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This Legacy journal article was published in Volume 1, May 1992, and has not been
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HEASARC Calibration Database
Ian George
HEASARC
Introduction
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 a dataset and its associated
calibration measurements is therefore critical.
This article briefly outlines the role of the HEASARC concerning the storage
and documentation of the calibration data of past, current, and future
missions. 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.
The Calibration Dataflow
A schematic showing the relationship between the various elements within the
calibration dataflow is shown in Figure 1. Those elements for which the HEASARC
is primarily responsible are shaded.
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.
Calibration File Types
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).
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 coordinates
the energy resolution of the detector
a theoretical parameterization of the instrument point spread function as a
function of energy, source intensity and detector coordinates
the change in sensitivity of the instrument as a function of time and
detector coordinates
a parameterization of the various components of the instrument background as
a function of time and in-orbit conditions
The Calibration Product Files can be divided into two types. The first
is those CPFs that are independent of a specific observation (i.e. 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 (e.g., a light curve, a 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. Examples of the type of calibration data included within
the CPFs are:
the instrument 'exposure map', describing the time each detector pixel spent
'on' and unobscured by any instrument support structure, etc., during a
given exposure.
the detector response matrix describing the conversion between pulse height
and energy for a given position on the detector during a given exposure.
a specific parameterization of the instrument point spread function as a
function of energy, detector coordinates, etc.
Calibration Updates
In the case of current and future missions, the Calibration Database 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
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 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
proprietary data period will 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.
Calibration Database Structure & Subroutine Libraries
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 coordination with the
various projects currently distributing calibration data. Full advantage will
be taken 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 as some of
the information may no longer be available. Full documentation of the format
of each calibration file for each instrument will be provided by the HEASARC
(see Documentation section below).
To facilitate access to the Calibration Database, a subroutine library will be
developed (in ANSI FORTRAN or C) and maintained by the HEASARC. The 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 (e.g., 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
users wishing to develop their own Stage 2 Calibration Software and/or Data
Analysis Package.
The Stage 2 Calibration Software
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 the HEASARC data archives once the
proprietary 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 available. Hence
users may therefore customize their own CPFs. The problems, changes and
effects of the various calibrations will be well-documented, to enable users to
make optimum use of the information and software.
Documentation
Substantial hard-copy documentation will be provided, and in the case of
current and future missions may form part of the Observer's Guide. Its main
purpose is to serve as a guide to HEASARC users and 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 in 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.
Implementation Plan
Work is currently underway on 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 HEASARC.has responsibilities, starting with the
EXOSAT ME, Einstein HRI and ROSAT HRI. It is anticipated
that 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. Anyone wishing to contribute original calibration data or
products.and/or experiences, is encouraged to contact the author.
Figure 1 A
schematic
representation of the calibration
dataflow showing the definitions of, and relationship between, the various
elements described in the text. In the case of current and future missions, the
responsibility for those elements (including documentation) above the dashed
line lies primarily with the hardware teams. In the case of past missions, the
HEASARC will be responsible for locating and documenting the required
information, although a substantial fraction of the necessary knowledge may
already have been lost to posterity. The responsibility for those elements
below the line jointly lies within the GOF, the HEASARC, and the software
teams.
References
Pence, W. 1992, Legacy, 1, 14.
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