1. 2005-05-16IVOA Interop Kyoto - VOTable1 Space-Time Coordinate Metadata for VOTable Arnold Rots Harvard-Smithsonian CfA / CXC T HE US N ATIONAL V IRTUAL O BSERVATORY

2. 2005-05-16IVOA Interop Kyoto - VOTable2 Overview Justification: what is driving this? The simple requirements Design: how do we solve this? Implementation Example

3. 2005-05-16IVOA Interop Kyoto - VOTable3 Scope The following coordinate axes are closely intertwined: –Space –Time –Spectral (frequency, wavelength, energy) –Redshift (Doppler velocity) Time is bound to a position and positions are time-variable Spectral and redshift data are tied to reference frames that may or may not be time-variable

4. 2005-05-16IVOA Interop Kyoto - VOTable4 Scope (2) Context-dependent defaults are fine –Issues are well-defined and clear for single- observatory observations – even when not all is explicitly specified But there are no global defaults –In the VO all implicit assumptions need to be made explicit since they will not be obvious anymore One must be able to transform the coordinates of two observations to a common coordinate system –Including far-field/near-field transformations

5. 2005-05-16IVOA Interop Kyoto - VOTable5 Requirement The requirement for Space-Time Coordinate metadata is that they: –provide sufficient and necessary information –are self-consistent We need to assure that coordinate transformations can be performed

6. 2005-05-16IVOA Interop Kyoto - VOTable6 The Metadata Components Coordinate system –Consists of one or more frames Frames typically consist of a reference position (origin) and a reference frame Coordinate values –Refers to a coordinate system Coordinate areas or ranges –To define a volume in coordinate space –Special case: Regions Specifically for spatial coordinates

7. 2005-05-16IVOA Interop Kyoto - VOTable7 Coordinate System Time frame –Reference position & time scale Spatial frame –Reference position and coordinate frame Spectral frame –Reference position in phase space Redshift frame –Definition and reference position

8. 2005-05-16IVOA Interop Kyoto - VOTable8 Reference Frames and Positions Examples of spatial Reference Frames: –FK4, FK5, ICRS, Ecliptic, Galactic, Geocentric, Geodetic, various solar and planetary frames, unknown, custom, … Time scales: –TT, TAI, UTC, TDB, TEB, TCG, TCB, … Reference Positions: –Topocenter, Geocenter, Heliocenter, Barycenter, Galactic center, LSR-K, LSR-D, planetary centers, unknown, custom, …

9. 2005-05-16IVOA Interop Kyoto - VOTable9 Coordinates A coordinate object contains a reference to a coordinate system It is a composite object that may hold –Name- Resolution –Value- Size –Error- Pixel size All quantities are scalar except for spatial (>1D) –In that case errors and resolutions get more complicated All coordinates include their units Spatial may include position and velocity (PM) Time has options (absolute or elapsed)

10. 2005-05-16IVOA Interop Kyoto - VOTable10 Coordinate Area Defines the volume in coordinate space that is represented Consists of one or more ranges in individual coordinates Spatial position has more options –Sphere –2-D Regions Shapes: polygon, sector, ellipse, convex, convex hull Operations: intersection, union, negation

11. 2005-05-16IVOA Interop Kyoto - VOTable11 Implementations STC-X: XML schemata current version: 1.20 STC-S: String version used in Resource Metadata

12. 2005-05-16IVOA Interop Kyoto - VOTable12 CatalogEntry A simple catalog of positions:

13. 2005-05-16IVOA Interop Kyoto - VOTable13 AstroCoordSystem One could optionally add a SpectralFrame TT

14. 2005-05-16IVOA Interop Kyoto - VOTable14 AstroCoordSystem (2) The coordinate system may also be specified through an XInclude: http://www.ivoa.net/xml/STC/ICRS-TT-TOPO.xml

15. 2005-05-16IVOA Interop Kyoto - VOTable15 AstroCoords The position vector points to an element with ID=Column3 – presumably a Field in the document that describes column 3 of the table; in addition, an error is given Column3 0.0002

16. 2005-05-16IVOA Interop Kyoto - VOTable16 AstroCoordArea This element is not required, but it is a mechanism to provide information on the coverage of the catalog 2000-01-01T00:00:00 2005-01-01T00:00:00

17. 2005-05-16IVOA Interop Kyoto - VOTable17 AstroCoordsArea (2) Time interval is only for illustrative purposes; only position may be useful; one could optionally add a spectral interval 0 0 90 30

18. 2005-05-16IVOA Interop Kyoto - VOTable18 XInclude The physical XML documents can be simplified through use of XInclude files that allow inclusion of frequently used elements, referenced through standardized IDREFs –Constructors can just insert the single lines –Parsers that do not care about coordinate systems or observatory positions can just ignore them and optionally rely on the IDREFs, while the document remains rigorously correct

19. 2005-05-16IVOA Interop Kyoto - VOTable19 Extensibility This was a simple example; the system can be extended to handle more sophisticated catalogs: –Multiple coordinate systems (e.g., l,b) –More coordinate axes: Proper motions Redshifts, radial velocities Spectral information Temporal information –Additional information: errors, sizes, etc. –Merged catalogs