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EXOMASTER - EXOSAT Master Observation Log


The EXOMASTER database contains the EXOSAT observation log. This is a complete list of all EXOSAT observations, observing modes, and principal investigators. The log can be used to find out which targets were observed by EXOSAT, who observed them and the observation configuration. In addition this database can also be used to check the availability of the FOT (Final Observation Tape) files (the original raw data files) and their reformatted FITS files.


Taylor B.G. et al., 1981 Sp. Sci. Rev., Vol. 30, p. 479.

White N.E. and Peacock A.P. 1988, X-ray Astronomy with
EXOSAT, [eds. N.E. White and R. Pallivicini], Memoria S. A. It., p. 59.


This database table was originally created in September/October, 1997. The HEASARC revised this database table in August, 2006, in order to fix the equatorial coordinates (which were in the wrong equinox) and to rename or convert some of the time-related fields to better conform with current HEASARC practices.

Data Products

For each database record there are associated two type of data: the FOT and for the LE only Fits file. The FOT (Final Observation Tape) are the original raw data file (in their native format) distributed to the users in a form of 1600 BPI tape. A single FOT contains data for only one instrument. All the tapes have been archived on disk by ESRIN/ESA and an exact copy has been given to the HEASARC. These data are available in the legacy.gsfc.nasa.gov FTP area. Data are missing For 53 observations the FOT data are missing for one or more instruments because the 1600 BPI tapes could not be read. To analyse the EXOSAT FOT data is recommended to use the EXOSAT Interactive Analysis (IA) System. This can be obtained from the FTP area at ESTEC/SSD on the astro.estec.esa.nl machine or from the FTP area at HEASARC on the legacy.gsfc.nasa.gov. There are several parameters associated to the FOT data (Fotid, Errors, Directory, File_Name, File_Size, Comp_Factor, Nfiles, File_Names, File_Bytes, Faino). Data locations and file names are in the parameters 'File_Name' and 'Directory'.

Using the IA, the FOT data for the LE instrument are now available as Fits file. For each observation there are several fits files: an observation catalog, an orbit file and the data and housekeeping files. Depending on the configuration and the number of time periods several the FITS data files and housekeeping files are available for each observation. The parameters associated to the FITS file are Fits_dir and Status. The first gives the directory location of FITS data on the legacy.gsfc.nasa.gov FTP area and the second records if any error occurs during the processing. The value of status is set to -1 if data in FITS are not available (currently for all the ME and GSPC observations).

Historical View

The European X-ray Observatory Satellite (EXOSAT) was operational from May 1983 to April 1986 and in that time made 1780 detailed observations of a wide variety of astronomical objects. The scientific payload consists of two energy telescopes (LEIT, LE1 and LE2), a medium energy proportional counter (ME), and a gas scintillator proportional counter (GSPC).

The data acquired for the four instruments were distributed to observers as FOT (Final Observation Tape) in a form of 1600 BPI tape. A single FOT contains data for only one instrument, and a FOT (observation) could consist of more than one 1600 BPI tape, which generally contained data from the pointed observation plus part of the in-going and out-going slews (only for the GSPC and ME instruments).

Since 1992, the total amount of ~8643 tapes (1600 BPI), for a volume of 150 Gbytes of data, has been archived at ESRIN/ESA, on an optical disk, by the ESIS project. An exact copy has been given to HEASARC/GSFC (Greenbelt, MD, USA). From ESRIN the data arrived at HEASARC on magnetic media, each containing a number of FOTs. At HEASARC the magnetic tapes were read into a jukebox organized into directories named from the magnetic tape number delivered from ESRIN. ESIS could not recover and archive the complete mission because some of the earliest 1600 BPI tapes (now over ten years old) were badly damaged. There are about 53 observations for which data recovery (for either one or more experiments) was not possible.

All the available FOTs were archived in the beginning of 1995. HEASARC then started reformatting the FOT data in FITS format, starting first with all the LE (LE1 and LE2) data.

Note on Database

EXOSAT carried on board four instruments (LE1, LE2, ME and GSPC). On October 28, 1983, the remaining detector on the optical path of the LE2 failed. Data acquired after that day were obtained only with the LE1, ME, and GSPC instruments. For any given observation, the information pertaining to each instrument (the configuration and data files) is kept in separate records; therefore, there are typically three (or four if the observation is before October 1983) records associated with each observation, one for each of the instruments.


The 'Name' parameter gives the target name. This name usually corresponds to the target name as specified by the original observer. If a source has been observed by different observers it may be entered under two or more names. It is recommended a search by 'name' and by coordinates.

Target name as recorded in the FOT (Final Observation Tape). Its value is usually similar or equal to the value in the parameter 'Name'.

The Right Ascension of the EXOSAT pointing.

The Declination of the EXOSAT pointing.

The Galactic Longitude of the EXOSAT pointing.

The Galactic Latitude of the EXOSAT pointing.

This parameter contains the start time of the observation. Times are accurate to the nearest minute. All times are stored internally as a Short History File key (SHF), which is the number of seconds before or after 1 January 1980 at 00:00 hrs UT.

The end time of the observation. The units and use of this parameter are the same as those for 'shfstart'.

The start time of the observation, stored as an MJD value. Browse will display this value as YYYY-MM-DD hh:mm:ss. Note that the time is only accurate to the nearest minute.

The end time of the observation, stored as an MJD value. Browse will display this value as YYYY-MM-DD hh:mm:ss. Note that the time is only accurate to the nearest minute.

The start time of the observation, as recorded in the FOT. This time is probably only accurate to the nearest minute.

The stop time of the observation, as recorded in the FOT. This time is probably only accurate to the nearest minute.

For each observation there is one entry per instrument. The 'Instrument' parameter identifies the instrument in use. The allow strings are 'LE1' or 'LE2' for the Low energy telescopes used in conjunction with either the CMA or PDS detectors; 'ME' for the Medium Energy proportional counter and 'GSPC' for the Gas Scintillation Proportional Counter. The two LE telescopes are treated as separate instruments because the filters and instrument modes can be different.

A code was assigned to each EXOSAT proposal that approximately defined the class of object to be observed. The codes are as follows:

                 AGN - active galactic nuclei
                 EXG - other kinds of galaxies
                 CLU - clusters of galaxies
                 HLX - high luminosity galactic sources
                 LLX - low luminosity galactic sources
                 SNR - supernova remnants
                 OPS - operational activities
                 CAL - calibration
                 DEE - deep field
                 MIS - miscellaneous
                 PV  - performance verification
                 TOO - target of opportunity

This parameter contains a string corresponding to the original proposal code associated with an observation.

The observer code is a number associated to an observer.

Name of the principal investigator for the observation.

This is a code that gives information concerning the instrument modes for each observation. The code depends on the instrument as follows:

    LE Filters:                        LE detector:
              P = PPL                             C = CMA with filter
              4 = 4000 Lexan                      G = CMA with grating
              A = Al/Pa                           P = PSD
              3 = 3000 Lexan
              B = Boron

   The orientation of the ME array halves:
                                 1 = half 1 offset for part of the period
                                 2 = half 2 offset for part of the period
                                 C = coaligned for part of the period.

   The number, as a hexadecimal digit, of ME high voltages on. If all 16
   detectors are turned on then no character is given. (The argon and
   xenon layers are treated here as separate detectors).

   The GSPC gain used:
                    1 = gain 1.0
                    2 = gain 2.0
                    + = non standard gain, probably 0.5.

The OBC modes define on-board computer programs that were running during the observation. Each program has a short name, e.g., HTR3. More details about these programs can be found in the EXOSAT FOT handbook, available upon request from ESTEC/SSD and from GSFC/HEASARC. The programs depend on the instrument as follows:

   LE:             O = old DIRECT
                   N = new DIRECT without spatial filter
                   D = new DIRECT with diamond filter
                   R = new DIRECT with rectangular filter

   ME:            T3 = HTR3 in full sub-mode
                  P1 = PULS (old version)
                  P2 = PULS2 (new version, with background spectrum)
                  E5 = HER5
                  EH = HER3 or HER4 (data per half array)
                  EQ = HER3 or HER4 (data per quadrant)
                  ED = HER3 or HER4 (data per detector)
                  E2 = HER2
                  D2 = MDIR2 (new direct)
                  I7 = HER7
                  T4 = HTR4
                  I6 = HER6
                  T3 = HTR3 OPT
                  T5 = HTR5

   GSPC:           0 = old DIRECT
                   N = new DIRECT
                   H = HEBL2 or HEBL4

The sequence number is the observation number in chronological order from the beginning of the mission.

Space is provided for up to 3 two-digit footnotes for remarks concerning each log entry. The meanings of the two-digit codes are as follows:

  					Footnote Number
  Solar aspect angle < 90 deg ............  11

  Slight attitude instability ............  12
   (for part of pointing)

  Partial telemetry loss due to ground ...  13
   segment or FOT production problem

  Solar flare affects data ...............  14

  ME HTR3 OPT or HTR5 in addition to 2
   other active OBC programs .............  15

  ME HTR3 OPT data present at 64 Hz
   while sampling at only 32 Hz (so T3
   records have non-standard layout) .....  16

  Unexplained anomaly in ME HTR3 data ....  17

  ME HER4 data partly lost/corrupted .....  18

  OBC data anomaly or crash for short
   period during observation .............  19

  Observation is a raster scan ...........  21

  Observation is an occultation ..........  22

  ME VHR used for at least part of
   observation ...........................  23

  LE1 HER3 used for part of pointing .....  24

  ME PULS or PULS2 active for part of
   an observation, although not the
   predominant configuration .............  27

  GS DIRECT used for part of observation
   although not the predominant mode .....  28

  The ME data for this period is split
   between 2 PI's: between 85/264 0010
   and 0820 the PI is W. Brinkmann .......  29

  Unstable pointing or scanning ..........  30

  Coordinated Observation ................  31

  Data contained on FOT's may not
   correspond to intended source .........  32

Many EXOSAT observations were scheduled to be coordinated (either simultaneously or contemporaneously) with observations from other facilities - ground based (radio, IR, optical), airborne (Balloon platforms, Kuiper Airborne Observatory) and space-based (IRAS, IUE, Voyager, Tenma). Where the coordinated observation flag (note number 31 above) is set, it means that the observatory was notified of planned coordinated observations _before_ they were performed. It is not always known whether individual coordinated observations using other facilities were in fact successful. Those interested should contact the PI directly.

This parameter contains the target number. Sources observed more than once during the mission lifetime have the same target number.

The accumulation time in seconds of the spectral data for the ME argon data (for ME only observations).

The accumulation time in seconds of the spectral data for the ME xenon data (for ME only observations).

The number of channels combined by the OBC; e.g., 1 means full resolution, and 4 means the summing of every 4 adjacent channels.

The ME PHA channel selection and compression:

               spectra                           intensity channels
               -------                           ------------------
            0 = all channels                      A = one argon, no xenon
            1 = argon 4-67 or 5-68, no xenon      B = two argon, no xenon
            2 = argon 4-67 and xenon 138-201      C = one argon, one xenon
            3 = argon 4-67 and xenon 132-195      D = four argon (HER7)
            4 = argon 4-35 and no xenon           E = 4 ranges, with one or
            5 = argon 0-63 and no xenon                more xenon
            6 = argon 0-127 and no xenon          X = one or two xenon, no
            ? = a non standard selection               argon

The minimum time resolution available for the intensity mode data, in seconds (for ME only observations). This time resolution is referring to the 'main' OBC program.

This parameter contains the observation start time and it was used for archive request submission of the FOT.

This parameter contains the observation stop time and it was used for archive request submission of the FOT.

The string in the 'FOTID' field is the identifier assigned to the FOT by ESOC/ESA (Germany).

The date of ESOC transcription.

This parameter flags data sets in which errors occurred during the reading transcription procedure of the 1600 BPI tape into the optical juke-box. If the parameter is 'YES', it is always followed by the number of bad blocks which occur in the data set. The parameter is 'NONE', the data set is not corrupted.

This parameter gives the directory name where the file is located in the legacy.gsfc.nasa.gov anonymous FTP. The parameter value 'tape#' ranges from 1 to 49. Note that this is not the full path. The directories where the FOT files are stored can be found under 'exosat/data/raw'. For example, if the file is located in directory 'tape5', under the anonymous FTP account the full path to the data will be 'exosat/data/raw/tape5'.

This gives the exact name of the file, as found in the FTP account. The string is always in the format 'FOTID.fot.zoo'.

This is file size in bytes of the 'zoo' compressed file. The user should be aware that the compression factor can be as great as six times the uncompressed value. The uncompressed file size is written in the parameter 'File Bytes'. Within a 'zoo' file, multiple files will be present if the original FOT observation consisted of more than one 1600 BPI tape.

This gives the factor of compression from the original file size to the 'zoo' file size.

An observation (FOT) distributed on magnetic tape could consist of more than one magnetic 1600 BPI tape. If multiple tapes were needed to store data for one observation (FOT), each of them has been archived as a separate file. In the compression procedures, all of the files that belong to an observation have been 'zoo'ed together. This parameter gives the number of files compressed within each 'zoo' file.

This parameter gives the list of names for each file in the 'zoo' file. The number of names should match the 'Nfiles' parameter.

Gives the original size in bytes of the uncompress data. More than one value is stored if multiple files have been 'zoo'ed together.

The FAI number is a parameter which connects the HEASARC database with the ESRIN database. It was used by HEASARC personnel to exchange information about particular data sets with ESRIN.

This contains the directory name on the legacy.gsfc.nasa.gov anonymous FTP server that contains the FITS files associated to this observation. The FITS directory value was obtained by summing the value of the sequence number to a numerical representation of the proposal type. The numerical designations of the proposal type have the following meanings:

      100000     PV  - performance verification
      200000     CAL,TOO,OPS - calib., target of opp.,operational act.
      300000     LLX - low luminosity galactic sources
      400000     HLX - high luminosity galactic sources
      500000     SNR - supernova remnants
      600000     EXG - other kinds of galaxies
      700000     AGN - active galactic nuclei
      800000     CLU - clusters of galaxies
      900000     MIS,DEE - miscellaneous, deep field
For example, 401039 is the directory containing the FITS files for this database record with sequence number 1039 and proposal type HLX object. Note that this is not the full path. The directories where the FITS files are stored can be found under 'exosat/data/rawfits/NNNNNN' where NNNNNN is the numerical designations of the proposal type. For example, if the 'FITS_Dir' parameter is 401039, then the full path to the data on the anonymous FTP server is 'exosat/data/rawfits/400000/401039'.

This field provides information on the availability of FITS files and whether there were any errors in processing these files. Only the raw LE data are available as FITS file. The parameter values range between -1 and 2. A value of -1 means no FITS files have been generated yet. A value of 0 means that FITS files for the LE are available and no errors were detected during the processing. A value of 1 means that FITS files were generated however error occurred in processing so the data set may not be complete. A value of 2 means that the FITS files were generated but there were errors in all of the files, little or no data may be available in FITS format. For the 1,984 LE entries processed, 77 had some error when converted into FITS files and of these only 7 have a status of 2.

Contact Person

Questions regarding the EXOMASTER database table can be addressed to the HEASARC User Hotline.
If you have any problems, please consult the help page or mail ledas-help@star.le.ac.uk