Contents
1 INTRODUCTION2 OVERVIEW 2.1 The Redistribution Matrix 2.2 Detector Effective Area 2.3 Implementation 2.4 Rationale 2.5 Additional Notes3 THE HEASARC STANDARD RMF FORMAT 3.1 The RMF Redistribution Extension 3.1.1 Extension Header 3.1.2 Data Format 3.1.3 Points to Note & Conventions 3.2 The RMF EBOUNDS Extension 3.2.1 Extension Header 3.2.2 Data Format 3.2.3 Points to Note & Conventions4 THE HEASARC STANDARD ARF FORMAT 4.1 The ARF Extension 4.1.1 Extension Header 4.1.2 Data Format: Type I - a single arf 4.1.3 Data Format: Type II - multiple arfs 4.1.4 Points to Note & Conventions5 USAGE: TYPICAL SCENARIOS6 SPECIAL CASES7 EXAMPLE FITS HEADERS 7.1 ASCA RMF 7.1.1 RSP_MATRIX Extension 7.1.2 EBOUNDS Extension 7.2 ASCA ARF 7.2.1 SPECRESP Extension
1 INTRODUCTION
Calibration files within the
HEASARC
Calibration Database (CALDB) have been classified into 2 types:
- BASIC CALIBRATION FILES (BCFs) containing all the basic
calibration information for a given instrument.
The BCFs will contain calibration information which is both
independent of time (in most cases
data originating from ground calibration measurements), and
information which is expected to
vary throughout the mission (mainly from in-orbit measurements).
In many BCFs the data will be stored in the form
of large n-dimensional arrays. A more detailed discussion of the
general formats and organization of the
BCFs can be found in the Office of General Investigator Programs (OGIP) Calibration Memo
CAL/GEN/92-003.
- CALIBRATION PRODUCT FILES (CPFs) are essentially rearrangements
of a subset
of the information within the BCFs suitable for a specific task within
a given Data Analysis Package. The extraction of the necessary
information from the BCFs and construction of CPFs
is performed by `Stage 2 Calibration s/w', with reference
to HK data (eg observation date, instrument mode
etc)
as necessary.
All BCF and CPF calibration files within the
High Energy Astrophysics Science Archive Research Center
(HEASARC)
adhere to the
Flexible Image Transport System (FITS)
standard. Note that the HEASARC Calibration Database can also be used to store data in non-standard, mission-specific format. Such files are called Primary Calibration Files (PCF) and are stored to provide an historical record of mission calibration and data processing, but are not meant for general data analysis.
An overview of the relationship between the BCFs, CPFs and other elements
within the generic calibration dataflow is given in
CAL/GEN/91-001 (George 1992).
This document describes in detail the FITS format adopted by the
HEASARC
for the calibration files required for the spectral analysis of X-ray spectral
data.
2 OVERVIEW
2.1 The Redistribution Matrix
X-ray spectral analysis consists of convolving a model spectrum with the response of the detection system, and comparison of this convolved model with the observed data in order to constrain model parameters and thus derive physical quantities (like absorption columns, fluxes, emission measures, etc.) The "detector response" R(I,E) is proportional to the probability that an incoming photon of energy E will be detected in the output detector channel I. As such, the response is a
continuous function of E, while the detector output consists of a discrete number of channels. The continuous response function is converted to a discrete function by creating a "response matrix" R
D(I,J) at discrete energies E
J such that
R
D is often referred to as the "Redistribution Matrix", since it describes how a photon of energy E
J−1 < E < E
J is "redistributed" into output detector channels. The file which contains the "Redistribution Matrix" has been called the "Redistribution Matrix File", or RMF for short.
It is sometime useful to split the redistribution matrix into
parts. For instance, X-ray calorimeters are high resolution but have
long tails in their response down to low energies. This means that
R(I,E) is triangular and can be very large due to the small energy
bins required because of the detector's high resolution. The size of
the redistribution matrix can be reduced by dividing it into two
parts. A high resolution part with small energy bins, which is
mostly zeroes, and low resolution part with larger energy bins, which
is triangular. For computational simplicity the larger energy bins
should be made by combining the small energy bins.
2.2 Detector Effective Area
In general the response of a detector to a source of photons depends not only on the redistribution of photons but also on the sensitivity of the detector to photons of known energy. For example, for many X-ray detectors, the sensitivity is a function of off-axis angle: a source observed on-axis usually appears brighter than the same source observed away from the detector optical axis. The sensitivity of a detector to a photon of a given energy E
J−1 < E < E
J can be described by an array of values A(J), and the file which contains this information is usually called the Ancillary (sometimes, Auxiliary) Response File, or ARF for short. The Ancillary Response File gives the "effective area" of the detector system, and usually includes such components as mirror vignetting, filters, etc.
2.3 Implementation
Spectral analysis of a file containing the X-ray spectrum of a source (often called, for historical reasons, a Pulse Height Analyzer or PHA file)
using
XSPEC
or similar spectral analysis software requires
the following Calibration Product Files:
- A DETECTOR REDISTRIBUTION MATRIX FILE (RMF)
- The RMF consists of one or more compressed 2-d (energy vs PHA channel)
FITS extensions (Section 3), and another
extension explicitly listing the nominal energy range of each PHA channel.
- The RMF is created by folding together individual components due to the:
- Detector Gain
- Detector Energy Resolution
(including the response to a monoenergetic source
eg. escape peaks, partial charge tail
etc.)
- AN ANCILLARY RESPONSE FILE (ARF)
- The ARF consists of a 1-D array vs. energy stored as a table extension in a FITS file (see Section 4), and includes the summed contribution of efficiency
components, ie those not involved in
the redistribution of
photons such as:
- the Effective Area of the Telescope/Collimator
(including vignetting),
- the Filter Transmission (if any)
- the Detector Window Transmission
- the Detector Efficiency
- any additional energy dependent effects
(eg correction factors for the p.s.f.).
Sometimes the calibration information contained in the redistribution matrix file and the ancillary response file is incorporated into a single file
(see Section
6), which is usually called the "response" file (and usually denoted by a
.rsp file extension). Combining the redistribution matrix and detector effective area information in a single response matrix was common for early missions (
EINSTEIN and
ROSAT, for example, or the output of the
pcarsp tool for analysis of RXTE Proportional Counter Array Data) but is generally not done for more modern missions.
Note that, in the spectrum (PHA) file and the associated redistribution matrix (RMF) and effective area (ARF) files, the detector channels used to specify the observed spectrum and the detector calibration information
refer to
unbinned detector channels.
The number of unbinned channels for a given detector is given explicitly
by the
DETCHANS keyword within the MATRIX extension (see Section
3).
Rebinning of the PHA data can be specified by the
GROUPING flag within the PHA file (see
OGIP/92-007).
It is recommended that rebinning of the data and the
calibration data supplied by the RMF & ARF be done by
spectral analysis software, so that the binning can be easily adapted to emphasize important regions in the spectrum.
2.4 Rationale
For many high-energy instruments, the detector gain (the mapping between detector channel and approximate X-ray energy) and energy resolution
do not vary significantly with detector coordinates or time
(in particular not within an "observation"). Therefore, usually a single RMF
can be constructed for a given observation and used to analyze all sources in the field-of-view.
On the other hand, analysis of individual sources from a single observation will usually require
customized ARFs, since the ARF depends on details of the location of the source on the detector. Spectral analysis software like
XSPEC
can use a common RMF along with source-specific ARF and PHA files. For complicated detectors there are
situations in which the use of a common RMF is not possible; such instances
are discussed in Section
5.
The benefit of isolating the effective area components (which are a simple function
of energy for a given observation, but which may depend on time, detector position, observing mode
etc)
into the Ancillary Response File allows these components to be listed individually within
the ARF if desired (see Section
4). While including the individual contributions to the ARF is optional (but recommended), the HEASARC requires that the combined product of these individual components must
always be provided in the ARF.
2.5 Additional Notes
- Use of "Pulse-Invariant" channels (usually "PI" channels) are sometimes defined from some conversion of PHA channels
onto a uniform scale appropriate for some standard detector
gain setting. The HEASARC recommends that these "PI" channels NOT be used for data storage or analysis2.
The detector gain specified within the RMF should provide, in almost all cases,
the necessary information needed to convert PHA channels to a standard, uniform energy scale.
- If significant gain changes occur during a given observation
(as was often the case for the Imaging Proportional Counter on the EINSTEIN Observatory),
separate PHA files & RMFs
should be constructed for times when the gain was relatively constant, prior
to spectral analysis. The PHA files should be then analyzed with the appropriate (time-dependent)
ARFs & RMFs.
- An RMF contains only information applicable to the redistribution
of photons into detector channels. The effects of the detector effective area, detector filter (if appropriate), and Window
Transmission function are included in the ARF. Additional effects, such as
obscuration by a Window Support Structure, absorption due to contaminants on the
detector
(eg water ice or other substances), etc., can also be included in the ARF.
This minimizes the number of RMFs
required for a analysis of given observation (generally to one in most cases), minimizing disk-space requirements, and
facilitating investigations of the effects of these individual components on the spectral analysis.
3 THE HEASARC STANDARD RMF FORMAT
The standard RMF format consists of a FITS file with a null primary array
and at least two extensions:
- The Redistribution (MATRIX) extension(s)
- an (EBOUNDS) extension containing the nominal energy
bounds of each channel
both employing the BINTABLE FITS format.
3.1 The RMF Redistribution Extension
In general, redistribution matrices are sparse, so storing the full matrix in an RMF is not very efficient in terms of file storage and file access. In order to minimize disk-space requirements,
the RMF Redistribution Matrix should be stored in a compressed format
in which all matrix elements below a given threshold (specified by the
LO_THRES FITS keyword in the RMF file) are ignored.
This format is very similar to that used originally by the
XSPEC
.rsp standard-format response files.
3.1.1 Extension Header
The header must include the following (mandatory) keywords/values:
- EXTNAME = 'MATRIX' or 'SPECRESP MATRIX' - the name (ie type) of the extension
- TELESCOP - the"telescope" (ie mission/satellite name).
- INSTRUME - the instrument/detector.
- FILTER - the instrument filter in use (if any)
- CHANTYPE - whether the detector channels given in the matrix
are uncorrected (ie as assigned by the detector electronics,
CHANTYPE = 'PHA'),
or have been corrected (eg are "pulse invariant",
CHANTYPE = 'PI').
- DETCHANS - the total number of raw detector PHA channels in
the full (uncompressed) matrix.
- HDUCLASS = 'OGIP' - file format is OGIP standard.
- HDUCLAS1 = 'RESPONSE' - extension contains response data.
- HDUCLAS2 = 'RSP_MATRIX' - extension contains a response
matrix.
- HDUVERS = '1.3.0' - version of the file format.
- TLMIN# - the first channel in the response. # is the column number
for the F_CHAN column (see below).
If there are multiple MATRIX extensions then each one should have a
different value of their EXTVER keyword (1 for the first extension, 2
for the second and so on).
The following optional keywords may be useful for programs reading the file
in that they specify the amount of memory various arrays will require.
- NUMGRP - the total number of channel subsets. The sum of the N_GRP column.
- NUMELT - the total number of response elements. The sum of the N_CHAN column.
The following optional keywords supply further information:
- PHAFILE - name of PHA file for which this file was produced
- LO_THRES - minimum probability threshold used to construct the matrix (matrix
elements below this value are considered to zero and are not stored)
- HDUCLAS3 - giving further details of the stored matrix
Allowed values are:
- 'REDIST'
for a matrix whose elements represent probabilities
associated with the photon redistribution process only
- 'DETECTOR'
for a matrix whose elements have been multiplied
by all energy-dependent effects associated with detector
(eg detector efficiency, window transmission etc).
- 'FULL'
for a matrix whose elements have been multiplied
by all energy-dependent effects associated with detector,
optics, collimator, filters etc.
The following keywords are now obsolete but may be included for the benefit
of old software. They should be commented as obsolete.
- RMFVERSN = '1992a'
- HDUVERS1 = '1.1.0'
- HDUVERS2 = '1.2.0'
Finally, the following keywords are mandatory if these calibration data
are ever to form an entry in a Calibration Index File
(CIF; see
CAL/GEN/92-008, George, Pence & Zellar 1992)
These keywords and their acceptable values are listed in more detail in
CAL/GEN/92-011 (George, Zellar & Pence 1992)
However, it should be noted that there is often no such requirement for
RMF or ARF files as they are usually specific to a given PHA file
(but see Section
6).
- CCLS0001 (= 'CPF')
- the OGIP-class of this calibration file.
- CCNM0001 (= 'MATRIX')
- the (CIF) codename for this type of calibration dataset.
- CDTP0001 (= 'DATA')
- the OGIP code for the form of the contents of the
file ('real' data, a taskname and associated parameter inputs etc)
- CVSD0001
- the UTC date (in yyyy-mm-dd format) when this calibration
data should first be used
- CVST0001
- the UTC time (in hh:mm:ss format) on the day CVSD0001
when this calibration data should first be used.
- CDES0001
- a string giving a brief descriptive summary of this dataset
3.1.2 Data Format
In the general case, the organization of the data within
this extension will be as follows (with the
matrix x-axis = raw PHA channel, y-axis = Energy) with each row of
the BINTABLE
referring to a single energy range
(thus the number of rows = number of energy bins)
and consist of the following columns:
- Elow, a 4-byte REAL scalar for each row
containing the lower energy bound of the
energy bin.
The FITS column name is ENERG_LO.
The recommended units are keV.
- Ehigh, a 4-byte REAL scalar for each row.
containing the upper energy bound of the
energy bin.
The FITS column name is ENERG_HI.
The recommended units are keV.
- Ngrp, a 2-byte INTEGER scalar for each row
containing the number of 'channel subsets'
for the energy bin (see below).
The FITS column name is N_GRP
(unitless).
- Fchan, a fixed- or variable-length 2-byte or 4-byte INTEGER array
for each row.
Contains the channel number of the start of
each "channel subset" for the energy bin.
The FITS column name is F_CHAN
(unitless).
- Nchan, a fixed- or variable-length 2-byte or 4-byte INTEGER vector
for each row.
Contains the number of channels within
each "channel subset" for the energy bin.
The FITS column name is N_CHAN
(unitless).
- Mat, a (fixed- or variable-length) REAL array. Each element Mat is 4-byte REAL number
containing
all the response probability
values for each 'channel subset' corresponding to the energy bin for a given row.
The FITS column name is MATRIX
(unitless).
These are summarized in Table
1.
Table 1: OGIP format (1992a) for storing photon redistribution
matrices within an RMF
to (filename).RMF
Name: RMF
Description: Photon Redistribution Matrix
Format: BINTABLE
A final column may be added for responses of grating instruments.
- Order, a (fixed- or variable-length) INTEGER vector
(array, each element within which is 2-byte)
for each row containing the dispersion order of each
'channel subset' in the energy bin.
The FITS column name is ORDER.
(unitless).
This column matches the F_CHAN and N_CHAN columns and requires that
every 'channel subset' be for a single order.
3.1.3 Points to Note & Conventions
- The ordering of the columns used here is recommended.
- Values of both ENERG_LO & ENERG_HI are given in each row (j)
for clarity and for efficiency of access.
The order should be monotonically increasing with increasing row number, starting from
the minimum ENERG_LO value.
In no case should there be any overlap between consecutive energy
bins, so that for row j, ENERG_LO(j) ≥ ENERG_HI(j-1).
In most RMFs,
ENERG_LO(j) = ENERG_HI(j-1)).
- The concept of "channel subsets" is included to minimize the
RMF storage requirements for instruments for which the 2-d
matrix is sparse, and consists of non-zero values in two or more (unconnected)
regions of channel-energy space. A channel subset therefore
consists of a number (Nchan) of contiguous channels for
which the matrix elements are above the LO_THRES threshold.
Thus, using the above notation, a given row of the MATRIX
array contains the elements appropriate to
- Generally, the F_CHAN,
N_CHAN and MATRIX columns will be FITS variable-length
arrays (Cotton, Tody & Pence, 1995).
For variable-length arrays, the number of elements within each array
varies from row to row. For a given row, the number of elements within the F_CHAN and
N_CHAN columnd for that row equals the
value of N_GRP for that row.
It should be noted that, if Ngrp ≤ 3 for all rows, it is usually more efficient in
terms of both disk storage
requirements3
and speed of access4
to designate that array as fixed-length format.
This criterion has been adopted as the general policy of all
files containing arrays of variable length
within the HEASARC calibration database.
The HEASARC recommends the following guidelines:
- All RMFs for a given instrument should employ
the same format.
- in most cases the
F_CHAN & N_CHAN columns will be fixed-length integer
arrays, since commonly, for instruments for which there is data
in the HEASARC archives,
N_GRP ≤ 3.
Note, in all cases the F_CHAN & N_CHAN
columns contain the same number of elements, thus should
both either be in fixed- or variable-length array format.
- Due to the greater read-efficiency, the MATRIX column
is also in fixed-length format unless this
leads to a significant increase (say > 1.5) in
disk-space requirements.
- Unused elements within an array should be padded with
`null data' values.
Spectral analysis software (like XSPEC) software should be able to handle both fixed- and variable-length arrays.
Use of the FITSIO interface by spectral analysis software
(Pence 1992) is recommended to read FITS files, since FITSIO can transparently interpret either format.
- If a column contains a constant
value in every row, then the column can be deleted from the table and
transformed into a keyword value.
Analysis software should first
look for a keyword value having the name of one of the required columns;
if the keyword is not found, then the software should look for a column
with that name.
- Each row within the RMF matrix will be normalized to 1 detected
photon, ie
each element of Mat will contain the probability of a
detected photon within the appropriate energy range giving rise to a
signal in that PHA channel, i.e., for a given row J,
Effects due to detector
efficiencies < 100%, absorption by mirrors, filters & the
detector window etc should be included within the ARF and not the RMF.
Note that, in practice, the sum of the probabilities in a given row of the MATRIX array may be less than unity.
due to the finite probability of a photon being registered
as either below or above the PHA discriminator thresholds, and/or if
LO_THRES > 0.0.
3.2 The RMF EBOUNDS Extension
The RMF EBOUNDS extension lists the (nominal) energy boundaries of each of the
(raw) detector channels within the redistribution matrix given above.
It should be stressed that these energies are
not necessarily the same as those
given in the
ENERG_LO &
ENERG_HI columns of the
MATRIX extension discussed above.
The (nominal) energy boundaries are required by spectral analysis packages when (say) the user would like
the results of spectral analysis (PHA data and best-fitting model)
to be displayed as a function of photon energy, rather than
detector channel. Providing this information in a separate FITS extension provides for more efficient access when plotting, especially if the response matrix is large.
The format described here is a simple
1-dimensional list (as a function of raw detector PHA channel) which gives the
nominal energy boundaries for each detector channel.
3.2.1 Extension Header
For clarity, the header must include the same mandatory keywords as
the RMF extension,
namely:
- EXTNAME (= 'EBOUNDS')
- the name (ie type) of the extension
- TELESCOP
- the"telescope" (ie mission/satellite name).
- INSTRUME
- the instrument/detector.
- FILTER
- the instrument filter in use (if any)
- CHANTYPE
- whether the detector channels given in the matrix
are PHA or PI channels (see above).
- DETCHANS
- the total number of raw detector PHA channels in the full
(uncompressed) matrix.
- HDUCLASS = 'OGIP' - file format is OGIP standard.
- HDUCLAS1 = 'RESPONSE' - extension contains response data.
- HDUCLAS2 = 'EBOUNDS' - extension contains a response
matrix.
- HDUVERS = '1.2.0' - version of the file format.
along with the optional keywords
- PHAFILE
- name of PHA file for which this file was produced
The following keywords are now obsolete but may be included for the benefit
of old software. They should be commented as obsolete.
- RMFVERSN = '1992a'
- HDUVERS1 = '1.0.0'
- HDUVERS2 = '1.1.0'
Finally, if these calibration data are ever to form an entry in
a Calibration Index File, the mandatory
C*** keywords listed
in Section
3.1.1 are also mandatory,
but in this case with
CCNM0001 = 'EBOUNDS'.
3.2.2 Data Format
A BINTABLE FITS format has been chosen whereby each each row refers to a
single detector channel. The number of rows is thus the
number of (raw) detector channels
and must correspond exactly
to the channels within the PHA file and hence also to the
value of the
DETCHANS keyword in the RMF
MATRIX extension
described above.
Thus, we have
- Chan, a 2-byte or 4-byte INTEGER scalar giving the raw channel number for
each row.
The FITS column name is CHANNEL.
(unitless)
- Emin, a 4-byte REAL scalar for each for each row
containing the nominal energy corresponding to the
lower
boundary of the detector channel.
The FITS column name is E_MIN.
The recommended units are keV.
- Emax, a 4-byte REAL scalar for each for each row
containing the nominal energy corresponding to the
upper
boundary of the detector channel.
The FITS column name is E_MAX.
The recommended units are keV.
Table
2 summarizes the organization of this extension
Table 2: OGIP format (1992a) for the EBOUNDS extension within RMFs
to (filename).RMF
Name: EBOUNDS
Description: Nominal energy boundaries for each detector channel
Format: BINTABLE
3.2.3 Points to Note & Conventions
- The ordering of the columns is arbitrary, however the order presented here
is strongly recommended.
- Note that E_MIN and E_MAX in the EBOUNDS extension are different from
ENERGO_LO and ENERGO_LO given in the MATRIX extension. E_MIN and E_MAX are determined by the characteristics of the detector; values of ENERG_LO and ENERG_HI are generally selected by the calibration scientist, who may choose to have much finer energy resolution than the detector offers in order to oversample the detector response.
- Because pulse-height analysers generally
oversample the true spectral response of most X-ray detectors, there is no guarantee that an incident X-ray with an energy
between E_MIN and E_MAX will be detected in the
corresponding detector channel given in the EBOUNDS extension. Determining in which channels an incident X-ray photon may be detected requires a full spectral analysis using the redistribution matrix and an assumed intrinsic source spectrum.
4 THE HEASARC STANDARD ARF FORMAT
The ARFs are relatively straightforward, consisting of a simple
1-dimensional
list (as a function of energy) of the product of the various components
required for spectral analysis not involved in the photon redistribution
process (see Section
2.4).
4.1 The ARF Extension
4.1.1 Extension Header
The header must include the following (mandatory) keywords:
- EXTNAME (= 'SPECRESP')
- the name (ie type) of the extension
- TELESCOP
- the "telescope" (ie mission/satellite name).
- INSTRUME
- the instrument/detector.
- FILTER
- the instrument filter in use (if any)
- HDUCLASS = 'OGIP' - file format is OGIP standard.
- HDUCLAS1 = 'RESPONSE' - extension contains response data.
- HDUCLAS2 = 'SPECRESP' - extension contains an ARF.
- HDUVERS = '1.1.0' - version of the file format.
The following optional keywords supply further information:
- PHAFILE
- name of PHA file for which this file was produced
The following keywords are now obsolete but may be included for the benefit
of old software. They should be commented as obsolete.
- ARFVERSN = '1992a'
- HDUVERS1 = '1.0.0'
- HDUVERS2 = '1.1.0'
As for the RMF file, if these calibration data are ever to form an entry in
a Calibration Index File, the
CC*** keywords listed
in Section
3.1.1 are also mandatory,
but in this case with
CCNM0001 = 'SPECRESP'.
If (in addition to the Prod array) the ARF
SPECRESP extension
also lists each of the individual contributing components (see below),
and these components are to be be listed in the Calibration Index File,
then each component must have its own unique set of
C*** keywords
(denoted by
CCNMXXXX
etc where XXXX is a number of the form
0002,
0003 etc
In this case, the
CCNMXXXX
must conform to the appropriate standards
given in
CAL/GEN/92-011 (George, Breedon and Corcoran 1992).
4.1.2 Data Format: Type I - a single arf
The general
HEASARC
standard for ARFs also makes use of the BINTABLE FITS
format, and thus the data again resides in a single extension of
a FITS file (though generally as a file separate from the RMF) with a null primary array.
As in the case of the RMFs, each row of the BINTABLE refers to a single
energy range.
The ARF SPECRESP extension must use the same energy binning as the RMF MATRIX extension to which its associated, that is, the ARF should have the same
ENERG_LO and
ENERG_HI values as its associated RMF file, and so the ARF SPECRESP and RMF MATRIX extensions should have the same number of rows.
In
all cases the following columns are included in the SPECRESP extension of the ARF
(preferably as the first 3 columns within the table):
- Elow, a 4-byte REAL scalar for each row
containing the lower energy bound of
the energy bin.
The FITS column name is ENERG_LO.
The recommended units are keV.
- Ehigh, a 4-byte REAL scalar for each row
containing the upper energy bound of
the energy bin.
The FITS column name is ENERG_HI.
The recommended units are keV.
- Prod, a 4-byte REAL scalar for each row
containing the product of all the components
(effective area,
filter transmission, correction factors
etc)
specific to a given PHA file
(ie the spectral response of the instrument as a whole).
The FITS column name is SPECRESP.
The recommended units are cm2.
Other columns can be added to show the various components out of which
Prod was constructed but these are optional.
Table
3 summarizes the organisation of an ARF.
Table 3: OGIP format (1992a) for ARFs
to (filename).ARF
Name: ARF
Description: Ancillary Response File
Format: BINTABLE
4.1.3 Data Format: Type II - multiple arfs
It is sometime convenient to store many ARFs in the same file so we
provide a type II format analogous to that for spectral files. As with
the case of a single ARF, the data are stored in a BINTABLE
extension. However, vector arrays are used in place of scalars for the
columns. Thus the energies and response are vectors and each row of
the table contains a single ARF. The notation below is the same as
that used in the description of the type I format.
- Num, a 2-byte INTEGER giving the reference number of the
spectrum stored in this row.
The FITS column name is ARF_NUM.
(unitless)
- Elow, a 4-byte REAL array containing the lower energy bound of
the energy bin.
The FITS column name is ENERG_LO.
The recommended units are keV.
- Ehigh, a 4-byte REAL array containing the upper energy bound of
the energy bin.
The FITS column name is ENERG_HI.
The recommended units are keV.
- Prod, a 4-byte REAL array containing the product of all the components
(effective area,
filter transmission, correction factors
etc)
specific to a given PHA file
(ie the spectral response of the instrument as a whole).
The FITS column name is SPECRESP.
The recommended units are cm2.
If the
TELESCOP,
INSTRUME,
DETNAM,
FILTER
differ between the ARFs then these keywords can be included as columns
although this is not recommended.
4.1.4 Points to Note & Conventions
- The ordering of the columns used here is recommended.
- Values of both ENERG_LO & ENERG_HI are given in each row (j) of the SPECRESP extension
for clarity and for efficiency of access.
The order of the energy bins should be monotonically increasing with increasing row number, starting from
the minimum ENERG_LO value.
In no case should there be any overlap between consecutive energy
bins, so that for row j, ENERG_LO(j) ≥ ENERG_HI(j-1).
In most RMFs,
ENERG_LO(j) = ENERG_HI(j-1)).
- The dimension of the data within the Prod column will be length2
(due to the inclusion of the effective area).
5 USAGE: TYPICAL SCENARIOS
For non-imaging devices (assuming a time-stable detector gain)
a User extracts a PHA file, then runs software to generate a redistribution matrix file and the ancillary response file appropriate for the observation.
The user than inputs these files along with the
PHA file, into (
eg XSPEC) or some other analysis software and chooses an appropriate model of the emission. The model is folded through the redistribution matrix multiplied by the effective area given in the ARF, and then compared directly to with the data. A new redistribution matrix and/or effective area curve can be generated to explore the effects of the observational parameters and/or to employ a new detector gain relation.
Spectral analysis could
continue using the original PHA file & RMF in conjunction with the
new ARF, for example.
In some cases,
for a given detector,
a single RMF may be applicable to several
datasets within the
HEASARC archive.
For imaging instruments (with a time- and position-stable
Detector Gain etc.) a User performs an almost identical set of actions
as above for each PHA file extracted (
ie from each source in the
image). Each PHA file therefore has an associated ARF file, which the
User creates and/or customizes using a response generator. However all
the PHA files
will normally
share a single RMF.
For imaging instruments (with a time-stable Detector Gain
etc, but
which varies with detector position) a User has to construct both a
RMF & ARF for each PHA file extracted (
ie
from each source in the image).
Users can
customize the individual ARFs as desired.
For instruments (of any type) for which the Detector Gain
etc
is not
stable with time (
ie significantly varies over the course of a pointing),
the observational dataset should be broken-down into a series of periods
for which all Detector-related quantities
are considered sufficiently
constant.
Separate PHA files, RMFs and associated ARFs can then be
constructed for each of these periods (with each RMF obviously containing
the matrix constructed using the gain setting appropriate to its time-window).
Spectral analysis is then performed on these files either individually
or simultaneously.
6 SPECIAL CASES
Sometimes the effective area curve has been combined with the redistribution matrix to produce a "spectral response" file, or the spectra in raw PHA channels has been "corrected" to a standard channel system (as denoted by the
CHANTYPE = 'PI' keyword
within the PHA file), requiring use of a "corrected" redistribution matrix. In such cases, the response file format should follow the redistribution matrix file format given above.
If no ARF is specified within the PHA file (via the
ANCRFILE keyword
- see
OGIP/92-007 (Arnaud, George & Tennant 1992),
then the Spectral Analysis Package should assume that the instrument spectral
response (
i.e. the Prod array from Table 3)
has been folded in with the redistribution matrix (Mat from Table 1),
and this information is what is stored in the RMF extension.
In this case the following changes to the list of mandatory keywords/values
given in Section
3.1.1 are necessary to the
header of the (new) RMF extension:
- EXTNAME (= 'SPECRESP MATRIX')
- the name (ie type) of the extension
- CCNM0001 (= 'SPECRESP MATRIX')
- the (CIF) codename for this type of
calibration dataset.
Again, it is emphasized that this is generally not recommended, especially
in the case of future missions.
7 EXAMPLE FITS HEADERS
As an example, below we list the relevant keywords from an
ASCA
SIS0 RMF and ARF.
7.1 ASCA RMF
7.1.1 RSP_MATRIX Extension
XTENSION= 'BINTABLE' / binary table extension
BITPIX = 8 / 8-bit bytes
NAXIS = 2 / 2-dimensional binary table
NAXIS1 = 34 / width of table in bytes
NAXIS2 = 1180 / number of rows in table
PCOUNT = 1031160 / Number of bytes acumulated in heap
GCOUNT = 1 / one data group (required keyword)
TFIELDS = 6 / number of fields in each row
TTYPE1 = 'ENERG_LO' / label for field 1
TFORM1 = 'E ' / data format of field: 4-byte REAL
TUNIT1 = 'keV ' / physical unit of field
TTYPE2 = 'ENERG_HI' / label for field 2
TFORM2 = 'E ' / data format of field: 4-byte REAL
TUNIT2 = 'keV ' / physical unit of field
TTYPE3 = 'N_GRP ' / label for field 3
TFORM3 = 'I ' / data format of field: 2-byte INTEGER
TTYPE4 = 'F_CHAN ' / label for field 4
TFORM4 = 'PI(2) ' / data format of field: variable length array
TTYPE5 = 'N_CHAN ' / label for field 5
TFORM5 = 'PI(2) ' / data format of field: variable length array
TTYPE6 = 'MATRIX ' / label for field 6
TFORM6 = 'PE(418) ' / data format of field: variable length array
EXTNAME = 'MATRIX ' / name of this binary table extension
TLMIN4 = 0 / First legal channel number
TLMAX4 = 511 / Highest legal channel number
TELESCOP= 'ASCA ' / mission/satellite name
INSTRUME= 'SIS0 ' / instrument/detector
FILTER = 'NONE ' / filter information
CHANTYPE= 'PI ' / Type of channels (PHA, PI etc)
DETCHANS= 512 / Total number of detector PHA channels
LO_THRES= 1.00E-07 / Lower probability density threshold for matrix
HDUCLASS= 'OGIP ' / Keyword information for Caltools Software.
HDUCLAS1= 'RESPONSE ' / Keyword information for Caltools Software.
HDUCLAS2= 'RSP_MATRIX ' / Keyword information for Caltools Software.
HDUVERS = '1.3.0 ' / Keyword information for Caltools Software.
HDUCLAS3= 'DETECTOR ' / Keyword information for Caltools Software.
CCNM0001= 'MATRIX ' / Keyword information for Caltools Software.
CCLS0001= 'CPF ' / Keyword information for Caltools Software.
CDTP0001= 'DATA ' / Keyword information for Caltools Software.
CVSD0001= '1993-02-20 ' / Keyword information for Caltools Software.
CVST0001= '11/11/11 ' / Keyword information for Caltools Software.
CDES0001= 'SISRMGv1.10:1180x512 S0C0 G"0234" V100 P40 E1.6 '
CBD10001= 'CHAN(0- 511) ' / Keyword information for Caltools Software.
CBD20001= 'ENER(0.2-12.0)keV' / Keyword information for Caltools Software.
CBD30001= 'GRADE("0234" ) ' / Keyword information for Caltools Software.
CBD40001= 'RAWX(-95- 325) ' / Keyword information for Caltools Software.
CBD50001= 'RAWY( 22- 444) ' / Keyword information for Caltools Software.
RMFVERSN= '1992a ' / Obsolete
HDUVERS1= '1.0.0 ' / Obsolete
HDUVERS2= '1.2.0 ' / Obsolete
END
7.1.2 EBOUNDS Extension
XTENSION= 'BINTABLE' / binary table extension
BITPIX = 8 / 8-bit bytes
NAXIS = 2 / 2-dimensional binary table
NAXIS1 = 10 / width of table in bytes
NAXIS2 = 512 / number of rows in table
PCOUNT = 0 / size of special data area
GCOUNT = 1 / one data group (required keyword)
TFIELDS = 3 / number of fields in each row
TTYPE1 = 'CHANNEL ' / label for field 1
TFORM1 = 'I ' / data format of field: 2-byte INTEGER
TUNIT1 = 'channel ' / physical unit of field
TTYPE2 = 'E_MIN ' / label for field 2
TFORM2 = 'E ' / data format of field: 4-byte REAL
TUNIT2 = 'keV ' / physical unit of field
TTYPE3 = 'E_MAX ' / label for field 3
TFORM3 = 'E ' / data format of field: 4-byte REAL
TUNIT3 = 'keV ' / physical unit of field
EXTNAME = 'EBOUNDS ' / name of this binary table extension
TLMIN1 = 0 / First legal channel number
TLMAX1 = 511 / Highest legal channel number
TELESCOP= 'ASCA ' / mission/satellite name
INSTRUME= 'SIS0 ' / instrument/detector
FILTER = 'NONE ' / filter information
CHANTYPE= 'PI ' / Type of channels (PHA, PI etc)
DETCHANS= 512 / Total number of detector PHA channels
SMOOTHED= 0 / 0 = raw, 1-12 = smooth, -1 = ep-lin, -2 = mean-
HDUCLASS= 'OGIP ' / Keyword information for Caltools Software.
HDUCLAS1= 'RESPONSE ' / Keyword information for Caltools Software.
HDUCLAS2= 'EBOUNDS ' / Keyword information for Caltools Software.
HDUVERS = '1.2.0 ' / Keyword information for Caltools Software.
CCNM0001= 'EBOUNDS ' / Keyword information for Caltools Software.
CCLS0001= 'CPF ' / Keyword information for Caltools Software.
CDTP0001= 'DATA ' / Keyword information for Caltools Software.
CVSD0001= '1993-02-20 ' / Keyword information for Caltools Software.
CVST0001= '11/11/11 ' / Keyword information for Caltools Software.
CDES0001= 'SISRMGv1.10:1180x512 S0C0 G"0234" V100 P40 E1.6 '
CBD10001= 'CHAN(0- 511) ' / Keyword information for Caltools Software.
CBD20001= 'ENER(0.2-12.0)keV' / Keyword information for Caltools Software.
CBD30001= 'GRADE("0234" ) ' / Keyword information for Caltools Software.
CBD40001= 'RAWX(-95- 325) ' / Keyword information for Caltools Software.
CBD50001= 'RAWY( 22- 444) ' / Keyword information for Caltools Software.
RMFVERSN= '1992a ' / Obsolete
HDUVERS1= '1.0.0 ' / Obsolete
HDUVERS2= '1.2.0 ' / Obsolete
END
7.2 ASCA ARF
7.2.1 SPECRESP 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 = 1180 / number of rows in table
PCOUNT = 0 / size of special data area
GCOUNT = 1 / one data group (required keyword)
TFIELDS = 3 / number of fields in each row
TTYPE1 = 'ENERG_LO' / label for field 1
TFORM1 = '1E ' / data format of field: 4-byte REAL
TUNIT1 = 'keV ' / physical unit of field
TTYPE2 = 'ENERG_HI' / label for field 2
TFORM2 = '1E ' / data format of field: 4-byte REAL
TUNIT2 = 'keV ' / physical unit of field
TTYPE3 = 'SPECRESP' / label for field 3
TFORM3 = '1E ' / data format of field: 4-byte REAL
TUNIT3 = 'cm**2 ' / physical unit of field
EXTNAME = 'SPECRESP' / name of this binary table extension
TELESCOP= 'ASCA ' / Telescope (mission) name
INSTRUME= 'SIS0 ' / Instrument name
FILTER = 'NONE ' / Instrument filter
HDUCLASS= 'OGIP ' / Organisation devising file format
HDUCLAS1= 'RESPONSE' / File relates to response of instrument
HDUCLAS2= 'SPECRESP' / effective area data is stored
HDUVERS = '1.1.0 ' / Version of file format
RESPFILE= 'test.rmf' / RMF file used to get the energies
WAOAA = 7.68984E+00 / WMAP-wgtd avg off-axis ang
HISTORY ARF created by ascaarf v3.00
HISTORY from test.sp
HISTORY using test.rmf
HISTORY with extended source algorithm
HISTORY XRT effec area from /FTP/caldb/data/asca/xrt/bcf/xrt_ea_v2_0.fits
HISTORY PSF from /FTP/caldb/data/asca/xrt/bcf/xrt_psf_v2_0.fits
HISTORY Input WMAP array has size 28 by 28 bins
HISTORY expanded to 28 by 28 bins
HISTORY First WMAP bin is at detector pixel 336 648
HISTORY 8 detector pixels per WMAP bin
HISTORY WMAP bin size is 0.21600 mm
HISTORY 0.21216 arcmin
HISTORY Selected region size is 1.8701 arcmin^2
HISTORY Optical axis is detector pixel 662.72 559.02
HISTORY 1180 energies from RMF file
HISTORY Effective area fudge applied
HISTORY Arf filter applied
ARFVERSN= '1992a ' / Obsolete
HDUVERS1= '1.0.0 ' / Obsolete
HDUVERS2= '1.1.0 ' / Obsolete
END
ACKNOWLEDGMENTS
We thank the numerous people, both inside and outside the OGIP, who have
contributed ideas and suggestions.
In particular we thank Alan Smale.
REFERENCES
Information regarding on-line versions of any of the following references
with an OGIP Memo number (
ie documents starting
OGIP/.. or
CAL/..) can most easily be found via the WorldWide Web by following
the links from the URL:
https://heasarc.gsfc.nasa.gov/docs/heasarc/caldb/caldb_doc.html
Most OGIP Calibration Memos of general community interest will eventually
appear as articles in
Legacy, but are also available on request
from The Office of Guest Investigator Programs, Code 660.2,
NASA/GSFC, Greenbelt, MD 20771, USA.
Arnaud, K.A., George, I.M. & Tennant, A.,
1992.
(OGIP/92-007)
Cotton, W.D., Tody, D. & Pence, W.D.
1995.
Astron. Astrophys. Suppl.,
113,159.
George, I.M.,
1992.
Legacy,
1, 56.
(CAL/GEN/91-001)
George, I.M., Pence, W. & Zellar, R.
1992.
(CAL/GEN/92-008)
George, I.M., Zellar, R. & Pence, W.,
1992.
(CAL/GEN/92-011)
George, I.M., Arnaud, K.A. & Ruamsuwan, L.,
1992.
(CAL/SW/92-004)
Griesen, E.W. & Harten, R.H.,
1981.
Astron. Astrophys. Suppl.,
44, 371.
Grosbol, P., Harten, R.H., Greisen, E.W. & Wells, D.C.,
1988.
Astron. Astrophys. Suppl.,
73,365.
Pence, W.,
1992.
Legacy,
1, 14..
Wells, D.C., Griesen, E.W. & Harten, R.H.,
1981,
Astron. Astrophys. Suppl.,
44, 363.
USEFUL LINKS