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This Legacy journal article was published in Volume 2, November 1992, and has not been updated since publication. Please use the search facility above to find regularly-updated information about this topic elsewhere on the HEASARC site.

The OGIP Spectral File Format

(PHAVERSN = 1992a)

Keith A. Arnaud (HEASARC), Ian M. George (HEASARC)
and Allyn F. Tennant (X-ray Branch, NASA/MSFC)


Version: 1992 Sept 21 OGIP Memo OGIP/92-007

Summary

The approach and OGIP standard format for storing PHA data for spectral analysis is outlined and discussed. We concentrate on the simple case of a single file containing a single PHA dataset, however the more general case of storing multiple datasets is briefly described. The requirements imposed on the resultant PHA file such that calibration data etc. can be easily constructed are also outlined.

Intended audience: primarily OGIP programmers, hardware teams & authors of spectral analysis s/w.

1 Introduction

Any general X-ray spectral analysis package requires two type of input:

  • a file containing the data. This is usually the number of counts observed in each spectral channel/bin of the detector, and commonly known as the PHA (Pulse Height Analyzer) file for historical reasons.

  • auxiliary information required during the spectral analysis process.
    This includes:
    • calibration data (detailing the transformation between an input X-ray spectrum and the number of counts in each detector channel, the spectral response of the detector etc.). The OGIP standard format for the calibration files associated with spectral analysis is given in OGIP Calibration memo CAL/GEN/92-002[1] (George et al. 1992a)
    • the background in the detector during the observation
    • any miscellaneous corrections applicable during the observation

The precise requirements for this type of input are obviously highly detector- and observation-specific.

A generic mission-independent X-ray spectral analysis package (e.g., XSPEC) requires that both the above types of information are provided prior to the spectral analysis process in the form of input files. The two types of information are separable, with this memo setting out the OGIP standard format for the PHA file. However it is important to note that the auxiliary information is invariably highly dependent upon the criteria used to construct a given PHA dataset (e.g., upon the observation date, the size and position of the extraction cell in detector co-ordinates etc.), along with the value of any number of housekeeping parameters during the observation. Thus the PHA data file must contain sufficient information to allow the auxiliary information to be generated either inside or outside the spectral analysis package. We use XSPEC to set the requirements since it is the most sophisticated package generally available to date, and in widespread use throughout the community.

The PHA data file formats specified here were developed with the following goals in mind:

  • The format must be FITS
  • The file must contain all information required by a spectral analysis package.
  • The file must contain a complete creation and modification history.
  • The file must contain enough information for the response matrix for the particular observation to be generated using this file and any associated the basic calibration files (BCFs --see George 1992). We use the OGIP program BLDRSP to set these requirements.
  • For those instruments for which a background prediction algorithm is used, the file must also contain enough information to do this.

As will be discussed in Section 3 below, within the OGIP standard format for PHA files, the information is divided between several standard FITS extensions.

2 Requirements

2.1 XSPEC

The input PHA files required by XSPEC Versions prior to 8.2 are in the so-called SF format[2]. This is a non-portable (sequential access) binary format which stores the required data as a number of "packages'' within the file. The FITS format must contain all the information contained in the mandatory SF "packages''. This information comprises:

  • The data plus optional errors and systematics. The data is either in counts in which case Poisson errors will be applied by XSPEC (and optional systematic errors can be given) or in counts s-1 in which case the file must also include errors (and optional systematic errors). The systematic errors should be expressed as a fraction of the data.

  • File information. This comprises a detector identification string, the number of channels in the PHA spectrum, the integration time, the area scaling factor, the background scaling factor (for an imaging instrument this should be the area on the sky over which the spectrum is accumulated), the correction scaling factor.

  • Associated file information. Also required are pointers to the default background, correction, redistribution matrix (RMF), and ancillary response (ARF) files.

Additional optional information can also be used by XSPEC. This includes:

  • Grouping information. This specifies any "raw" detector channels that should be ignored by default and also includes any user-specified binning of the data to be performed. In the SF files the actual stored data is binned and the grouping card describes how this was done so that XSPEC can bin up any background and response in the same way. In the FITS format this will be modified so that the data is always stored in unbinned mode and the grouping card indicates to XSPEC how data should be binned up when it is read in.

  • Filter information. This consists of a set of character strings which are used to specify how the data was selected. This might include time ranges, spatial region specifiers, phase specifiers etc. This information is used in XSPEC to perform joint spectral/spatial and spectral/temporal analysis.

2.2 BLDRSP

A Stage 2 Cal s/w task (BLDRSP; see CAL/SW/92-004, George et al. 1992b) is currently under development, based on the existing XANADU VIMAT task, which constructs the redistribution matrix (RMF) & ancillary response (ARF) files. These files contain the calibration information required by XSPEC for spectral analysis of the PHA data, and their formats are discussed in CAL/GEN/92-002 (George et al. 1992a). In most cases the RMF & ARF are specific to a given PHA dataset (see Section 6 of CAL/GEN/92-002). Thus in order to minimize demands on the User, wherever possible that PHA file should contain all the information required by BLDRSP to construct the RMF & ARF.

BLDRSP requires a large variety of information which is different for different detectors. This information is provided in a number of additional FITS extensions within the PHA file, as a combination of instrument-specific header keywords and data arrays. Currently it is proposed that i/p PHA files to BLDRSP can contain all/any of the following: a Good Time Interval (GTI) extension; a Detector extension; and a (Selector) History extension. The format of these will conform to OGIP standards which will be described elsewhere.

BLDRSP will be modular, divided on a detector-by-detector basis. It is intended that successive versions of BLDRSP will be released as the calibration data, necessary algorithms etc. for successive detectors becomes available.

3 The OGIP Standard PHA File Format

The standard spectral file will consist of a FITS file with a null primary array and a number of extensions. Each PHA dataset will be contained within a

  • Data Extension

    However, each file may also include a number of (optional) associated, additional extensions which contain the detector- and observation-specific information required for construction of the calibration data etc. (i.e., describing how that PHA dataset was selected, housekeeping parameters during the extraction etc.). These include:

  • a Good Time Interval (GTI) extension
  • a Detector extension
  • a (Selector) History extension

    The format of these extensions will also conform to OGIP standards and be identical to those required by BLDRSP (above). It should be noted that none of the latter optional extensions are strictly mandatory for specification of the PHA dataset, and hence as an input to XSPEC. Thus these extensions are not discussed in detail here, and if disk space becomes a problem may be deleted by the user prior to using XSPEC. However, it should be stressed that any analysis package may require information in the GTI and Detector extensions when joint spectral/temporal and/or spectral/spatial analysis is to be performed. Information from the Detector extension will also be required when background modelling is to be performed. Such complex, detector-specific tasks will be discussed individually elsewhere. However, in most cases the XFLTXXXX keyword (discussed below) provides an adequate description of the required information.

    We first discuss the case where a single PHA dataset is stored. Then, briefly, a more general format whereby multiple PHA datasets can be stored in a single extension, as is required for some analysis tasks.

    3.1 Type I: The PHA Data Extension for a single dataset

    This is a BINTABLE extension containing the data. In this simplest of cases, the data is stored as a 1-dimensional list (as a function of detector channel). The number of rows in the BINTABLE is therefore the number of (raw) detector channels.

    3.1.1 Extension Header

    The header for the PHA file Data Extension includes all the information required by XSPEC (and by extension any other general spectral analysis package) to describe the stored PHA data. This consists of the following (mandatory) keywords/values:

  • EXTNAME (= SPECTRUM) - the name (i.e., type) of the extension
  • TELESCOP - the "telescope" (i.e., mission/satellite name).
  • INSTRUME - the instrument/detector.
  • FILTER - the instrument filter in use (if any)
  • EXPOSURE - the integration time (in seconds) for the PHA data (assumed to be corrected for deadtime, data drop-outs etc.)
  • AREASCAL - the area scaling factor (see below).
  • BACKFILE - the name of the corresponding background file (if any)
  • BACKSCAL - the background scaling factor.
  • CORRFILE - the name of the corresponding correction file (if any)
  • CORRSCAL - the correction scaling factor.
  • RESPFILE - the name of the corresponding (default) redistribution matrix file (RMF; see George et al. 1992a), supplied by BLDRSP.
  • ANCRFILE - the name of the corresponding (default) ancillary response file (ARF; see George et al. 1992a), supplied by BLDRSP.
  • XFLTXXXX - the XSPEC selection filter descriptor, where XXXX is a number (see below).
  • PHAVERSN - the OGIP version number of the FITS format in use to store the PHA data (in this case 1992a).
  • POISSERR - whether Poissonian errors are appropriate to the data (see below).
  • CHANTYPE - whether the channels used in the file have been corrected in anyway (see below).
  • DETCHANS - the total number of detector channels available.

    The AREASCAL keyword gives the scaling factor of the PHA data. In most cases AREASCAL = 1.0, since the instrumental effective area is provided within the spectral calibration files (specifically by the SPECRESP column of the ARF in the general case -- see George et al. 1992a).

    The XFLTXXXX keyword is required by XSPEC to aide joint spectral/temporal or spectral/spatial fitting when the input consists of several individual PHA files. The keyword will contain the values of the appropriate time (or phase etc.), or spatial parameters describing the PHA dataset. Thus, in the case of joint spectral/temporal analysis for example, the XFLT0001 keyword will consist of a string containing the start & stop time of the PHA dataset. These times will be in the units required by the spectral/temporal model to be used within XSPEC (and thus could be in units of orbital phase of a binary system). It is the responsibility of the User to consult the relevant GTI and/or Detector extensions (below), and place the required information into the relevant XFLTXXXX keyword. Further examples of such complex spectral analysis tasks will be given elsewhere.

    The CHANTYPE keyword specifies whether the channels referred to in the file are as assigned by the detector electronics at the time of data collection (in which case CHANTYPE = PHA), or whether any corrections have been applied. An example of the latter case is when the data has been re-mapped onto a standard "pulse-Invariant" (PI) channel grid, in which case CHANTYPE = PI (see also CAL/GEN/92-002, George et al. 1992a, Section 7).

    The following optional keywords supply further information:

  • OBJECT - the observed object.
  • RA-NOM - the nominal Right Ascension of the object (in decimal degrees)
  • DEC-NOM - the nominal declination of the object (in decimal degrees)
  • EQUINOX - the equinox of the above celestial co-ordinate (RA & DEC) specifications
  • RADECSYS - the coordinate frame used for EQUINOX
  • DATE-OBS - nominal U.T. date when integration of this PHA data started (dd/mm/yy)
  • TIME-OBS - nominal U.T. time when integration of this PHA data started (hh:mm:ss)
  • DATE-END - nominal U.T. date when integration of this PHA data ended (dd/mm/yy)
  • TIME-END - nominal U.T. time when integration of this PHA data ended (hh:mm:ss)

    Finally, a number of optional keywords are provided to supply the user with information on the precise start & stop times of the integration of the PHA dataset. These will be listed elsewhere and adhere to the standard OGIP method for defining times (see Angelini et al. 1992). Such keywords are necessary since times calculated using the standard (DATE-OBS + TIME-OBS) & (DATE-END + TIME-END) keywords are not guaranteed exact (since they involve a conversion from spacecraft clock to UT etc.), and in any case do not allow sub-second temporal resolution. Times specified by these keywords are in the original temporal units supplied by the spacecraft/pre-processing system. Thus, in the case of (say) sub-second, phase-resolved spectroscopy users are able to cross-check that their selected PHA dataset does indeed correspond to that required from the ephemeris, taking into account any glitches in the spacecraft clock, leap seconds, barycentric corrections, etc. It is strongly recommended that the information supplied by these keywords is included in all PHA datasets, even if not strictly necessary from the scientific viewpoint. Of course, the exception to this recommendation is when the times have no meaning --- e.g., when a phase-folded or source-intensity-selected PHA dataset has been constructed.

    3.1.2 Data Format

    The BINTABLE columns are:

  • Chan, a 2-byte INTEGER scalar giving the channel number for each row.
    The FITS column name is CHANNEL.
    (unitless).

  • Data, either

    • a 2-byte INTEGER scalar giving the number of counts observed in that channel.
    • The FITS column name is COUNTS, and the recommended units are counts.
    • a 4-byte REAL scalar giving the number of counts per second observed in that channel.
    • The FITS column name is RATE, and the recommended units are counts s-1.
  • Stat_err, a 4-byte REAL scalar giving the statistical error on the value within Data.
    The FITS column name is STAT_ERR.
    The recommended units are as for Data above.

  • Sys_err, an (optional) 4-byte REAL scalar giving the fractional systematic error to be applied to the data in this row (channel).
    The FITS column name is SYS_ERR.
    (unitless).

  • Qual, an (optional) 2-byte INTEGER scalar giving the data quality flag for this row (channel):

    • Qual=0 if the data quality is "good"
    • Qual!=0 if the data quality is "bad", with the option of the integer value specifying the origin of the flag:

      • Qual=1; channel defined "bad" by s/w
      • Qual=2; channel defined "dubious" by s/w
      • Qual=3,4; spare (i.e., flag value not used)
      • Qual=5; channel set "bad" by user
      • Qual=-1; reason for "bad" flag unknown

      The FITS column name is QUALITY.
      (unitless).

  • Grpg, an (optional) 2-byte INTEGER scalar giving the data grouping flag for this row (channel):

    • Grpg=+1 if the channel is the start of a new bin.
    • Grpg=-1 if the channel is part of a continuing bin.
    • Grpg=0 if the data grouping is undefined for all channels (see below)

      The FITS column name is GROUPING.
      (unitless).

    These are summarized in Table 1.

    It should be noted that an alternative scheme whereby the Qual and Grpg columns are combined into a single quantity, dqf (with the column name DQF) is also allowed, though no longer recommended. Under this scheme (which is used on the 1992 June 01 CD-ROM distributed by the HEASARC containing Einstein SSS and MPC data), dqf=0 corresponds to "bad" data, and dqf=+1 & dqf=-1 flag whether the channel is (respectively) the start of a new bin, or part of a continuing bin (see Section 4.2).

    Table 1 OGIP format (1992a) for a single PHA dataset

    3.1.3 Points to Note & Conventions

  • The ordering of the columns is of course arbitrary, however that used here is strongly recommended.
  • The order of Chan should be sequential, starting from the lowest.
  • Alternate internal formats (e.g., DOUBLE PRECISION) for the specification of the values within Data & Stat_err are of course allowed but the full precision may not be used by the analysis program.
  • In the case of Data in units of Counts per channel (only), if appropriate the Stat_err column can be deleted from the data table, and POISSERR=T specified as a keyword within the extension header. XSPEC will then assume Poissonian errors are appropriate to the data stored in Data.
  • Sys_err is the (optional) fractional systematic error (relative to the value of Data in each row) which XSPEC will add in quadrature to the error given by Stat_err (or the calculated Poissonian error). If no such systematic error is to be applied, then SYS_ERR = 0 will be specified as a keyword within the extension header, and the Sys_err column deleted from the data table.
  • If all rows contain the value Qual = 0 ("good"), then QUALITY = 0 will be specified as a keyword within the extension header, and the Qual column deleted from the data table. Within XSPEC, Qual=1 and Qual=5 rows will never be accessible and Qual=2 may be ignored using the "ignore bad" command.
  • If the data has not been "grouped" (i.e., the rebinning of the channels has not been defined), then GROUPING = 0 will be specified as a keyword within the extension header, and the Grpg column deleted from the data table.

    3.2 Type II: The PHA Data Extension for multiple datasets

    For a number of analysis tasks in which multiple PHA datasets are to be stored and/or analyzed simultaneously, it is generally considered cumbersome if each resides in a separate file. This is particularly the case for (say) sub-second, time-resolved spectroscopy when a single observation may give rise to a very large number of datasets. Thus guidelines for the storage of multiple PHA datasets are currently being devised. It should be stressed however that this is for datasets from the same instrument. Users wishing to simultaneously analyze PHA data from different instruments should always use multiple input files. Finally, it is worth noting that users who prefer to use the multiple file approach for such analysis (especially if only a small number of datasets are to be considered) can still do so by using the XFLTXXXX keyword described above.

    As in the single dataset case, the data will be stored as a BINTABLE extension. However here, vector arrays are used in place of scalars for several columns. Thus the PHA data, errors etc. are vectors and each row contains a single PHA dataset. The other columns list all other parameters which vary between the datasets. Here, purely for the purpose of demonstration, we consider the example in which the format is customized for time-resolved spectroscopy.

    3.2.1 Extension Header

    The mandatory and optional keywords are as defined above; the exceptions are those keywords which are now column names (the value of the PHAVERSN keyword will also of course be different).

    3.2.2 Data Format

    An example of the format customized for time-resolved spectroscopy would contain the following columns:

  • Num, a 2-byte INTEGER scalar giving the reference number of the spectrum stored in this row.
    The FITS column name is SPEC_NUM.
    (unitless).
  • Tstart, a 4-byte REAL scalar giving the start time of the accumulation.
    It is strongly recommended that the FITS column name and units is consistent with the relevant keyword within the standard OGIP method for defining times
  • either:
    • Tbin, a 4-byte REAL scalar giving the duration of the accumulation.
      The FITS column name is EXPOSURE.
      The recommended units are seconds.
    • or Tstop, a 4-byte REAL scalar giving the end time of the accumulation.
      The recommended FITS column name and units are as for Tstart.

  • Data, either:
    • a 2-byte INTEGER array containing the number of counts observed in all channels during this time interval;
    • or a 4-byte REAL array containing the number of counts per second observed in all channels during this time interval.
      In both cases the notation is as described in Section 3.1.2.

  • Stat_err, a 4-byte REAL array giving the statistical error on the values within Data. The notation is as described in Section 3.1.2.

  • Sys_err, an (optional) 4-byte REAL array giving the fractional systematic error to be applied to each value within Data. The notation is as described in Section 3.1.2.

  • Qual, an (optional) 2-byte INTEGER array giving the data quality flag for each value within Data. The notation is as described in Section 3.1.2.

  • Grpg, an (optional) 2-byte INTEGER array giving the data data grouping flag for each channel. The notation is as described in Section 3.1.2.

  • Bkgdfile, an (optional) 2-byte CHARACTER string giving the name of the background file (if any) associated with the dataset contained in Data.
    The FITS column name is BACKFILE

  • Backscal, an (optional) 4-byte REAL giving the scaling factor to be applied to Bkgdfile.
    The FITS column name is BACKSCAL

  • Corrfile, an (optional) 2-byte CHARACTER string giving the name of the correction file (if any) associated with the dataset contained in Data.
    The FITS column name is CORRFILE

  • Corrscal, an (optional) 4-byte REAL giving the scaling factor to be applied to Corrfile.
    The FITS column name is CORRSCAL

  • Respfile, an 2-byte CHARACTER string giving the name of the redistribution matrix file (RMF) associated with the dataset contained in Data.
    The FITS column name is RESPFILE

  • Ancrfile, an (optional) 2-byte CHARACTER string giving the name of the ancillary response file (ARF; if any) associated with the dataset contained in Data.
    The FITS column name is ANCRFILE

    3.2.3 Points to Note & Conventions (in addition to those given in Section 3.1.2)

  • As with all OGIP files, any of the above variables which have the same value in every row can be removed from the table, and specified as a keyword within the header. Here this is most likely in the case of Bkgdfile, Backscal, Corrfile, Corrscal, Respfile and Ancrfile.
  • In the case that any/all of arrays Sys_err, Qual and Grpg contain elements with differ as a function of PHA channel, but not as a function of row, then these columns can be removed from the table and put in a separate extension within the same file.

    4 Example FITS Headers

    Here we give an example of keywords used for the Primary, Data and Detector extensions (only) appropriate for BBXRT. Note that several of the keywords are repeated in both the Data and Detector Extension for clarity and user reassurance. In Section 4.2 we also give the (old) PHA file format used for Solid State Spectrometer (SSS) data products distributed on the HEASARC CD-ROM 1992 June 01 for direct comparison.

    4.1 BBXRT

    This example demonstrates a PHA dataset which is stored in counts per channel, requires Poissonian errors (with no systematic errors) to be included during spectral analysis, and has not been rebinned.

    4.1.1 Primary Header

    SIMPLE  =                    T / file does conform to FITS standard
    BITPIX  =                    8 / number of bits per data pixel
    NAXIS   =                    0 / number of data axes
    EXTEND  =                    T / FITS dataset may contain extensions
    CONTENT = 'SPECTRUM'           / spectrum file contains time intervals and
    event
    FILENAME= 'a0cygx2j.sp'        / File that fits was produced from
    ORIGIN  = 'NASA/GSFC'          / origin of fits file
    DATE    = '18/09/92'           / FITS creation date (dd/mm/yy)
    TELESCOP= 'BBXRT   '           / Telescope (mission) name
    INSTRUME= 'A0      '           / Instrument name
    OBJECT  = 'CYG X-2 '           / Name of observed object
    RA-NOM  =           3.2565E+02 / Right Ascension of target (deci. deg)
    DEC-NOM =           3.8091E+01 / Declination of target (deci. deg)
    DROLLANG=           3.0100E+02 / Mean roll angle (deci. deg)
    OBS-MODE=                    1 / observing mode 1=point,2=slew,3=calibration
    DATE-OBS= '06/12/90'           / Date observations were made (dd/mm/yy)
    TIME-OBS= '06:10:51'           / Time observations were made (hh:mm:ss)
    DATE-END= '06/12/90'           / Date observations ended (dd/mm/yy)
    TIME-END= '06:25:15'           / Time observations ended (hh:mm:ss)
    END  
    
    4.1.2 Data Extension

    XTENSION= 'BINTABLE'           / binary table extension
    BITPIX  =                    8 / 8-bit bytes
    NAXIS   =                    2 / 2-dimensional binary table
    NAXIS1  =                    6 / width of table in bytes
    NAXIS2  =                  512 / Modified
    PCOUNT  =                    0 / size of special data area
    GCOUNT  =                    1 / one data group (required keyword)
    TFIELDS =                    2 / number of fields in each row
    TTYPE1  = 'CHANNEL '           / label for field   1
    TFORM1  = 'I       '           / data format of the field: 2-byte INTEGER
    TTYPE2  = 'COUNTS  '           / label for field   2
    TFORM2  = 'E       '           / data format of the field: 4-byte REAL
    TUNIT2  = 'COUNTS  '           / physical unit of field
    EXTNAME = 'SPECTRUM'           / name of this binary table extension
    STAT_ERR=                    0 / no statistical error specified
    POISSERR=                    T / Poissonian statistical errors to be assumed
    SYS_ERR =                    0 / no systematic error specified
    GROUPING=                    0 / no grouping of the data has been defined
    QUALITY =                    0 / no data quality information specified
    
    Additional Mandatory keywords for XSPEC (in addition to those above)

    TELESCOP= 'BBXRT   '           / Telescope (mission) name
    INSTRUME= 'A0      '           / Instrument name
    FILTER  = 'none    '           / Instrument filter in use
    EXPOSURE=           8.6400E+02 / Exposure time
    AREASCAL=           1.0000E+00 / nominal effective area
    BACKSCAL=           1.0000E+00 / background scale factor
    CORRSCAL=           0.0000E+00 / Correlation scale factor
    BACKFILE= 'a0cygx2j.bk'        / background FITS file for this object
    CORRFILE= 'none    '           / correlation FITS file for this object
    RESPFILE= 'a0bb10.rs'          / redistribution matrix file (RMF)
    ANCRFILE= 'none    '           / ancillary response file (ARF)
    XFLT0001= 'none    '           / XSPEC selection filter description
    CHANTYPE= 'PHA     '           / Channels assigned by detector electronics
    DETCHANS=                  512 / Total no. detector channels available
    PHAVERSN= '1992a   '           / OGIP classification of FITS format style
    
    Additional Optional (but recommended) keywords (some specific to BBXRT)

    OBJECT  = 'CYG X-2 '           / Name of observed object
    FILENAME= 'a0cygx2j.sp'        / File that fits was produced from
    ORIGIN  = 'NASA/GSFC'          / origin of fits file
    DATE    = '18/09/1992'         / FITS creation date (dd/mm/yy)
    RA-NOM  =           3.2565E+02 / Right Ascension of target (deci. deg)
    DEC-NOM =           3.8091E+01 / Declination of target (deci. deg)
    EQUINOX =               1950.0 / Equinox of celestial coord system
    RADECSYS= 'FK4     '           / coord frame used for EQUINOX
    DROLLANG=           3.0100E+02 / Mean roll angle (deci. deg)
    DATE-OBS= '06/12/90'           / Date observations were made (dd/mm/yy)
    TIME-OBS= '06:10:51'           / Time observations were made (hh:mm:ss)
    DATE-END= '06/12/90'           / Date observations ended (dd/mm/yy)
    TIME-END= '06:25:15'           / Time observations ended (hh:mm:ss)
    OBS-MODE=                    1 / observing mode 1=point,2=slew,3=calibration
    COMMENT   This table contains a PHA histogram
    COMMENT   obtained with the Broad Band X-ray telescope, part
    COMMENT   of the Astro-1 payload on STS-35, which was
    COMMENT   in orbit 1990 Dec 2 - Dec 11.
    COMMENT   For each of the 512 detector channels,
    COMMENT
    COMMENT   CHANNEL     detector channel number (1-512)
    COMMENT   COUNTS        observed photon count rate
    COMMENT
    COMMENT   The keywords STAT_ERR=0 and POISERR=T combined indi-
    COMMENT   indicates that Poissonian errors are to be assumed on
    COMMENT   COUNTS. SYS_ERR=0 indicates that no systematics are
    COMMENT   added in quadrature. GROUPING=0 indicates that
    COMMENT   no grouping flag has been assigned
    END
    
    4.1.3 Detector Extension

    For BBXRT, the detector extension includes

    XTENSION= 'BINTABLE'           / binary table extension
    BITPIX  =                    8 / 8-bit bytes
    NAXIS   =                    2 / No binary table is actually present
    NAXIS1  =                    0 / width of table in bytes
    NAXIS2  =                    1 / number of rows in table
    PCOUNT  =                    0 / size of special data area
    GCOUNT  =                    1 / one data group (required keyword)
    TFIELDS =                    0 / number of fields in each row
    EXTNAME = 'DETECTOR'           / name of this binary table extension
    TELESCOP= 'BBXRT   '           / Telescope (mission) name
    INSTRUME= 'A0      '           / Instrument name
    FILTER  = 'none    '           / Instrument filter in use
    OBJECT  = 'CYG X-2 '           / Name of observed object
    FILENAME= 'a0cygx2j.sp'        / File that fits was produced from
    ORIGIN  = 'NASA/GSFC'          / origin of fits file
    DATE    = '18/09/92'           / FITS creation date (dd/mm/yy)
    RA-NOM  =           3.2565E+02 / Right Ascension of target (deci. deg)
    DEC-NOM =           3.8091E+01 / Declination of target (deci. deg)
    EQUINOX =               1950.0 / Equinox of celestial coord system
    RADECSYS= 'FK4     '           / coord frame used for EQUINOX
    DROLLANG=           3.0100E+02 / Mean roll angle (deci. deg)
    DATE-OBS= '06/12/90'           / Date observations were made (dd/mm/yy)
    TIME-OBS= '06:10:51'           / Time observations were made (hh:mm:ss)
    DATE-END= '06/12/90'           / Date observations ended (dd/mm/yy)
    TIME-END= '06:25:15'           / Time observations ended (hh:mm:ss)
    OBS-MODE=                    1 / observing mode 1=point,2=slew,3=calibration
    PHACHANS=                  512 / number of pha channels
    MINCHAN =                    1 / lowest pha channel included
    MAXCHAN =                  512 / highest pha channel included
    MGUARDR =            1.433E+03 / Mean guard rate during observation
    OFFAXISA=              9.0E-01 / Off-axis angle (arcminutes)
    END                  
    
    4.2 The Einstein SSS

    This example PHA dataset is from the HEASARC CD-ROM distributed on 1992 June 01 containing data from the Einstein Observatory. It should be stressed that it is NOT in the OGIP standard format (PHAVERSN = 1992a) described above, though can be read by XSPEC version 8.0 upwards. It is provided only for direct comparison with the new standard.

    4.2.1 Primary Header

    SIMPLE  =                    T / file does conform to FITS standard
    BITPIX  =                  -32 / number of bits per data pixel
    NAXIS   =                    0 / number of data axes
    EXTEND  =                    T / FITS dataset may contain extensions
    CONTENT = 'SPECTRUM'           / file contains spectrum
    DATE    = '24/04/92'           / date this FITS file was created (dd/mm/yy)
    ORIGIN  = 'HEASARC/GSFC'       / organization which created this file
    TELESCOP= 'EINSTEIN'           / also known as HEAO-2
    INSTRUME= 'SSS     '           / Solid State Spectrometer
    OBJECT  = 'FAIRALL 9'          / observed object
    RA      =           20.4583340 / right acension of target (decimal degrees)
    DEC     =          -59.0663872 / declination of the target (decimal degrees)
    EQUINOX =               1950.0 / equinox of celestial coord. system
    DATE-OBS= '10/05/79'           / date observations were made (dd/mm/yy)
    TIME-OBS= '04:55:38'           / time observations were made (hh:mm:ss)
    DATE-END= '10/05/79'           / date observations ended (dd/mm/yy)
    TIME-END= '09:45:05'           / time observations ended (hh:mm:ss)
    END      
    
    4.2.2 Data Extension

    XTENSION= 'BINTABLE'           / binary table extension
    BITPIX  =                    8 / 8-bit bytes
    NAXIS   =                    2 / 2-dimensional binary table
    NAXIS1  =                   12 / width of table in bytes
    NAXIS2  =                  128 / number of rows in table
    PCOUNT  =                    0 / size of special data area
    GCOUNT  =                    1 / one data group (required keyword)
    TFIELDS =                    4 / number of fields in each row
    TTYPE1  = 'CHANNEL '           / detector channel number
    TFORM1  = 'I       '           / data format of the field: 2-byte INTEGER
    TTYPE2  = 'RATE    '           / observed event count rate
    TFORM2  = 'E       '           / data format of the field: 4-byte REAL
    TUNIT2  = 'COUNTS/S'           / physical unit of field
    TTYPE3  = 'STAT_ERR_RATE'      / statistical rms error on the count rate
    TFORM3  = 'E       '           / data format of the field: 4-byte REAL
    TUNIT3  = 'COUNTS/S'           / physical unit of field
    TTYPE4  = 'DQF     '           / data quality flag
    TFORM4  = 'I       '           / data format of the field: 2-byte INTEGER
    EXTNAME = 'SPECTRUM'           / name of this binary table extension
    TELESCOP= 'EINSTEIN'           / also known as HEAO-2
    INSTRUME= 'SSS     '           / Solid State Spectrometer
    OBJECT  = 'FAIRALL 9'          / observed object
    RA      =           20.4583340 / right acension of target (decimal degrees)
    DEC     =          -59.0663872 / declination of the target (decimal degrees)
    EQUINOX =               1950.0 / equinox of celestial coord. system
    DATE-OBS= '10/05/79'           / date observations were made (dd/mm/yy)
    TIME-OBS= '04:55:38'           / time observations were made (hh:mm:ss)
    DATE-END= '10/05/79'           / date observations ended (dd/mm/yy)
    TIME-END= '09:45:05'           / time observations ended (hh:mm:ss)
    STRTTIME=            -20372662 / obs. start time (seconds from 01/01/80)
    STOPTIME=            -20355295 / obs. stop time (seconds from 01/01/80)
    EXPOSURE=         4300.7998047 / integration time (seconds)
    MEANEPOC=          495.3051758 / mean obs time (decimal day of year 1978)
    MAJORFRM=                 7633 / major frame start time
    MINORFRM=                    0 / minor frame start time
    NEXPOSE =                    6 / total number of exposures
    EXPOS1  =          901.1199951 / length of exposure (seconds)
    EXPOS2  =          327.6799927 / length of exposure (seconds)
    EXPOS3  =         1105.9199219 / length of exposure (seconds)
    EXPOS4  =            0.0000000 / length of exposure (seconds)
    EXPOS5  =         1310.7199707 / length of exposure (seconds)
    EXPOS6  =          655.3599854 / length of exposure (seconds)
    MEPOC1  =          495.2131042 / mean epoch of exposure (day of 1978)
    MEPOC2  =          495.2373047 / mean epoch of exposure (day of 1978)
    MEPOC3  =          495.2858887 / mean epoch of exposure (day of 1978)
    MEPOC4  =          495.0000000 / mean epoch of exposure (day of 1978)
    MEPOC5  =          495.3490906 / mean epoch of exposure (day of 1978)
    MEPOC6  =          495.4108887 / mean epoch of exposure (day of 1978)
    EFFAREA =               180.00 / on-source detector area (cm**2)
    BACKSCAL=                1.000 / background scale factor
    CORRSCAL=                0.000 / correction scale factor
    JCGPARM =            171.00000 / background prediction parameter
    DEADTIME=           17600000.0 / dead time parameter
    MEAN_ICE=                 1.33 / mean water ice coating on detector
    BACKFILE= 'fair9a.bk'          / background FITS file for this object
    RESPFILE= 'fair9a.rs'          / response FITS file for this object
    CORRFILE= 'fair9a.cr'          / correction FITS file for this object
    COMMENT   This table summarizes an X-ray spectrum observed with the Solid
    COMMENT   State Spectrometer on the EINSTEIN (HEAO-2) satellite. For each
    COMMENT   of the 128 detector channels the table lists the following values:
    COMMENT
    COMMENT   CHANNEL       detector channel number (1-128)
    COMMENT   RATE          observed photon count rate before background
    subtraction
    COMMENT   STAT_ERR_RATE statistical rms error of the count RATE
    COMMENT   DQF           data quality flag (0=bad, 1=good)
    COMMENT
    COMMENT   The first 14 channels and the last 20 channels out of the 128 SSS
    COMMENT   detector channels did not produce any useful data, therefore the
    COMMENT   corresponding quantities for these channels are all set equal to
    COMMENT   an IEEE NaN value (all bits set to 1).
    HISTORY   Input PHA spectrum file = SFAIR9A.PHA
    END
    
    Acknowledgements

    We thank the numerous people, both inside and outside the OGIP, who have contributed ideas and suggestions. In particular we thank Alan Smale, Nick White, Bill Pence & Lorella Angelini for their critical reading of the various drafts.

    References

    Angelini, L., et al., 1992. In preparation (OGIP/92-010).
    George, I.M., 1992. Legacy, 1, 56, (CAL/GEN/91-001).
    George, I.M., Arnaud, K.A., Pence, W. & Ruamsuwan, L., 1992a. Legacy, 2, 51 (CAL/GEN/92-002).
    George, I.M., et al., 1992b. In preparation. (CAL/SW/92-004).


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