultra-soft X-ray sources such as cataclysmic variables and flare stars.
After 12 years in orbit, the ALEXIS satellite reached the end of its career. Its solar arrays degraded in charge producing ability, and 2 out of the 4 battery packs failed. As of 29 April 2005, its solar arrays were intentionally tipped away from the sun, placing the Alexis system in the lowest power state for safety purposes, after which it stopped being tracked.