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Referencing STC in VOTable
Version 1.01

IVOA Note 2008-10-18

This version:
http://www.ivoa.net/Documents/Notes/VOTableSTC/VOTableSTC-20081018.html

Latest version:
http://www.ivoa.net/Documents/latest/VOTableSTC.html

Previous version(s):
http://www.ivoa.net/Documents/Notes/VOTableSTC/VOTableSTC-20080910.html

Editor(s):
François Ochsenbein

Author(s):
    François Ochsenbein   Observatoire Astronomique de Strasbourg, France
    Jonathan McDowell   Harvard-Smithsonian Center for Astrophysics, Cambridge MA, USA
    Arnold Rots   Harvard-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.

Contents:

1  Introduction

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]:

  1. 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)
  2. what is the exact role of the numbers listed in a specific table, 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:

  1. the coordinate system (AstroCoordSystem) is specified as a GROUP element containing the necessary information to describe completely a spatial—temporal—spectral—redshift coordinate system. This group will generally include sub-GROUPs for the specification of the temporal, spatial, spectral or redshift axes, which in turn include the necessary PARAMeters to completely specify the coordinate system, like the equinox for an equatorial frame, the number of axes for a non—2-D spatial frame, the reference position for the time axis, etc. This GROUP must have the attributes

    This group may be omitted in the case where the coordinate system is part of the STC library (see example 1). When present, the location of this group may be any of the acceptable locations of the GROUP element in the VOTable hierarchy [1]: a direct element of VOTABLE, or a sub-element of RESOURCE or TABLE.

  2. the coordinates components are grouped in a GROUP element with the properties:

    This second GROUP may include additional PARAMeters to qualify the coordinates if needed (e.g. an epoch of observation), and/or FIELDref references (see the examples below). Since it describes the components of the coordinates in a given table, this second group must be located within a TABLE element.

The individual coordinates provided in the table are defined as FIELD elements and must have the attributes

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 (several GROUPs with different ID attributes).

5  Examples

Three examples are given here. The first one omits the CoordSys group and refers to the STClib described in the Appendix C of the STC model [2]; the other two illustrate the defiinition of non-standard coordinate system.

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 within a TABLE:

  1. 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)
  2. 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 (on the sky) of the CCD center, date/time of observation, and the exposure time. The XML serialisation looks like the following:

<?xml version="1.0"?>
<VOTABLE version="1.2" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 xmlns:stc="http://www.ivoa.net/xml/STC/stc-v1.30.xsd"
 xsi:noNamespaceSchemaLocation="http://www.ivoa.net/xml/VOTable/v1.2">

  <RESOURCE name="myObservations">
    <TABLE name="results">
      <DESCRIPTION>Observation log</DESCRIPTION>
      <PARAM name="Telescope" datatype="float" ucd="phys.size;instr.tel"
             unit="m" value="3.6"/>
      <GROUP ID="Coo1" utype="stc:AstroCoords" >
        <PARAM name="cooframe" datatype="char" arraysize="*" ucd="pos.frame"
               utype="stc:AstroCoords.coord_sys_id"
               value="UTC-ICRS-TOPO" /> <!-- UTC-ICRS-TOPO defined in STC-Lib -->
        <FIELDref ref="ObsStart" /> <!-- second referencing, optional -->
        <FIELDref ref="RAJ2000"/>   <!-- second referencing, optional -->
        <FIELDref ref="DEJ2000"/>   <!-- second referencing, optional -->
      </GROUP>
      <FIELD name="RAJ2000" ucd="pos.eq.ra;meta.main" ref="Coo1"
             ID="RAJ2000" utype="stc:AstroCoords.Position2D.Value2.C1"
             datatype="float" precision="4" unit="deg" />
      <FIELD name="DEJ2000" ucd="pos.eq.dec;meta.main" ref="Coo1"
             ID="DEJ2000" utype="stc:AstroCoords.Position2D.Value2.C2"
             datatype="float" precision="4" unit="deg" />
      <FIELD name="ObsStart" ucd="time.start;obs" datatype="char"
             arraysize="19" unit="s" ID="ObsStart" ref="Coo1"
             utype="stc:AstroCoords.Time.TimeInstant.ISOTime" />
      <FIELD name="ExpTime" ucd="time.duration;obs.exposure"
             datatype="float" width="6" precision="1" unit="s"/>
      <DATA>
        <TABLEDATA>
        <TR>
          <TD>035.0798</TD><TD>-05.2336</TD><TD>2005-11-01T12:00:55</TD><TD>1200</TD>
        </TR>
        <TR>
          <TD>035.0547</TD><TD>-05.2253</TD><TD>2005-11-01T12:25:20</TD><TD>600</TD>
        </TR>
        <TR>
          <TD>035.0463</TD><TD>-05.2503</TD><TD>2005-11-01T12:36:20</TD><TD>600</TD>
        </TR>
        </TABLEDATA>
      </DATA>
    </TABLE>
  </RESOURCE>
</VOTABLE>

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). The exact specification of this location could be specified by a PARAM or GROUP having the utype="stc:AstroCoordSystem.SpaceFrame.ReferencePosition" attribute.

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.

<?xml version="1.0"?>
<VOTABLE version="1.2" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 xmlns:stc="http://www.ivoa.net/xml/STC/stc-v1.30.xsd"
 xsi:noNamespaceSchemaLocation="http://www.ivoa.net/xml/VOTable/v1.2">

  <GROUP utype="stc:AstroCoordSystem" ID="HIP" >
    <DESCRIPTION>
           The original Hipparcos catalogue uses different origins for
           spatial (barycentric) and temporal (geocentric) components. 
    </DESCRIPTION>

    <!-- The time axis has the position origin at the center of Earth -->
    <GROUP utype="stc:AstroCoordSystem.TimeFrame" >
      <PARAM name="TimeScale" datatype="char" arraysize="*"
             utype="stc:AstroCoordSystem.TimeFrame.TimeScale" value="TT"  />
      <PARAM name="refPosition" datatype="char" arraysize="*"
             utype="stc:AstroCoordSystem.TimeFrame.ReferencePosition"
             value="GEOCENTER" />   <!-- reference position for time -->
    </GROUP>

    <!-- The spatial frame is barycentric -->
    <GROUP utype="stc:AstroCoordSystem.SpaceFrame" >
      <PARAM name="CooFrame" datatype="char" arraysize="*"
             utype="stc:AstroCoordSystem.SpaceFrame.CoordRefFrame"
             value="ICRS"  />  <!-- Orientation of spatial axes -->
      <PARAM name="CooFrameOrigin" datatype="char" arraysize="*"
             utype="stc:AstroCoordSystem.SpaceFrame.ReferencePosition"
             value="BARYCENTER" />  <!-- Origin of spatial axes -->
      <PARAM name="CooType" datatype="char" arraysize="*"
             utype="stc:AstroCoordSystem.SpaceFrame.CoordFlavor"
             value="SPHERICAL"  /> <!-- 2-D is the default -->
    </GROUP>
  </GROUP>

  <RESOURCE name="hip_main">
    <TABLE name="hip_main_excerpt">
      <DESCRIPTION>A (tiny) excerpt from the Hipparcos catalog</DESCRIPTION>
      <GROUP ID="HIPcoo" utype="stc:AstroCoords" ref="HIP">
        <PARAM name="epoch" datatype="char" arraysize="*" unit="yr"
          utype="stc:AstroCoords.SpaceFrame.Epoch"
          value="J1991.25" />  <!-- Julian time scale ?? (1) -->
        <FIELDref ref="RA1"/>  <!-- second referencing, optional -->
        <FIELDref ref="DE1"/>  <!-- second referencing, optional -->
        <FIELDref ref="pm1"/>  <!-- second referencing, optional -->
        <FIELDref ref="pm2"/>  <!-- second referencing, optional -->
      </GROUP>
      <FIELD name="HIP" ucd="meta.id;meta.main" datatype="int" width="6">
        <DESCRIPTION>Identifier (HIP number) (H1)</DESCRIPTION>
      </FIELD>
      <FIELD name="Vmag" ucd="phot.mag;em.opt.V" datatype="float"
             width="5" precision="2" unit="mag">
        <DESCRIPTION>Magnitude in Johnson V (H5)</DESCRIPTION>
        <VALUES null="" />
      </FIELD>
      <FIELD name="RA(ICRS)" ucd="pos.eq.ra;meta.main" ref="HIPcoo"
             ID="RA1" utype="stc:AstroCoords.Position2D.Value2.C1"
             datatype="double" width="12" precision="8" unit="deg">
        <DESCRIPTION>alpha, degrees (ICRS, Epoch=J1991.25) (H8)</DESCRIPTION>
      </FIELD>
      <FIELD name="DE(ICRS)" ucd="pos.eq.dec;meta.main" ref="HIPcoo"
             ID="DE1" utype="stc:AstroCoords.Position2D.Value2.C2"
             datatype="double" width="12" precision="8" unit="deg">
        <DESCRIPTION>delta, degrees (ICRS, Epoch=J1991.25) (H9)</DESCRIPTION>
      </FIELD>
      <FIELD name="Plx" ucd="pos.parallax.trig" datatype="float" width="7"
             precision="2" unit="mas">
        <DESCRIPTION>? Trigonometric parallax (H11)</DESCRIPTION>
      </FIELD>
      <FIELD name="pmRA" ucd="pos.pm;pos.eq.ra" ref="HIPcoo"
             ID="pm1" utype="stc:AstroCoords.Velocity2D.Value2.C1"
             datatype="double" width="8" precision="2" unit="mas/yr">
        <DESCRIPTION>Proper motion mu_alpha.cos(delta) ICRS(H12)
                     (for J1991.25 epoch)</DESCRIPTION>
      </FIELD>
      <FIELD name="pmDE" ucd="pos.pm;pos.eq.dec" ref="HIPcoo"
             ID="pm2" utype="stc:AstroCoords.Velocity2D.Value2.C2"
             datatype="double" width="8" precision="2" unit="mas/yr">
        <DESCRIPTION>? Proper motion mu_delta, ICRS (H13)
                      (for J1991.25 epoch)</DESCRIPTION>
      </FIELD>
      <FIELD name="e_Plx" ucd="stat.error" datatype="float" width="6"
             precision="2" unit="mas">
        <DESCRIPTION>Standard error in Plx (H16)</DESCRIPTION>
      </FIELD>
      <DATA>
        <TABLEDATA>
          <TR>
            <TD>1</TD><TD>9.10</TD><TD>0.00091185</TD><TD>1.08901332</TD>
            <TD>3.54</TD><TD>-5.20</TD><TD>-1.88</TD><TD>1.39</TD>
          </TR>
          <TR>
            <TD>2</TD><TD>9.27</TD><TD>0.00379737</TD><TD>-19.49883745</TD>
            <TD>21.90</TD><TD>181.21</TD><TD>-0.93</TD><TD>3.10</TD>
          </TR>
          <TR>
            <TD>3</TD><TD>6.61</TD><TD>0.00500795</TD><TD>38.85928608</TD>
            <TD>2.81</TD><TD>5.24</TD><TD>-2.91</TD><TD>0.63</TD>
          </TR>
          <TR>
            <TD>4</TD><TD>8.06</TD><TD>0.00838170</TD><TD>-51.89354612</TD>
            <TD>7.75</TD><TD>62.85</TD><TD>0.16</TD><TD>0.97</TD>
          </TR>
          <TR>
            <TD>5</TD><TD>8.55</TD><TD>0.00996534</TD><TD>-40.59122440</TD>
            <TD>2.87</TD><TD>2.53</TD><TD>9.07</TD><TD>1.11</TD>
          </TR>
        </TABLEDATA>
      </DATA>
    </TABLE>
  </RESOURCE>
</VOTABLE>

Note (1) the STC-schema specifies a numeric datatype for the Epoch; we neverthless added the J prefix which indicates values expressed in Julian years (365.25days) as opposed to Besselian years (about 365.2422 days).

5.3  Ephemeride of a Comet

This example gives the position of the comet 125P as a function of time. Notice that the planetary model used is specified by a PARAM having
utype="stc:AstroCoordSystem.SpaceFrame.ReferencePosition.PlanetaryEphem"

<?xml version="1.0"?>
<VOTABLE version="1.2" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 xmlns:stc="http://www.ivoa.net/xml/STC/stc-v1.30.xsd"
 xsi:noNamespaceSchemaLocation="http://www.ivoa.net/xml/VOTable/v1.2">

  <GROUP utype="stc:AstroCoordSystem" ID="JPL-DE405">

    <!-- Times refer to the local observations -->
    <GROUP utype="stc:AstroCoordSystem.TimeFrame" >
      <PARAM name="TimeScale" datatype="char" arraysize="*"
             utype="stc:AstroCoordSystem.TimeFrame.TimeScale" value="UTC" />
      <PARAM name="refPosition" datatype="char" arraysize="*"
             utype="stc:AstroCoordSystem.TimeFrame.ReferencePosition"
             value="GEOCENTER" />
    </GROUP>

    <!-- Distance is geocentric -->
    <GROUP utype="stc:AstroCoordSystem.SpaceFrame" >
      <PARAM name="CooFrame" datatype="char" arraysize="*"
             utype="stc:AstroCoordSystem.SpaceFrame.CoordRefFrame"
             value="ICRS"  />      <!-- Orientation of spatial axes -->
      <PARAM name="CooFrameOrigin" datatype="char" arraysize="*"
             utype="stc:AstroCoordSystem.SpaceFrame.ReferencePosition"
             value="GEOCENTER" />  <!-- Origin of spatial axes -->
      <PARAM name="Ephemeris" datatype="char" arraysize="*"
         utype="stc:AstroCoordSystem.SpaceFrame.ReferencePosition.PlanetaryEphem"
         value="JPL-DE405">
         <DESCRIPTION>
           Planetary theory used to compute the ephemeris of the planets
         </DESCRIPTION>
      </PARAM>
      <PARAM name="CooType" datatype="char" arraysize="*"
             utype="stc:AstroCoordSystem.SpaceFrame.CoordFlavor"
             value="SPHERICAL" />
      <!-- default number of axes is 2, but here we are in 3-D -->
      <PARAM name="Naxes" datatype="int" value="3"
             utype="stc:AstroCoordSystem.SpaceFrame.coord_naxes" />
    </GROUP>
  </GROUP>

  <RESOURCE name="125P">
    <TABLE name="Ephemeris">
      <DESCRIPTION>Ephemeris of comet 125P in July 2007</DESCRIPTION>
      <GROUP ID="Ephem" utype="stc:AstroCoords" ref="JPL-DE405">
        <PARAM name="Ephemerides" datatype="char" arraysize="*"
               utype="stc:AstroCoords.coord_sys_id"
               value="UTC-ICRS-GEO"/>
        <FIELDref ref="RA1"/>   <!-- second referencing, optional -->
        <FIELDref ref="DE1"/>   <!-- second referencing, optional -->
        <FIELDref ref="Dist1"/> <!-- second referencing, optional -->
      </GROUP>
      <FIELD name="UTC" ucd="time.epoch" datatype="double" ref="Ephem"
                   utype="stc:AstroCoords.Time.TimeInstant.JDTime"
                   width="9" precision="1">
        <DESCRIPTION>Date UTC </DESCRIPTION>
      </FIELD>
      <FIELD name="RA" ucd="pos.eq.ra" ref="Ephem"
             ID="RA1" utype="stc:AstroCoords.Position3D.Value3.C1"
             datatype="double" width="12" precision="8" unit="deg">
        <DESCRIPTION>alpha, degrees</DESCRIPTION>
      </FIELD>
      <FIELD name="DE" ucd="pos.eq.dec" ref="Ephem"
             ID="DE1" utype="stc:AstroCoords.Position3D.Value3.C2"
             datatype="double" width="12" precision="8" unit="deg">
        <DESCRIPTION>delta, degrees</DESCRIPTION>
      </FIELD>
      <FIELD name="Dist" ucd="pos.distance" ref="Ephem"
             ID="Dist1" utype="stc:AstroCoords.Position3D.Value3.C3"
             datatype="double" width="12" precision="9" unit="AU">
        <DESCRIPTION>Distance from geocenter </DESCRIPTION>
      </FIELD>
      <FIELD name="Vmag" ucd="phot.mag;em.opt.V" datatype="float"
             width="5" precision="2" unit="mag">
        <DESCRIPTION>Visual magnitude</DESCRIPTION>
      </FIELD>
      <DATA>
        <TABLEDATA>
          <TR>
            <TD>2454284.0</TD><TD>180.1745177</TD><TD>+10.2710289</TD>
            <TD>1.468825725</TD><TD>17.06</TD>
          </TR>
          <TR>
            <TD>2454286.0</TD><TD>181.2970157</TD><TD>+09.8077289</TD>
            <TD>1.476798829</TD><TD>17.07</TD>
          </TR>
          <TR>
            <TD>2454288.0</TD><TD>182.4335757</TD><TD>+09.3344139</TD>
            <TD>1.484770578 </TD><TD>17.09</TD>
          </TR>
          <TR>
            <TD>2454290.0</TD><TD>183.5839061</TD><TD>+08.8513908</TD>
            <TD>1.492750033</TD><TD>17.11</TD>
          </TR>
          <TR>
            <TD>2454292.0</TD><TD>184.7477408</TD><TD>+08.3589893</TD>
            <TD>1.500745242</TD><TD>17.13</TD>
          </TR>
        </TABLEDATA>
      </DATA>
    </TABLE>
  </RESOURCE>
</VOTABLE>

6  Summary of Interpretation and Defaults

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:

<FIELD name="ObsStart" utype="stc:AstroCoords.Time.TimeInstant.ISOTime" ref="Coo1" .../>

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 with an explicit reference to a group having the attribute utype="stc:AstroCoordSystem" (or utype="stc:CoordSys") included in the same document (ID/ref mechanism); or with a reference to the STClib component designated by the content of the value attribute of a PARAM element having the attributeutype="stc:AstroCoordSystem.coord_system_id".

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

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
TDB Barycentric Dynamical Time synchronous with TT
LST Local Sideral Time (needs ObservationLocation)
LOCAL Local definition  

Most important ReferencePosition values
TOPOCENTER local position (generally on the Earth surface) 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 coordinate flavors
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">

  <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 summarizes the most frequently frames used in astrometry.

Most important astronomical frame values
ICRS International Reference System Compatible with FK5(J2000)
FK5 Equatorial frame based on FK5 Requires the Equinox (default J2000)
FK4 Equatorial frame based on FK4 Requires the Equinox (default B1950)
ECLIPTIC Ecliptic frame Requires the Equinox ((default J2000)
GALACTIC Galactic frame Also named GALACTIC_II
SUPER_GALACTIC Local Group  
J2000 Equatorial on FK5, Equinox=J2000Coincides with ICRS
B1950 Equatorial on FK4, Equinox=B1950 

A few STC-library values
TT-ICRS-TOPO Terrestrial time reported at local place, ICRS orientation of axes with origin at local place
TT-FK5-TOPO Coincides with TT-ICRS-TOPO at Equinox J2000.0
UTC-ICRS-TOPO Universal Coordinated Time reported at local place, ICRS orientation of axes with origin at local place
UTC-FK5-TOPO Coincides with UTC-ICRS-TOPO at Equinox J2000.0
TT-ICRS-GEO Terrestrial time reported at center of Earth, ICRS orientation of axes with origin at center of Earth
TT-FK5-GEO Coincides with UTC-ICRS-TOPO at Equinox J2000.0
UTC-ICRS-GEO Universal Coordinated Time reported at center of Earth, ICRS orientation of axes with origin at center of Earth
UTC-FK5-GEO Coincides with TT-FK5-GEO at Equinox J2000.0
TDB-ICRS-BARY Barycentric Dynamical Time reported at Solar System Barycenter, ICRS orientation of axes with origin at Solar System Barycenter
TDB-FK5-BARY Coincides with TDB-ICRS-BARY at Equinox J2000.0
TDB-ECLIPTIC-BARY Barycentric Dynamical Time reported at Solar System Barycenter, along the Ecliptic axes (the equinox has to be specified)

7  History and Modifications

8  References

[1] François Ochsenbein, et al. VOTable Recommendation
http://ivoa.net/Documents/latest/VOTable.html

[2] Arnold Rots Space-Time Coordinate Metadata for the Virtual Observatory (v1.30)
http://ivoa.net/Documents/latest/STC.html

[3] The Hipparcos and Tycho Catalogues, ESA SP1200 June 1997, Vol.1, section 1.2 (also available in PDF)