Jonathan McDowellHarvard-Smithsonian Center for Astrophysics, Cambridge MA, USA
Arnold RotsHarvard-Smithsonian Center for Astrophysics, Cambridge MA, USA
Abstract
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.
Comments to this note are welcome; it is expected that
the expression of other data models into VOTable terms
will be added in the future.
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 attribute
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 a 2-D astronomical coordinate.
The prefix (stc) should also be declared with the name space
convention (as xmlns:stc=http://...) to specify the version
of the STC data model used.
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;
these definitions may however be omitted in the
case of coordinate systems referenced in the
STC library (see the example in example 1)
what is the exact role of the numbers listed in the document,
which is defined by the AstroCoords element
in the astronomical context,
or by the Coords element in a more generic context.
In a VOTable context, the utype attribute is used to specify
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 specified in a VOTable GROUP containing
the necessary PARAMeters or PARAMrefs
to completely specify the coordinate system;
this GROUPmust have the attributes
utype="stc:AstroCoordSystem"
ID="coordSys_name",
where coordSys_name is any name
which satisfies the ID/IDREF requirements
(begin with a letter, and contain alphanumeric characters
or the underscore without any whitespace).
such that it can be referenced
from the utype="stc:AstroCoords" group (item#2)
This group may be omitted in the case where the coordinate
system is part of the STC library [2].
the coordinates components are grouped in a GROUP
element with the properties:
the attribute utype="stc:AstroCoords"
an attribute ref="coordSys_name"
where coordSys_name is the ID assigned in
the first GROUP.
A coordinate system existing in the STC library
may be referenced by a ivo://STClib/CoordSys
identifier (see example 1)
an attribute ID="coordGroup_name"
such that the group can be referenced from FIELD elements;
like coordSys_name, the coordGroup_name
assigned name must satisfy the ID/IDREF requirements.
This GROUP may include PARAMeters to qualify the
coordinates if needed, and/or FIELDref references
(see the examples below).
finally the individual coordinates,
defined as FIELD elements, must have the attributes
utype="stc:AstroCoords.cootype.coorep.comp"
which specifies 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="coordGroup_name" which refers to the GROUP
defined in item#2 (having the utype="stc:AstroCoords").
It is quite possible to have several groups of coordinates in a single
table, either in a common system (a common value of the ref attribute
of the GROUPs having utype="stc:AstroCoordSystem"),
or in several different systems.
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. We draw attention to the possibility of the
double referencing, that is:
each FIELD which represents 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 FIELDref 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 role in
the STC model;
their ref="Coo1" attribute designates the GROUP having
the attribute ID="Coo1".
In this example the coordinate system (UTC-ICRS-TOPO) is
known in the STC library(see Appendix C of [2]), which is enough to fully define
the time reference (UTC), the frame orientation (ICRS)
and the reference point (TOPOcentric).
5.2 Extract of the Hipparcos catalog
The Hipparcos catalog is the primary realization of the International
Celestial Reference System (ICRS) for optical wavelengths.
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.
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.ReferencePosition.PlanetaryEphem"
A VOTABLE resource may contain multiple TABLEs and each of
these tables may contain many header PARAMs and many FIELD
columns, possibly giving several sets of positions and
times in several different coordinate systems.
For any PARAM or FIELD that gives an STC utype, there must
be a ref attribute
pointing to the GROUP that contains the STC AstroCoords
(or Coords) definition.
This is how we tie a particular utype to a particular STC instance.
For example:
says that the column in question contains the Time value for an AstroCoords
instance whose frame is defined in the group which begins with
<GROUP ID="Coo1" utype="stc:AstroCoords" .../>.
An AstroCoords instance may contain all of space, time, spectral and
redshift coordinates.
It is often meaningful to omit the spectral and redshift coordinates. Space and
time coordinates are usually more tightly coupled. While in practice for some
common coordinate systems and astronomical contexts it is reasonable to give a
position with no time, or a time not explicitly tied to a position, this is not
recommended.
This AstroCoords instance in turn contains a reference to
the relevant AstroCoordSystem (or CoordSys) used,
either to a group having a stc:AstroCoordSystem (or
stc:CoordSys) utype, or with an appropriate refence to the
bf STClib component.
Most important stc:AstroCoords components
in FIELD
utype
ucd
Comments
stc:AstroCoords.Time...
time components(1).
TimeInstant.ISOTime
time.start;obs.exposure
Date with ISO-8601 convention (e.g.2008-10-25T12:14:15)
TimeInstant.JDTime
time.start
Julian date, applies to e.g. a variable phenomenon
(e.g.2454765.09323)
TimeInstant.TimeOrigin.JDTime
time
Reference date
TimeInstant.TimeOffset
time
Time elapsed since the TimeOrigin
stc:AstroCoords.Position2D... position
on the celestial sphere
Value2.C1
pos.eq.ra
Right ascension or longitude
Value2.C2
pos.eq.dec
Declination or Latitude
Error2Radius
stat.error;pos.eq
Error radius of the spherical position
Error2.C1
stat.error;pos.eq.ra
Error on the Right Ascension or Longitude
Error2.C2
stat.error;pos.eq.dec
Error on the Declination or Latitude
stc:AstroCoords.Velocity2D... proper motion
on the celestial sphere
Velocity1D
pos.pm
Total proper motion
Velocity2D.C1
pos.pm;pos.eq.ra
Proper motion in RA or Longitude
Error2.C1
stat.error;pos.pm;pos.eq.ra
Error on a proper motion in RA or Longitude
stc:AstroCoords.Position3D... position
in a 3-D spherical system(2), as in example 3
Value3.C1
pos.ecliptic.lon
Ecliptic longitude
Value3.C2
pos.ecliptic.lat
Ecliptic latitude
Value3.C3
pos.distance;pos.barycentric
Distance to solar system barycenter
(1)
the reference position to which the time applies
is defined in a parameter having a utype
stc:AstroCoordSystem.SpaceFrame.ReferencePosition
(2)
requires to specify a SPHERICAL frame
(in stc:AstroCoordSystem.SpaceFrame.CoordFlavor component)
and a value of 3 for the number of axes
(in stc:AstroCoordSystem.SpaceFrame.coord_naxes)
6.1 stc:AstroCoords components
The actual coordinate components which are present in a VOTable
as FIELDs are characterized by
a reference to a GROUP gathering the coordinate components,
e.g. ref="Coo1"
their utype, e.g.
utype="stc:AstroCoords.Position2D.Value2.C1"
for the right ascension.
their ucd attribute, e.g.
ucd="pos.eq.ra"
... and the other attributes necessary for a correct interpretation
of the data: datatype, unit, width,
precision, etc...
Illustrations with actual examples are given in the examples above;
the most important utype and ucd attributes
of use in this context
are listed in the summary table of space-time coordinates.
Many more parameters are actually available in the STC model [2], which should
be consulted for details.
6.2 stc:AstroCoordSystem components
The coordinate systems used in a VOTable, if these are not
part of the STC Standard Library (Appendix C of [2]),
must be defined in GROUP elements having the
utype="stc:AstroCoordSystem" attribute,
and an ID attribute
to which the group of coordinate elements defined above can
refer.
Most important TimeScale values
TT
Terrestrial time
TAI
Temps Atomique International
TT+32.184s
UTC
Coordinated Universal Time
TAI+leap seconds
LST
Local Sideral Time
(needs ObservationLocation)
LOCAL
Local definition
Most important ReferencePosition values
TOPOCENTER
local position, on the surface of Earth
Location should be specified
GEOCENTER
center of Earth
HELIOCENTER
center of Sun
BARYCENTER
barycenter of the Solar System
LSR
Local Standard of Rest
GALACTIC_CENTER
center of our Galaxy
Most important astronomical frame types
SPHERICAL
Spherical coordinates, 2-D (2 angles) and 3-D(additional
distance) coord_naxes required for 3-D
CARTESIAN
Cartesian frame
POLAR
Longitude + polar distance
The temporal, spatial (and eventually the spectral, velocity / redshift)
components
are defined as subgoups with the appropriate utypes, following the schema:
<GROUP utype="stc:AstroCoordSystem" ID="myFrame">
<!-- If one wants to give an specific name to this frame-->
<PARAM name="myName" datatype="char" arraysize="*"
utype="stc:AstroCoordSystem.coord_system_id"
value="UTC-FK5-TOPO"/>
<GROUP utype="stc:AstroCoordSystem.TimeFrame" ID="myTime">
<PARAM name="TimeScale" datatype="char" arraysize="*"
utype="stc:AstroCoordSystem.TimeFrame.TimeScale"
value="(see time_scale)" />
<PARAM name="refPosition" datatype="char" arraysize="*"
utype="stc:AstroCoordSystem.TimeFrame.ReferencePosition"
value="(see ref_position)" />
</GROUP>
<GROUP utype="stc:AstroCoordSystem.SpaceFrame" ID="mySpace">
<PARAM name="CooFrame" datatype="char" arraysize="*"
utype="stc:AstroCoordSystem.SpaceFrame.CoordRefFrame"
value="(see standard_frame)" />
<PARAM name="CooFrame" datatype="double"
utype="stc:AstroCoordSystem.SpaceFrame.CoordRefFrame.Equinox"
value="(equinox only for Equatorial or Ecliptic rame)" />
<PARAM name="CooFrameOrigin" datatype="char" arraysize="*"
utype="stc:AstroCoordSystem.SpaceFrame.ReferencePosition"
value="(see ref_position)" />
<PARAM name="CooType" datatype="char" arraysize="*"
utype="stc:AstroCoordSystem.SpaceFrame.CoordFlavor"
value="(see frame type)" />
</GROUP>
</GROUP>
The most frequent values of the parameters defining the time
are summarized in time_scale(time scales) and
ref_position(reference location
of time measurement); among the parameters which define
a spatial frame, ref_position
defines the origin of the frame (which may differ from the
reference location of time measurements),
frame type defines the frame type and
dimensionality (standard frames used in astrometry are 2-D spherical),
and standard_frame summarized the
most frequently frames used in astrometry.
