The HEASARC Calibration Database
Ian M. George
Version: 1995 Mar 29
(a brief overview)
Code 668,
NASA/GSFC,
Greenbelt, MD20771
|
| Release | Sections Changed | Brief Notes |
| Date | ||
| 1991 Oct 21 | Original Version | |
| 1995 Jan 11 | All | Made compatible with LaTeX2HTML software |
| 1995 Feb 13 | Figure 1 | Made EPSF and included in text |
| 2004 Apr 1 | All | Made compatible with tth |
The availability of the instrument calibration and an understanding of its
limitations is fundamental to the scientific analysis of data.
In the past, calibration data has generally
been made available in a somewhat ad hoc fashion.
In most cases it has been a `black box' part of an analysis
task, hidden from the user.
There is usually little documentation available, nor any record of
previous versions of the calibrations.
It is unlikely that the majority of
HEASARC
users will have an intimate
understanding of all the detector details of all the instruments from
which they obtain data.
The transfer of the necessary information such that a
user can make sensible judgements concerning their dataset and associated
calibration measurements is therefore critical.
The HEASARC
intends to locate, document and store on-line a full and
up-to-date set of calibration data for each instrument for which
scientific data is contained within the archive.
The HEASARC
will define the standard types and formats for the
calibration data files, and develop a subroutine library to access them.
The overall motivation is to present users and programmers with calibration
data available for each instrument in a standard, flexible and
user-friendly format, along with information concerning its use, importance
and quality.
A schematic showing the inter-relationship between
the various elements within the calibration dataflow are shown in
Figure 1.
Those elements for which the HEASARC
is primarily responsible are shown
hatched.
The Stage 1 Calibration Software (developed and maintained by
the hardware teams) combines ground and in-orbit calibration measurements
for a given instrument
with any necessary theoretical modelling and algorithms to produce the
Basic Calibration Files (BCFs).
The data within the BCFs is then convolved with
further algorithms, and if appropriate housekeeping data
from the satellite,
by the Stage 2 Calibration Software (developed and maintained by the
relevant project) to produce the Calibration Products Files (CPFs).
Together, the BCFs and CPFs form the contents of the `HEASARC Calibration
Database' and provide the calibration input to the
various Data Analysis Packages.
The Basic Calibration Files will ideally contain all the data
(excluding any necessary housekeeping information) required
to construct the CPFs.
In the case of past missions this may, unfortunately, not always be possible.
The BCFs will contain calibration information
which is both independent of time and in-orbit conditions
(in most cases data originating from ground calibration measurements),
and information which is expected to vary throughout the mission
(mainly from in-orbit measurements).
The Calibration Product Files can be divided into two types.
The first is those CPFs that are independent of
a specific observation (ie for which housekeeping data is not required).
These CPFs are primarily a rearrangement of the
information contained within the BCFs suitable for a
specific purpose within an individual Data Analysis Package.
The second type of CPF is that for which housekeeping data is required
and hence observation specific.
These CPFs are not strictly part of the HEASARC Calibration Database
as each is associated with specific analysis products file
(eg a light curve, spectrum). They will therefore form part of the
HEASARC Products Database.
The number and use of the CPFs for a specific instrument may
increase as experience dictates.
In the case of current and future missions,
the Calibration Databases will be `live' in the sense that it
will be continually updated as necessary.
Periodic updates to those BCFs which contain data which
explicitly depend on time
will be obviously be necessary.
As the mission progresses, the appropriate files will thus be
extended as each new set of calibration measurements becomes
available.
Updates to the BCFs will also obviously be necessary in the event of an
error.
However in all cases previous (including erroneous)
versions of the calibration data will be retained
within the same file to ease direct comparison of results.
The format of the BCFs and CPFs will therefore be such that updates are
easily incorporated.
In the case of a current mission, the project/GOF
will automatically inform Guest Observers of calibrations updates,
and during the propriety data period re-release appropriate
BCFs and (reprocessed) CPFs.
In the case of a past mission, all efforts will be extended to include
earlier calibrations into the appropriate databases, although this task
will be given a relatively low priority.
Notification of calibration updates
and a brief description of their impact will be made available on
a bulletin board and within future issues of Legacy
to enable users to assess their impact on previous results.
All the data files within the
HEASARC Calibration Database
will be in standard FITS format and stored on-line.
Users will therefore be able to browse the various calibration data
available and extract data.
The detailed format of individual files will be defined by the
HEASARC in
discussion with the various projects currently distributing calibration
data.
It is intended to take full advantage of the similarities between different
individual instruments of similar type, and hence make the format of
the corresponding calibration files identical wherever possible.
However, it is recognized that at some level, the type and format of the
calibration data required and available may differ between instruments.
This is particularly the case for past missions for which it may be
impossible to always present the calibration in an identical form as some
of the information may no longer be available.
Full documentation of the format of each calibration file for each
instrument will of course be provided by the
HEASARC(see section 6 below).
To facilitate accesss to the
Calibration Database a
subroutine library will be developed
(in ANSI FORTRAN or C) and maintained by the
HEASARCThe subroutines will employ the
FITSIO package
(Pence 1991) and be used
by the Stage 2 Calibration Software and the
HEASARC supported Data
Analysis Packages to return
all the calibration results required by the Data Analysis Packages
(eg the effective area of the instrument at a specific time, energy
and detector position).
The source code for this library will also be made available for
user wishing to develop their own Stage 2 Calibration Software
and/or Data Analysis Package.
The existence of several updates to the calibration data within the
BCFs introduces an added complexity to the construction of the CPFs.
By default, the Stage 2 Calibration Software will use the calibration
data within the BCFs deemed at the time of processing to be the most
appropriate. The resultant CPFs will be sent to the GO in the case of
a current mission, and in the absence of later updates, transferred to
theHEASARC data archives
once the propriety data period
expires. However, the Stage 2 Calibration Software will also be
sophisticated enough to allow a user to interactively display and
choose between the various alternate calibration measurements
avaliable. Hence a user may therefore customise their own CPFs. The
problems, changes and effects of the various calibrations will be well
documented to enable the user to make optimum use of the information
and software.
Substantial
hard-copy & on-line documentation
will be provided, and in the case
of current and future missions may form part of the Observer's Guide.
It's main purpose is to serve as a guide to HEASARC users & staff
as to the status of the calibration data of a given instrument,
the origin and quality of the data, the detailed contents
of the calibration files, and a recipe for the use of the calibration data.
The detailed format of each of the calibration files will also be included,
as will
notes on relevant experiences, problems and worries associated with the
calibration of the science data.
The guide should be written at a level appropriate for a user
with only limited experience of X-ray astronomy, and hence be accessible
to users from the general astronomical community.
Copies will be made available to the community on request as appropriate.
A reduced form of the guide will also be kept on-line in
the HEASARC Documents Database.
Work is currently underway locating, reformatting and documenting the
Einstein IPC, EXOSAT LE/CMA and
ROSAT PSPC calibration data.
A draft definition of the formats will be circulated to interested
parties in the near future for comments.
The subroutine library will simultaneously be developed.
Thereafter work will commence on the other datasets for which the
has responsibilities, starting with the EXOSAT ME, Einstein HRI
and ROSAT HRI.
It is anticipated that ASCA (formerly ASTRO-D)
data will be distributed in the
standard format from the start.
The EXOSAT LE Calibration Guide is in preparation and will be used as
a guide to the production of the documentation of the other instruments.
Anybody wishing to contribute original calibration data or products
and experiences are encouraged to contact the author.
1 INTRODUCTION
2 THE CALIBRATION DATAFLOW
3 CALIBRATION FILE TYPES
Examples of the type of calibration data included within the BCFs are:
- the theoretical effective area of the instrument
as a function of energy and detector co-ordinates.
- the energy resolution of the detector.
- a theoretical parameterization of the
instrument point spread function as a
function of energy, source intensity and detector
co-ordinates.
- the change of sensitivity of the instrument as a function
of time and detector co-ordinates.
- a parameterization of the various components of the
instrument
background as a function of time and in-orbit conditions.
Examples of the type of calibration data included within the CPFs are:
- the instrument `exposure map' describing the time each
detector pixel spent unobscurred by any instrument support
structure etc during a given exposure.
- the detector response matrix describing the conversion
between pulse height and energy as a function of detector
co-ordinates during a given exposure.
- a specific parameterization of the instrument point spread
function as a function of energy, detector co-ordinates
etc.
4 CALIBRATION UPDATES
5 CALIBRATION DATABASE STRUCTURE & SUBROUTINE LIBRARIES
6 THE STAGE 2 CALIBRATION SOFTWARE
7 DOCUMENTATION
8 IMPLEMENTATION PLAN
FIGURES
Figure
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