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The Original EXOSAT raw data were distributed in a FOT
(FInal Observaton Tape). For a given observation, each FOT contains
data from one experiment. HEASARC has reformatted the FOT data from
the Low energy experiment (LE) into FITS file.
The reformatting procedure used the Interactive Analysis software to read
the data.
For each LE FOT there are 4 different basic types of FITS files :
- Catalog file - an overview of the data contained within a FOT
- Orbit file - coordinates of EXOSAT and Sun at epoch & 1950
- Event file - photon events and exposure information
- HouseKeeping file - spacecraft orientation information
While there is one catalog and one orbit file per FOT,
the event and HK files are a variable number depending on the
number of good time periods.
This document describes the content of each file type.
There are a number of ways time is expressed in the FOTs. These time
formats are carried over into the Raw FITS file.
- Short History File (SHF) key. This is stored as a 4-byte integer
and is equal to the number of seconds after the beginning of the year
1980.
- Ground station time. Stored originally in the FOT housekeeping
records, the Ground station time is the only means of relating data to
Universal Time with and accuracy of better than 1 second. Originally
stored as 16 4-bit numbers they represent the the decimal digits of
time at which the beginning of the format was received on the ground.
- Spacecraft Clock. This may be considered as a 39-bit hardware
register on-board, with resolution 2-14 seconds, which increments
starting from an arbitrary value at the beginning of an observing period.
- Modified Julian Date (MJD). In the Catalog FITS files this is the
IAU-defined standard Julian date-2,400,000.5. In the FOT the MJD was
defined such that MJD=0 at Oh GMT on 1 Jan 1950.
Next: EXOSAT RAW LE Catalog
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Leonard Garcia
1999-05-26