Array of Low Energy X-ray Imaging Sensors
EUV telescope satellite that scanned half the sky for transients.
The Array of Low Energy X-ray Imaging Sensors (ALEXIS), also known as P89-1B, was an X-ray telescope satellite funded by the United States Department of Energy and built by Los Alamos National Laboratory in collaboration with Sandia National Laboratories and the Space Sciences Laboratory. Launched on April 25, 1993, aboard a Pegasus Booster provided by the United States Air Force Space Test Program, ALEXIS used curved mirrors with multilayer coatings to reflect and focus low-energy X-rays and extreme ultraviolet (EUV) light. The satellite bus was built by AeroAstro, Inc. of Herndon, VA, and the mission was entirely controlled from a small ground station at Los Alamos National Laboratory.
- Launch date
- April 25, 1993
- End of mission
- April 29, 2005
- Mass
- 100 pounds
- Power
- 45 watts
- Data rate
- 10 kilobits/second
- Onboard memory
- 78 Megabytes
- Design life
- 1 year (exceeded to 12 years)
Lore & Background
ALEXIS featured six EUV telescopes arranged in three co-aligned pairs, each covering overlapping 33° fields-of-view. At each rotation, it monitored the entire anti-solar hemisphere. Each telescope used a spherical mirror with a Mo-Si layered synthetic microstructure coating, a curved microchannel plate detector, a UV background-rejecting filter, electron rejecting magnets, and image processing readout electronics. The geometric collecting area of each telescope was about 25 cm², with spherical aberration limiting resolution to about 0.25°. The peak on-axis effective collecting area ranged from 0.25 to 0.05 cm², and the peak area-solid angle product ranged from 0.04 to 0.015 cm²-sr.
Reader's Guide
ALEXIS's scientific goals included mapping the diffuse background in three emission line bands with the highest angular resolution to date, performing a narrow-band survey of point sources, searching for transient phenomena in the ultrasoft X-ray band, and providing synoptic monitoring of variable ultrasoft X-ray sources such as cataclysmic variables and flare stars. It complemented NASA's Extreme Ultraviolet Explorer and the ROSAT EUV Wide Field Camera by offering wide fields-of-view and well-defined wavelength bands. Ground-based amateur astronomers were well suited to observe visual counterparts to EUV transients seen by ALEXIS, as large telescopes could not quickly scan large enough sky areas. The satellite operated for 12 years, far exceeding its one-year design life, and was decommissioned on April 29, 2005.
Did You Know?
- The satellite used a 'wavetrap' feature on its multilayer mirrors to reduce reflectance of He II 304 Angstrom geocoronal radiation.
- ALEXIS produced 10 kilobits/second of data and stored it in an on-board solid state memory of 78 Megabytes.
- The mirrors were produced by Ovonyx, Inc. and were highly curved with very uniform multilayer coatings.
Frequently Asked Questions
What is the Array of Low Energy X-ray Imaging Sensors (ALEXIS)?
ALEXIS, also catalogued as P89-1B, was a compact 100-pound satellite dedicated to scanning roughly half the sky for transient X-ray and extreme-ultraviolet events. Funded by the U.S. Department of Energy, it operated from its 1993 launch until its mission concluded in 2005.
How did ALEXIS capture soft X-rays and EUV light?
Instead of absorbing photons, ALEXIS used curved mirrors coated with thin multilayer films to reflect and focus low-energy X-rays and extreme ultraviolet radiation onto its imaging sensors. This grazing-incidence design was essential because those wavelengths would simply pass through or be swallowed by conventional optics.
When did ALEXIS fly, and how long did it last?
The spacecraft was lofted on a Pegasus Booster on April 25, 1993, under the U.S. Air Force Space Test Program, and remained active until April 29, 2005. That worked out to roughly twelve years of continuous all-sky monitoring.
What were ALEXIS's main technical specifications?
The satellite ran on 45 watts of power, carried 78 megabytes of onboard memory, and downlinked data at 10 kilobits per second. Its bus was fabricated by AeroAstro, Inc., in Herndon, Virginia.
Who built and operated ALEXIS?
Los Alamos National Laboratory spearheaded the project in partnership with Sandia National Laboratories and the Space Sciences Laboratory, all under Department of Energy sponsorship. The satellite bus came from AeroAstro, Inc., and the Pegasus launch vehicle was provided through the Air Force Space Test Program.
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