OGIP Calibration Memo CAL/GEN/92-019
THE OGIP FORMAT FOR
EFFECTIVE AREA FILES
Ian M George
&
Ron S Zellar
(EARVERSN = 1992a)
Mail Code 668,
NASA/GSFC,
Greenbelt,
MD 20771.
Version: 1995 Jul 31
|
Release | Sections Changed | Brief Notes |
Date | ||
1992 Jul 24 | First Draft | (within memo CAL/GEN/92-003) |
1993 Oct 03 | All | Separation from CAL/GEN/92-003 |
1993 Nov 25 | All | Added HDUCLASn info |
1994 Jul 24 | 2.2 | Clarified CSYSNAME conventions |
1994 Aug 09 | All | General Review/up-dates |
1995 Jan 19 | All | Made compatible with LaTeX2HTML software |
2004 Apr 01 | All | Made compatible with tth |
The following documents may also be of use:
Historically, the term 'Effective Area' has been used somewhat ambiguously.
Within the HEASARC caldb,
the term is reserved
exclusively to denote that of the telescope mirror/collimator assembly
(only), but including any vignetting and obscuration effects applicable
for off-axis angles of incidence1.
In the general case an Effective Area calibration dataset consists of a
3-dimensional grid, EffArea,
with 1 axis giving the photon energy (E),
and 2 axes defining the position relative to the optical axis
- invariably the off-axis angle (θXMA) & azimuthal
angle (φXMA).
However, there are two basic methods in which this information can be
stored within the caldb:
The data formats described in Section 2
are able to accomodate both Methods A& B,
with the CBDnxxxx keyword(s) providing downstream software with the necessary
information as to whether further calibration i/p (i.e. off-axis Vignetting
& Obscuration factors) is required.
The construction, format used (within the limitations discussed here)
and delivery of the data to the HEASARC (including any updates)
is the responsibility of the h/w teams and/or GOF.
However, below, are the recommendations of the HEASARC calibration team
based on their experience.
General
Pre-launch
Post-launch
Due to the complexity of the Effective Area of an instrument as a
function of energy (as the result of sharp discontinuities due to atomic
processes etc. ), such a calibration dataset is not easily parameterized.
Thus, whilst theoretically possible, it is recommended that
an Effective Areas dataset is not described by a virtual calibration
file.
Data Files:
As discussed in the general guidelines for calibration files
(CAL/GEN/92-003),
it is strongly recommended
that the energy grid is of sufficient resolution and carefully chosen
such that interpolation of this parameter is not required.
However, in cases where interpolation is required, as simple
1-dimensional linear interpolation will be performed
(which will clearly be inaccurate close to sharp features).
Virtual Files:
Downstream s/w should assume further calibration input is required
for an Effective Area dataset under the following conditions:
An Effective Area dataset is used in the construction of the following
calibration datasets:
The HEASARC FITS Working Group (HFWG)
Header-Data Unit (HDU) keywords and values for this type of dataset are:
The following versions of file formats for an EffArea
dataset have been defined:
Description:
Note: this format was formally known as EARVERSN = '1992a', and
is still occasionally refered to as such.
Extension Header
Data Format:
These are summarized in Table 1.
RELATED DOCUMENTATION
CAL/GEN/92-003 (George & Zellar)
CAL/GEN/92-008 (George, Pence & Zellar)
CAL/GEN/92-011 (George, Zellar & Pence)
CAL/GEN/92-013 (George, Zellar & White)
CAL/GEN/92-021 (George & Zellar)
CAL/GEN/92-022 (George & Zellar)
1 Introduction
1.1 Storage Options
Method A provides the clear advantage to users and s/w requiring an
off-axis Effective Area that all the necessary information is contained within a
single dataset. However, this method has the potential disadvantage that
the Total Vignetting function (including the effects of vignetting and
obscuration) has been folded in, requiring the entire dataset to be
recreated if (say) the Total Vignetting function only is
improved in-flight.
By the same token
Method B has a disadvantage to users and s/w requiring an off-axis
Effective Area that additional calibration information is required -
namely the Total Vignetting function, Vignet,
appropriate for the desired off-axis position and energy range, i.e.
However, since the on-axis Effective Area is sometimes
known to a higher accuracy than the Total Vignetting function,
the advantage of Method B is that the isolation of the latter
component means that updates can be made without the
necessity of updating the (on-axis) Effective Area BCF.
The pros and cons of the two methods are further discussed in
Section 1.2.
EffArea (E,θXMA,φXMA)
=
EffArea (E,0,0)
× Vignet (E,θXMA,φXMA) (1)
1.2 Dataset Origins & Storage Recommendations
For completeness and maximum flexibility, in the case of instruments for
which the calibration is well understood and stable, the HEASARC would
prefer that an effective area dataset is stored using Method A, but that
the associated vignetting and obscuration functions are also stored in
CCNMxxxx = VIGNET and CCNMxxxx = OBSCFACT datasets.
Prior to launch, the on-axis effective area is usually accurately
measured at a (limited) number of photon energies during ground calibration
experiments, and combined with a theoretical model to produce a good
estimation of the effective area at all energies within the required band.
The effects of vignetting and obscuration at off-axis positions are
also measured at a (usually more limited) number of photon energies during
such experiments and/or combined with theoretical
(e.g. ray-tracing) models to produce the off-axis correction factors.
Thus in most cases, Method B (Section 1.1)
is the natural method of storing the necessary calibration
datasets at this time.
The effective area of the optics only cannot be measured in-orbit.
Instead, observations of standard cosmic sources (e.g. the Crab) combined
with spectral modelling enables the Spectral Response of the instrument
(i.e. the effective area of the optics multiplied by the transmission of any
filters & windows and by the detector efficiency as a function of energy)
to be determined.
The results of such calibration observations should be stored as a
CCNMxxxx = SPECRESP dataset.
However, should such measurements reveal that a discrepancy with previous
calibrations which is identified with (or interpreted as) a mis-calibration
of the effective area of the optics, h/w & GOF
teams are urged to isolate and also
supply an updated Effective Area dataset to the HEASARC.
Since such discrepancies are often the result of uncertainties in the
Vignetting function, Method B (Section 1.1)
is recommended for initial in-flight datasets also.
Only when the calibration is considered to be well understood
and stable, is it recommended that the Effective Area information
be reformatted to use Method A.
When such a time occurs should be the descision of the h/w & GOF teams,
and a new Effective Area BCF using Method A
(and any new CCNMxxxx = VIGNET and CCNMxxxx = OBSCFACT BCFs necessary)
be delivered to the HEASARC.
1.3 Dataset vs Task Summary
1.4 Software Considerations
In both Methods A & B outlined in Section 1,
interpolation between the θXMA,φXMA
grid points is usually required.
By default, downstream software will use a simple 2-dimensional
linear interpolation when calculating the Effective Area between
θXMA,φXMA grid points.
Thus the θXMA,φXMA grid should be of sufficient resolution
to enable this to be a reasonable approximation.
Not applicable (see Section 1.3).
1.5 Relationships to Other Calibration Datasets
an off-axis effective area is required, and
CBDn0001 = THETA(0)unit and CBDm0001 = PHI(0)unit
keywords are present (where n and m are integers, and unit
is any string) indicating the contents of the
Effective Area BCF datset is only applicable on-axis
(see CAL/GEN/92-003).
requirement:
a CCNMxxxx = TVIGNET calibration dataset
(or equivalently
CCNMxxxx = VIGNET and CCNMxxxx = OBSCFACT datsets)
giving the Total Vignetting function at the requested off-axis
position.
alternative:
If no such calibration sets are available within the
CALDB,
then
either the user should be informed and the s/w task stopped, or
(if appropriate) a CCNMxxxx = SPECRESP dataset
searched for within the CALDB
with
the corresponding implications to the subsquent operation of
downstream s/w.
2 Effective Data File Formats
These are valid for all datasets described in this section, and
should be present in the header of the extension containing the
EffArea dataset.
- the name of the organization that defined this file format.
- the name of the document describing the format
(ie this document)
- giving the HDUCLAS hierarchy for this format.
2.1 Summary of Effective Area file formats versions
This format is currently still VALID.
2.2 The Effective Area Extension (HDUVERS = '1.0.0')
One file for each telescope containing a single BINTABLE FITS
extension. The BINTABLE only has a single row,
using arrays for the 5 necessary columns.
Beyond the standard FITS keywords required,
and the HDU keywords/values given in Section 2,
the following keywords/values are mandatory:
and the following keywords/values are mandatory for CIF purposes
(see
CAL/GEN/92-011):
Allowed values are given in
CAL/GEN/92-011.
Allowed values are given in
CAL/GEN/92-011.
In the case of collimated
detectors, this is usually the name of the detector itself.
Only the EFFAREA column here
(with nnn = 5 in the example below).
(see
CAL/GEN/92-003
for further details).
Only the EFFAREA column here
(with nnn = 5) giving:
in the example below.
(CSYSNAME = 'XMA_POL' is assumed in the example
below)
and the following are optional to supply further information:
The data within the extension is organised as a BINTABLE with the
following columns:
The FITS column name is ENERG_LO.
The recommended units are keV.
The FITS column name is ENERG_HI.
The recommended units are keV.
The FITS column name is THETA (but see below)
The recommended units are arcmin.
The FITS column name is PHI (but see below).
The recommended units are arcmin.
The FITS column name is EFFAREA.
The order of data storage is
EffArea (E,θXMA,φXMA),
where E represents the
Elow and Ehigh arrary (see below).
The recommended units are cm2.
Extension |
HDUCLASS: OGIP
HDUDOC: CAL/GEN/92-019
HDUVERS: 1.0.0
HDUCLAS1: RESPONSE
HDUCLAS2: EFF_AREA
EXTNAME : EFFECTIVE AREA (suggested, not required)
Description: Effective areas (including vignetting and obscurration
effects) as a function of energy and off-axis & azimuthal angle.
An alternate spatial coordinate frame may also be used (see text).
Optional columns containing the statistical and systematic error arrays
are not shown.
Format: BINTABLE
column 1 2 3 4 5 contents Low energy High energy Off-axis Azimuthal Effective Areas bounds bounds angles angles Elow Ehigh θXMA φXMA EffArea format of each column 4-byte 4-byte 4-byte 4-byte 4-byte real real real real real array array array array array total number of elements per row i i j k i ×j ×k column name ENERG_LO ENERG_HI THETA PHI EFFAREA
Points to Note & Conventions
As noted in Section 1.3, given the difficulty parameterizing the Effective Area in energy-space, it is not recommended that such datasets are stored as Virtual Calibration Files.
The following list of subroutines/tasks are available:
Below are several examples of files currently available within the HEASARC caldb. Note that the authors of datasets are encouraged to supply copious COMMENT cards to aide human readers.
Follows is the header from an extension containing an EARVERSN=1992a dataset. The effective area data is stored in column 4 and is a function of 729 energies and 14 off-axis angles, THETA. There is no PHI dependence for this dataset. From the iCTYP4 (and also implied by the TDIM4) keyword, it can be seen that ENERGY is the 1st axis/dimension of the EFFAREA data array, and THETA the 2nd axis/dimension.
XTENSION= 'BINTABLE' / binary table extension BITPIX = 8 / 8-bit bytes NAXIS = 2 / 2-dimensional binary table NAXIS1 = 46712 / 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 = 4 / number of fields in each row TTYPE1 = 'ENERG_LO' / Lower boundaries of energy bins TFORM1 = '729E ' / data format of the field: 4-byte REAL TUNIT1 = 'keV ' / physical unit of field TTYPE2 = 'ENERG_HI' / Upper boundaries of energy bins TFORM2 = '729E ' / data format of the field: 4-byte REAL TUNIT2 = 'keV ' / physical unit of field TTYPE3 = 'THETA ' / Spatial coord grid: dimension 1 TFORM3 = '14E ' / data format of the field: 4-byte REAL TUNIT3 = 'arcmin ' / physical unit of field TTYPE4 = 'EFFAREA' / Effective Area dataset TFORM4 = '10206E ' / data format of the field: 4-byte REAL TUNIT4 = 'cm**2 ' / physical unit of field EXTNAME = 'EFFECTIVE AREA' / name of this binary table extension HDUCLASS= 'OGIP ' / format conforms to OGIP standard HDUDOC = 'CAL/GEN/92-019' / format definition document HDUVERS = '1.0.0 ' / Version of family of formats HDUCLAS1= 'RESPONSE' / dataset relates to instrument response HDUCLAS2= 'EFF_AREA' / dataset is an effective area CSYSNAME= 'XMA_POL ' / spatial coord system used in this dataset 1CTYP4 = 'ENERGY' / 1st axis of EFFAREA is energy 2CTYP4 = 'COORD-1' / 2nd axis of EFFAREA is coord-1 CREF4 = '(ENERG_LO:ENERG_HI,THETA)' TELESCOP= 'ROSAT ' / mission/satellite name INSTRUME= 'XRT ' / instrument/detector name FILTER = 'NONE ' / filter in use COMMENT Dataset assumed to be independent of PHI TDIM4 = '(729,14)' / Ordering of n-d EFF_AREA array 1CTYP4 = 'ENERGY ' / Axis of 1st dimension of EFF_AREA array 2CTYP4 = 'THETA ' / Axis of 2nd dimension of EFF_AREA array EARVERSN= '1992a ' / OGIP classification of FITS format HISTORY Extension written by WTEAR1 1.1.0 COMMENT COMMENT The following keywords are required for the OGIP CALDB CCLS0001= 'BCF ' / OGIP class of calibration file CDTP0001= 'DATA ' / OGIP type of dataset (DATA, TASK etc) CCNM0001= 'EFFAREA ' / OGIP codename for this type of cal file CVSD0001= '01/06/90' / Dataset validity start date (UTC) CVST0001= '00:00:00' / Dataset validity start time (UTC, of day CVSD) CDES0001= 'XRT Effective Area (version 1); 729 energies vs 14 off-axis angles' CBD10001= 'THETA(0-60.0)arcmin'/ dataset parameter boundary CBD20001= 'PHI(0-360)deg' / dataset parameter boundary CBD30001= 'ENERG(0.0546-3.01)keV'/ dataset parameter boundary COMMENT COMMENT NOTES: 1994 Jul 21 (Ian M George, HEASARC) COMMENT ------------------ COMMENT This dataset was converted to OGIP FITS format by COMMENT Ian M George (HEASARC) from the ASCII file COMMENT area_b_1.asc COMMENT supplied by Steve Snowden (ROSAT GOF, NASA/GSFC). COMMENT The area_b_1.asc file consists of the (total) spectral COMMENT response for PSPCB. The current dataset was therefore created COMMENT by DIVIDING the area_b_1.asc dataset by: COMMENT the gas efficiency: pspc_v1.gas_eff COMMENT the window transmission: pspcb_v1.wind_trans COMMENT COMMENT The area_b_1.asc dataset has also been converted to COMMENT an OGIP FITS format as is stored in file COMMENT pspcb_v1.spec_resp COMMENT END
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:
/docs/heasarc/caldb/caldb_docs_index.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 668, NASA/GSFC, Greenbelt, MD 20771, USA.
George, I.M. & Zellar, R.S.,
1992.
(CAL/GEN/92-003)
George, I.M., Arnaud, K.A., Pence, W. & Ruamsuwan, L.,
1992. Legacy, 2, 51.
(CAL/GEN/92-002)
George, I.M., & Arnaud, K.A.,
1992.
(CAL/GEN/92-002a)
George, I.M., Zellar, R.S. & Pence, W.,
1992.
(CAL/GEN/92-011)
George, I.M., Pence, W. & Zellar, R.S.,
1992.
(CAL/GEN/92-008)
George, I.M., Zellar, R.S. & White, N.E.,
1992.
(CAL/GEN/92-013)
George, I.M. & Zellar, R.S.,
1992.
(CAL/GEN/92-021)
George, I.M. & Zellar, R.S.,
1992.
(CAL/GEN/92-022)
The following useful links are available (in the HTML version of this document only):
1The result of
multiplying the Effective Area of the mirror/collimator
assembly by the transmission due to any filters & detector windows
along the optical path, and by the efficiency of the detector, is referred
to as the Spectral Response of the instrument within the
HEASARC caldb