EXOFOT - EXOSAT Final Observation Tapes
The EXOFOT database contains the log of the EXOSAT final observation tapes
(FOTs). This database helps to locate the desired EXOSAT data file within the
HEASARC anonymous FTP account. The database itself contains a minimum number of
parameters regarding the observation which correspond to the first 255 bytes
which label the FOT. Those parameters are target name, experiment, and start
and stop time. The other parameters in the database are more related to the
organization of the archive.
The data can be accessed through the anonymous FTP account on
`legacy.gsfc.nasa.gov`. Because of the large volume of the mission, data have
been compressed using the public software `zoo`. Before retrieving the data,
the user should remember that the compression factor can be up to six times the
current size of the file. If the user does not have the `zoo` software, a
version for Unix machines can be retrieved under this ftp account in
The data format is the original binary format, the description of which can be
found in the FOT-handbook. This document was distributed by ESA, with the data
to the original PI. The document does not exist in electronic form; however, a
printed copy may be obtained from the HEASARC upon request.
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), 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
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Gbytes of data, has been archived at ESRIN/ESA, on an optical disk, by the ESIS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
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. Currently
there are about 50 observations for which data recovery was not possible.
Letters have been sent to the original PIs of those observation in the hope
that their copies of the data were better maintained. For a number of
observations (in particular for the early ones), more than one FOT is
available. This is because the FOT was regenerated for various reasons. The
record seems to be missing as to which is the correct FOT.
The following three parameters are used to identify the data set within this
database: target name, experiment, and start and stop time. The user should
be aware that a) within this database is not possible search by coordinates,
and b) one entry does not necessary correspond to a single entry within the
EXOSAT observation log. To search for your data within this database, it is
recommended to use first the EXOSAT observation log database (EXOLOG) to
retrieve the experiment, target name, start and stop time. With this
information, search within this database using the parameters TIME and/or TIME
END using a time window which includes the value found in the EXOLOG database
for the parameters FOT TIME and FOT END. In this way at least three (four
early in the mission when the LE2 was still operating) entries should be
selected, one for each instrument (LE1, LE2, ME, GSPC).
The FAI number.
The FAI number is a parameter which connects the HEASARC database with the
ESRIN database. It is useful to HEASARC personnel to exchange information
about particular datasets with ESRIN.
Each data file (FOT) that can be retrieved from this database contains data
from only one instrument. The values for the `instrument` parameter are
`LE1`, `LE2`, `ME` or `GSPC`. The two LE telescopes are treated as separate
instruments because the filters and other instrument modes can be different.
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.
There are a number of `special` strings for the target field of which the user
must be aware. Those are as follows: a) 2 TARGETS, b) 3 TARGETS, c) 4 TARGETS,
d) ???? and e), a blank string.
In the a), b), and c) cases, the file associated with the entry in the database
contains observations for more than one celestial object. In the d) and/or e)
cases, either it was not possible to retrieve the value from the tape header or
that value was found blank.
The `Time` parameter refers to the start time within a FOT. The data in the
FOT can be related to one or more entries in the EXOLOG database. For those
cases in which the `Time` parameter does not match precisely the start time of
an entry in the EXOLOG database, it is recommended to retrieve data such that
start time for the identify entry in the EXOLOG is included in the `Time`
The time is listed as `yr.day`, where `yr` is the last two digits of the year
and `day` is the day of the year; for example, day 300 of 1984 is 84.300.
End Time of Observation
Date of ESOC Transcription
The string in the `FOTID` field is the identifier assigned to the FOT by
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
parameters is set `YES` it is always followed by the number of bad blocks which
occur in the dataset. The parameter is set to `NONE` if the data set is not
This parameter gives the directory name where the file is located under the
anonymous FTP account on the `legacy` machine. The parameter value `tape#`
ranges from 1 to 49. Note that this is not the full path. In the anonymous FTP
account, the directories (this parameter) 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`.
Name of Archive File
This is file size in bytes of the compressed file. The user would 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 can be present if the original FOT observation consisted of
more than one 1600 BPI tape.
An observation (FOT) distributed on magnetic tape could consist of more than
one magnetic 1600 BPI tape. If multiple tapes were required 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'd` together. The parameters `NFiles` gives the number of files
compress within a `zoo` file.
`File Names` gives the listing of names of each single file that can be found
in the `zoo` file. The number of names should match the parameter `NFiles`.
Gives the original size in bytes of the uncompress data. More than one value is
stored if multiple files have been `zoo'd` together.
Questions regarding the EXOFOT database table can be addressed to the
HEASARC User Hotline.
If you have any problems, please consult the help
page or mail email@example.com