A1POINT - HEAO 1 A1 Lightcurves
A given source near the ecliptic was viewed for a few days while sources near the ecliptic pole were scanned nearly continuously during the entire mission. The satellite has limited pointing capability that was used to produce this data, giving continuous coverage of selected sources. The pointings began about 100 days into the mission.
The A1 instrument, also known as the NRL Large Area Sky Survey Experiment (LASS) covered the energy range 0.25 to 25.0 keV. The experiment consisted of seven detectors, six mounted on the -Y side of the spacecraft, the seventh on the +Y side. Two detectors, with a FWHM of 1 deg x 0.5 deg and open area of 1350 cm2 were tilted a third of a degree either towards or away from the Z (Sun-pointed) four other -Y side detectors has a FWHM of 1 deg x 4 deg and an open area of 1650 cm2. The single +Y detector has a FWHM of 2 deg x 8 deg and an open area of 1900 cm2. The experiment had sufficient sensitivity to detect sources as faint as as 0.25 uJy at 5 keV for sources with a Crab-like spectrum. Data was collected in either a 5 or a 320 millisecond timing resolution mode: Full sky coverage for both time resolutions was achieved before the mission's end. Wood et al. (1984) discuss the experiment and a catalog of sources in further detail.
Each file consists of two FITS extensions: the lightcurve count data and pointing information for use in timing correction analysis, and housekeeping and flag information preserved from the original 18432 byte ELE records.
Each lightcurve record consists 40.96 second records, constituting a Major Frame. The 40.96 record is consistent in each extension of the FITS file.
Within each 40.96 second row in the first extension there is one 8192 bin count vector. These are the 5 millisecond time count bins stored as the COUNT column.
The Module 7 data is useful because it can be cross-correlated with the A-2 experiment data. Module 7 was the only A-1 instrument coincident with the A-2 experiment. All other A-1 Modules are on the opposite side of the spacecraft.
There seems to have been a consistent error in the times associated with each record. This manifested itself in the following manner: Each record in the original NASA ELE format data file contained a start time. The start time for successive records were nominally 40.96 seconds apart, as long as the major frame number was consecutive to the previous record. Occasionally, the start time in a record would not be 40.96 seconds after the previous record's start time, and often the time was LESS. Subsequent cross-correlation of the data with A2 data suggested that when this occurred, the time should just be of the form:
TIME = (current_major_frame_number - last_major_frame_number) * 40.96
It is unknown why these time inconsistencies occur, but the remainder of the data in a record has been shown to be consistent in time and coordinates with both the parallel A2 observations and observation logs for the A1 instrument.
The start time of the observation. The time is displayed in the format 'yy.ddd' where yy is the last two digits of the year and ddd is the day number within the year (cf: 78.318, is 1978, day 318)
The stop time of the observation. The time is displayed in the format 'yy.ddd' where yy is the last two digits of the year and ddd is the day number within the year (cf: 78.318, is 1978, day 318)
Elapsed time between start of an observation file and the end of an observation file. Inclusive time, in seconds of entire observation.
The sum of all the continuous, uninterrupted major frames within an observation in seconds. Exposure is approximately the "up-time" of the instrument during each observation.
The start major frame number for this observation.
The stop major frame number for this observation.
The Right Ascension (1950) in degrees of the optimum observation direction. With a FWHM of 2 deg x 8 deg, lightcurves associated with a particular target should be within this range.
The Declination (1950) in degrees of the optimum observation direction. With a FWHM of 2 deg x 8 deg, lightcurves associated with a particular target should be within this range.
The Galactic Longitude (1950) in degrees of the optimum observation direction. With a FWHM of 2 deg x 8 deg, lightcurves associated with a particular target should be within this range.
The Galactic Latitude (1950) in degrees of the optimum observation direction. With a FWHM of 2 deg x 8 deg, lightcurves associated with a particular target should be within this range.
The Observation identification number. Numbered from 0001 to 0324.
The source name. If one does not exists, this parameter defaults to 'N/A'.
The BROWSE object classification flag.
The name of the events file in the archive associated with each observation. File is of the form a1_YYMMDD_HHMM_nnnX.fits, where a1 is the instrument designation, YYMMDD is the start year, month and day of the observation, HHMM is the approximate start hour and minute of the observation, nnn is the original celestial point sequence number from 001 to 324, X is 'a' or 'b' if there were multiple observations within a sequence number (e.g. 243a, 298b).