J. Bloch and J. Theiler
Astrophysics and Radiation Measurement Group, Nonproliferation and International Security Division, Los Alamos National Laboratory, Los Alamos, NM 87545
The Array of Low Energy X-ray Imaging Sensors ( ALEXIS) satellite contains two experiments: a VHF radio-frequency ionospheric experiment called ``BLACKBEARD,'' and the ALEXIS experiment itself, which consists of six low-energy X-ray telescopes in three pairs: named 1A, 1B, 2A, 2B, 3A, and 3B. These are multilayer mirror telescopes with micro-channel plate detectors, and each is tuned to a relatively narrow () energy bandpass (see Table ).
Table: The bandpass energy and wavelength for each of the six narrow-band telescopes on the ALEXIS satellite.
The scientific objective of the ALEXIS experiment is to map out the sky in these narrow energy bands and search for transient EUV sources as well as map out the diffuse EUV background (Priedhorsky et al. 1990; Bloch et al. 1994). The project is a collaborative effort between Los Alamos National Laboratory, Sandia National Laboratory, and the University of California--Berkeley Space Sciences Laboratory. The satellite is controlled entirely from a small ground station located at Los Alamos.
The six telescopes are arranged in three pairs. The satellite is always in scanning mode, and during each ( s) rotation period they scan most of the anti-solar hemisphere of the sky. No pointed observations are possible. Each f/1 telescope consists of a spherical, multilayer-coated mirror and a photon counting detector containing a curved microchannel plate located at the prime focus. In front of each detector is a thin metal or plastic UV rejection filter. The spacing in the periodic layers of the multilayer coatings determine the bandpasses of the telescopes. The field of view of each telescope is with a spatial resolution of 0.o 25, limited by spherical aberration. Peak effective on-axis collecting areas range from 0.05 to 0.25cm, while the peak area-solid angle products of each telescope range from 0.01 to 0.05cmsr.
Each X-ray photon or background event from each telescope is tagged with its time of arrival and location in the field of view. Ground processing must combine these event lists with the satellite aspect solution to place each photon back onto the proper place on the sky.
The mission was launched on 1993 April 25 by a Pegasus booster dropped from a B-52 into a nautical mile, inclination orbit. Unfortunately the satellite suffered damage to one of its solar arrays on launch, and the single spacecraft magnetometer, which was located on the damaged panel is dead. The missing magnetometer and the modified (and time varying) spacecraft mass properties have forced the project team to invent new methods for both controlling the orientation of the satellite and to determine spacecraft attitude. As of 1994 September, ALEXIS has generated over 55 CD-ROMs (650 MB each) of telemetry data. About 50% of that data are from the six telescopes. In the year since launch, the team has developed an attitude algorithm which is providing attitude solutions close to the originally desired precision.
The current data processing system for ALEXIS relies on two internally generated data formats that date back to the beginning of the project in 1988, Generalized Data Format (GDF) files and IDL Data Format (IDF) files (Bloch et al. 1992, 1993). In order to use ALEXIS data with various software packages that are available in the high energy astrophysics communities, we need to define the correspondence between ALEXIS data in GDF or IDF files, and FITS files.
The following is our current proposal for keywords to use for storing raw and processed ALEXIS mission data in FITS format.
TELESCOP= 'ALEXIS '
DETNAM = 'A '
DETNAM = 'B '
DETNAM = 'AB '
We will use the filter keyword to specify the fixed mirror, filter, and detector complement of each telescope, since they all determine the telescope's bandpass response. These will be specified by concatenating strings identifying the mirror ID, the filter material, the filter ID, the photocathode material, and the detector ID. The mirror/filter/detector combinations that are on the telescopes now (and therefore are fixed) are:
FILTER = 'XRO4770_Lex/Ti/B-03639-2_MgF2-AF01'
FILTER = 'XRO5371_Al(Si)/C-03479-1_NaBr-AF06'
FILTER = 'XRO4763_Lex/Ti/B-03606-1_MgF2-AF03'
FILTER = 'XRO5339_Al(Si)/C-03483-1_NaBr-AF08'
FILTER = 'XRO5369_Al(Si)/C-03479-2_NaBr-AF05'
FILTER = 'XRO5306_Al(Si)/C-03483-2_NaBr-AF07'
Three observation modes have been identified as relevant to the ALEXIS project. We will use:
SLEWkeyword is usually reserved for satellites that are capable of a stable pointing.
DATAMODE= 'RAWTEL '
DATAMODE= 'REDTEL '
DATAMODE= 'COMPTEL '
Currently, photon lists are stored in an in-house format (either
GDF or IDF). In migrating these existing files to FITS formats, we
propose the following non-inclusive list of generic column headers (these
will be of the form
DETYare the integer X and Y detector locations, each ranging from 0 to 127.
DECare currently stored in terms of XYZ direction cosines in event GDF records, and will be converted to right ascension and declination as part of the file transfer.
WAVELENGTHare essentially fixed by the narrow bandpass in ALEXIS; we will occasionally use this column when combining photons from several telescopes into one data file. The units would be in eV or Å.
The following are ALEXIS-specific column header names:
TELIDidentifies with which telescope a given photon was observed. This knowledge will be important since many of our data sets combine data from several different telescopes.
RAWZare integer values for the raw Wedge, Strip, and Zigzag anode values.
HIVOLTis the single precision high voltage on the detector at the time the photon was observed.
This work was supported by the Department of Energy.
Klarkowski, J. R. M. 1992, ALEXIS DPU Software Requirements Specifications, Sandia National Laboratories Document AD038
Bloch, J. J., Smith, B. W., & Edwards, B. C. 1992, in Astronomical Data Analysis Software and Systems I, ASP Conf. Ser., Vol. 25, eds. D.M. Worrall, C. Biemesderfer, & J. Barnes (San Francisco, ASP), p. 502
Bloch, J. J., Smith, B. W., & Edwards, B. C. 1993, in Astronomical Data Analysis Software and Systems II, ASP Conf. Ser., Vol. 52, eds. R.J. Hanisch, R.J.V. Brissenden, & J. Barnes (San Francisco, ASP), p. 243
Bloch, J., et al. 1994, in EUV, X-Ray, and Gamma-Ray Instrumentation for Astronomy V, SPIE Vol. 2280, p. 297