This note describes the recommended way to reference the coordinate systems
in usage in Astronomy for an accurate specification of
spatial and temporal reference frames. It was made in agreement between
the VOTable and the Data Models Working Groups.
Status of this document
This is an IVOA Note. It attempts to clarify how to reference
unambiguously, in a
VOTable document
containing space- and/or time-related data,
the reference systems in which these data are expressed,
systems which are described by
the STC data model.
This document is a preliminary note; comments are welcome.
Acknowledgments
This document is based on the W3C documentation standards, but has been adapted
for the IVOA.
Astronomical catalogues are a very common source of tabular
data, and among these the catalogs collecting astronomical sources
observed at specific sky locations and epochs have routinely to
be compared to other catalogues containing informations
related to the same sources, but observed or modelled in different
contexts: observations at different epochs, different wavelengths,
simulated results, etc. An accurate knowledge of the meaning of
the parameters included in these catalogues is therefore essential if one
expects to derive scientifically significant conclusions from the
comparisons of these data.
The VOTable format is an IVOA standard which ensures that large data
collections,
which can be structured as a set of tables, can efficiently be exchanged
within the Virtual Observatory framework with a full understanding of
the data meaning (the metadata). A lot of details may be
required for a full description of the elements stored in these data
collections, but the locations in
space and time (the coordinates, spatial and temporal)
are essential if any comparison of the sources observed is attempted.
The data in the context of the Virtual Observatory are described by data models,
which formalize the context of the data for their accurate interpretation.
Data described in this way can then be more efficiently compared
with data of similar nature but coming out from other horizons,
and valid conclusions or scientific results can be drawn with
higher confidence.
2 The space-time coordinates
In the context of astronomical catalogues, spatial (location in the sky)
and temporal (date/time) parameters are one characterisation of virtually
any observation, and these parameters are therefore fundamental
if one wishes to make any
comparison of the behaviour of the astronomical sources described in
different catalogs - typically catalogs of observations made in widely
different wavelength regimes. Since several systems are
currently in use to express the spatial coordinates, a dedicated
COOSYS element
was introduced in the original version (1.0) of VOTable:
it did specify some minimal level of details about the coordinate system
used in the table, and was merely an enumeration of the most
commonly coordinate frames used in astronomy (ICRS, historical equatorial
frames, galactic, ecliptic, etc) associated to a couple of parameters
(equinox and epoch). This VOTable-specific way of specifying the frames
used for spatial location is deprecated in VOTable version 1.2, in favor
of a more generic way of specifying the space and time coordinates
using the utype attributes and the GROUP constructs[1].
3 The utype attribute
VOTable is not a data model, it is more a container for data which
can be structured as tables, with emphasis on a single description
(the schema or metadata) for a potentially large number of elements
(the tuples or rows). The description of the data cannot include all
components of all the data models developed in the Virtual Observatory framework,
but their accurate description requires to reference the data model,
and to say how the various FIELDs composing the table are
related to this data model. This relation -- referencing a data model --
is specified by the utype attribute, introduced in the version 1.1
of VOTable for this purpose; utype can also be an attibute
of the PARAM, GROUP, TABLE and RESOURCE
elements [1].
The syntax of the utype attribute has not yet been fixed in
the Virtual Observatory context. This note assumes a syntax made from the
model name (stc) followed by a colon (:), and the dot-separated
hierarchy of elements, as e.g. stc:AstroCoords.Position2D.Value2.C1
to refer to first component of value of the position of the astronomical
coordinate.
4 Specifying coordinate components in VOTable
The specification of an astronomical spatial and/or temporal coordinates
requires essentially 2 definitions [2]:
the definition of the coordinate system (the frame in which
the coordinates are expressed), which can be
an AstroCoordSystem (for astronomically defined frames)
or a more generic CoordSys entity.
what do the actual numbers represent in the coordinate system,
which is defined by AstroCoords in the astronomical context,
or in a more generic context by the Coords specification.
In a VOTable context, the utype attribute is used to precise
the exact role of a specific field or parameter within the STC data model.
The utype is an attribute of the FIELD element,
and for instance the right ascension and declination components
are defined by the attributes utype="stc:AstroCoords.Position2D.Value2.C1"
and utype="stc:AstroCoords.Position2D.Value2.C2" respectively.
The full specification of the STC components in a VOTable is
constructed with 2 GROUPs which follow the STC hierarchy:
the coordinate system (AstroCoordSystem)
is precised in a VOTable GROUP containing
the necessary PARAMeters or PARAMrefs
to completely define the coordinate system;
this GROUP must have the attributes
attribute utype="stc:AstroCoordSystem"
an ID attribute such that it can be referenced
from the utype="stc:AstroCoords" group (item#2)
the coordinates are grouped in a GROUP element with the
properties:
attribute utype="stc:AstroCoords"
a ref attribute designating the coordinate system, i.e.
the contents of this ref attribute is identical to
the contents of the ID attribute of the group defining
the AstroCoordSystem (item#1)
an ID attribute such that it can be referenced
from FIELDs;
This GROUP may include PARAMeters to qualify the
coordinates if needed, and/or FIELDref references, see
examples below.
finally the individual coordinates,
defined as FIELD elements, have the attributes
utype="stc:AstroCoords.cootype.coorep.comp"
which precises the role of the field in the STC data model.
The italicized components are:
cootype can be "Position2D" for spherical
coordinates, "Position3D" for 3-D coordinates,
"Time" for the time axis;
coorep is the coordinate representation:
"TimeInstant" for the time axis,
"Value2" for spherical coordinates, "Value3"
for 3-D coordinates (cartesian or spherical,
depending on the parameter defining the flavour,
which is characterized by
utype="stc:AstroCoordSystem.SpaceFrame.CoordFlavor").
comp indicates the component: "C1",
"C2", "C3" for the spatial axes,
"ISOTime", "MJDTime" or one of the other
times defined in the STC [2] for the axis time.
ref="cooID" which refers to the GROUP
defined in item#2 having the utype="stc:AstroCoords".
A few examples are given below to illustrate the referencing mechanism.
5 Examples
Three examples are given here. The first one uses the STClib
described in the Appendix C of the STC model [2], the other
two give a more complete definition of the coordinate system used.
In all examples, we emphasize with different colors the
STC-related utypes and the
corresponding ID/ref attributes
to visualize their role in the XML output below. One could also notice the possibility of the
double referencing, that is:
each FIELD which is a component of the space-time coordinates
is referencing the "AstroCoords" GROUP. This reference
is required to ensure a correct interpretation
(made by ref="Coo1" in the first example below)
within the "AstroCoords" GROUP, the components
may be enumerated by means of PARAMref elements.
This second referencing is optional.
5.1 List of observations
This simple example of a VOTable document lists 3 observations
made with a telescope; each observation is characterized by
the location of the CCD center, date/time of observation,
and the exposure time. The XML serialisation looks like the following:
The spherical coordinates (RAJ2000, DEJ2000) defined as FIELDs
have their utype attribute which specify their position in
the STC model;
their ref="Coo1" attribute designates the GROUP having
the attribute ID="Coo1",
which in turn gives the details about the
coordinate system: here it refers to GROUP having the
attribute ID="mainFrame".
NOTE: one could (should?) remove the first GROUP with
utype="stc:AstroCoordSystem" which is highly redundant.
5.2 Extract of the Hipparcos catalog
The Hipparcos catalog is the catalog which represents the basis of
the ICRS system in the optical regime. Its positions and proper motions are
given for the epoch J1991.25 (roughly the mid-point of the mission).
The astrometric frame is barycentric (the barycenter of the Solar
system is assumed to be the origin of the frame); and the times used
throughout the Hipparcos catalog (for the epoch photometry)
is the geocentric terrestrial time [3].
Notice that the GROUP defining the coordinate system
(first group below) is itself made of 2 groups, one for the definition
of the time axis, and the second for the definition of the spatial axes.
the utype="stc:AstroCoordSystem.SpaceFrame.epoch"
would better be a component of utype="stc:AstroCoords",
since it is not a characteristic of the frame itself.
the utype="stc:AstroCoordSystem.SpaceFrame.epoch"
does not specify what is the time scale used for the
epoch. It could be given in any time flavor:
JD, MJD, Julian/Besselian year, etc...
5.3 Ephemeride of Comets
This example gives the position of the comet 125P as a function of time.
Notice that the planetary model used is added there
via a PARAM having
utype="stc:AstroCoordSystem.SpaceFrame.stdRefPosType.PlanetaryEphem"
