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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):
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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