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Space-Time Coordinate (STC) Metadata Model

 

Version 1.30

IVOA Note  30 October 2007

 

 

 

This version:

http://www.ivoa.net/Documents/Notes/STC-Model/STC-Model-20071030.html

Latest version:

http://www.ivoa.net/Documents/latest/STC-Model.html

Previous version(s):

None

 

Author(s):

            A. H. Rots

           

 

Abstract

This document provides a summary of the model for the description of the VO Space-Time Coordinate metadata.  It shows the hierarchical relation between the various components with brief descriptions where needed. The Tables that are referred to may be found in the STC document.  XML element names are shown in bold Courier font.

The objective is to provide a framework that allows a complete and internally consistent specification of coordinate metadata – primarily the intertwined temporal, spatial, spectral, and redshift coordinates – that is extensible to accommodate all future applications.

Status of This Document

This is a Note. The first release of this document was 2007-10-30.

It serves as an explanatory supplement to the Space-Time Coordinate Metadata Recommendation (version 1.3) that may be found at http://www.ivoa.net/Documents/latest/STC.html.

This is an IVOA Note expressing suggestions from and opinions of the authors. It is intended to share best practices, possible approaches, or other perspectives on interoperability with the Virtual Observatory. It should not be referenced or otherwise interpreted as a standard specification.

A list of current IVOA Recommendations and other technical documents can be found at http://www.ivoa.net/Documents/.

 

 

 

 

Table of Contents

1                  STC Metadata Description  6

1.1     Coordinate System (CoordSys, AstroCoordSystem, PixelCoordSystem) 6

1.1.1       Generic Coordinate Frame (CoordFrame) 7

1.1.1.1        Reference Position (CoordRefPos) 7

1.1.1.2        Generic Coordinate Reference Frame (CoordRefFrame) 7

1.1.1.2.1    ScalarRefFrame  7

1.1.1.2.2    Cart2DRefFrame  7

1.1.1.2.3    Cart3DRefFrame  7

1.1.1.2.4    SphericalRefFrame  7

1.1.1.3        Coordinate Flavor (CoordFlavor) 7

1.1.2       Time Frame (TimeFrame) 8

1.1.2.1        Reference Position (ReferencePosition) 8

1.1.2.1.1    Standard Reference Position  8

1.1.2.1.1.1    Planetary Ephemeris (PlanetaryEphem; conditional) 8

1.1.2.1.2    Custom Reference Position (CoordRefPos) 8

1.1.2.2        Time Scale  8

1.1.2.3        Time Reference Direction (TimeRefDirection; conditional) 8

1.1.3       Space Frame (SpaceFrame) 9

1.1.3.1        Reference Position (ReferencePosition) 9

1.1.3.1.1    Standard Reference Position  9

1.1.3.1.1.1    Planetary Ephemeris (PlanetaryEphem; conditional) 9

1.1.3.1.2    Custom Reference Position (CoordRefPos) 9

1.1.3.2        Space Reference Frame  9

1.1.3.2.1    Standard Reference Frame  9

1.1.3.2.1.1    Equinox (conditional) 9

1.1.3.2.2    (Custom) Coordinate Reference Frame (CoordRefFrame) 9

1.1.3.2.2.1    ScalarRefFrame  9

1.1.3.2.2.2    Cart2DRefFrame  10

1.1.3.2.2.3    Cart3DRefFrame  10

1.1.3.2.2.4    SphericalRefFrame  10

1.1.3.3        Coordinate Flavor (CoordFlavor) 10

1.1.3.4        Offset Center (OffsetCenter; optional) 10

1.1.4       Spectral Frame (SpectralFrame) 11

1.1.4.1        Reference Position  11

1.1.4.1.1    Standard Reference Position  11

1.1.4.1.1.1    Planetary Ephemeris (PlanetaryEphem; conditional) 11

1.1.4.1.2    Custom Reference Position (CoordRefPos) 11

1.1.5       Redshift Frame (RedshiftFrame) 12

1.1.5.1        Reference Position  12

1.1.5.1.1    Standard Reference Position  12

1.1.5.1.1.1    Planetary Ephemeris (PlanetaryEphem; conditional) 12

1.1.5.1.2    Custom Reference Position (CoordRefPos) 12

1.1.5.2        Doppler Definition (DopplerDefinition) 12

1.1.6       Pixel Frame (PixelFrame) 13

1.1.6.1        Reference Position (CoordRefPos) 13

1.1.6.2        Coordinate Reference Frame (CoordRefFrame) 13

1.1.6.2.1    ScalarRefFrame  13

1.1.6.2.2    Cart2DRefFrame  13

1.1.6.2.3    Cart3DRefFrame  13

1.1.6.2.4    SphericalRefFrame  13

1.1.6.3        Coordinate Flavor (CoordFlavor) 13

1.1.6.4        Reference Pixel (ReferencePixel) 13

1.2     Coordinates (Coords, AstroCoords, PixelCoords) 14

1.2.1       Generic Coordinate (GenCoordinate) 15

1.2.1.1        ScalarCoordinate  15

1.2.1.2        Vector2DCoordinate  15

1.2.1.3        Vector3DCoordinate  15

1.2.1.4        StringCoordinate  15

1.2.2       Time (Time) 16

1.2.2.1        Absolute Time (AbsoluteTime) 16

1.2.2.2        Time Offset (TimeOffset; optional) 16

1.2.3       Spatial Position (Position) 17

1.2.3.1        Position1D  17

1.2.3.2        Position2D  17

1.2.3.3        Position3D  17

1.2.4       Spatial Velocity (Velocity) 17

1.2.4.1        Velocity1D  17

1.2.4.2        Velocity2D; 17

1.2.4.3        Velocity3D; 17

1.2.5       Orbital Elements (Orbit; alternative) 18

1.2.5.1        Semi-major axis (a; conditional) 18

1.2.5.2        Periapsis Distance (q; conditional) 18

1.2.5.3        Eccentricity (e) 18

1.2.5.4        Inclination (i) 18

1.2.5.5        Longitude of Ascending Node (Node) 18

1.2.5.6        Argument of Periapsis (Aop) 18

1.2.5.7        Mean Anomaly (M; optional) 18

1.2.5.8        Orbital Period (P; optional) 18

1.2.5.9        Epoch (T)  18

1.2.6       Spectral Coordinate (Spectral) 19

1.2.7       Redshift Coordinate (Redshift) 19

1.2.8       Coordinate File (CoordFile) 20

1.2.8.1        FITSFile  20

1.2.8.2        FITSTime (optional) 20

1.2.8.3        FITSPosition (optional) 20

1.2.8.4        FITSVelocity (optional) 20

1.2.8.5        FITSSpectral (optional) 20

1.2.8.6        FITSRedshift (optional) 20

1.2.9       Pixel Coordinate (Pixel) 21

1.2.9.1        Pixel1D  21

1.2.9.2        Pixel2D  21

1.2.9.3        Pixel3D  21

1.3     Coordinate Area (CoordArea, AstroCoordArea, PixelCoordArea) 22

1.3.1       Generic Scalar Range (CoordInterval) 23

1.3.1.1        CoordScalarInterval  23

1.3.1.2        Coord2VecInterval  23

1.3.1.3        Coord3VecInterval  23

1.3.2       Time Interval (TimeInterval) 23

1.3.3       Spatial Area (PositionInterval) 24

1.3.3.1        1-Dimensional Intervals (PositionScalarInterval) 24

1.3.3.2        2-Dimensional Rectangles (Coord2VecInterval) 24

1.3.3.3        2-Dimensional Region  24

1.3.3.3.1    AllSky  24

1.3.3.3.2    Circle  24

1.3.3.3.3    Ellipse  24

1.3.3.3.4    Polygon  24

1.3.3.3.5    Box  24

1.3.3.3.6    Sector  24

1.3.3.3.7    Convex  24

1.3.3.3.8    ConvexHull  25

1.3.3.3.9    SkyIndex  25

1.3.3.3.10    Union  25

1.3.3.3.11    Intersection  25

1.3.3.3.12    Negation  25

1.3.3.3.13    Difference  25

1.3.3.4        3-Dimensional Intervals (Coord3VecInterval) 25

1.3.3.5        3-Dimensional Sphere (Sphere) 25

1.3.4       Spatial Velocity  26

1.3.4.1        1-Dimensional Intervals (VelicityScalarInterval) 26

1.3.4.2        2-Dimensional Rectangles (Velocity2VecInterval) 26

1.3.4.3        3-Dimensional Intervals (Velocity3VecInterval) 26

1.3.4.4        3-Dimensional Sphere (VelocitySphere) 26

1.3.5       Spectral Interval (SpectralInterval) 27

1.3.6       Redshift Interval (RedshiftInterval) 27

1.3.7       Pixel Interval (PixelCoordInterval) 27

1.3.7.1        PixelCoordScalarInterval  27

1.3.7.2        PixelCoord2VecInterval  27

1.3.7.3        PixelCoord3VecInterval  27

2                  Units  28

2.1     Time Units  28

2.2     Spectral Coordinate Units  28

2.2.1       Frequency  28

2.2.2       Wavelength  28

2.2.3       Energy  28

2.3     Spatial Coordinate Units  28

2.3.1       Angular Units  28

2.3.2       Linear Units  29

2.3.3       Three-units Strings  29

2.3.4       Position Angles  29

3                  Usage  30

3.1     Referencing  30

3.2     Use Context 30

3.2.1       Resource Profile (STCResourceProfile) 30

3.2.2       Search (SearchLocation) 30

3.2.3       Catalog Data (CatalogEntryLocation) 30

3.2.4       Observational Data (ObsDataLocation) 30

 

1         STC Metadata Description

An STC metadata description element consists of three components:

  1. Coordinate System
  2. Coordinates

3.      Coordinate area

Missing components or subcomponents shall be considered UNKNOWN; this is to be interpreted as: it is up to the client to decide whether or not to accept the metadata, and to assign a sensible default value if the component is relevant.

 

1.1      Coordinate System (CoordSys, AstroCoordSystem, PixelCoordSystem)

A Coordinate System consists of one or more Coordinate Frames.

A Coordinate Frame typically consists of a Reference Frame (orientation) and a Reference Position (or Origin), but some Frames contain more, or less, information.  There are six kinds of Frames.

The three kinds of Coordinate System are:

  1. (Generic) CoordSys; contains:
    1. One or more Generic Coordinate Frames
  2. AstroCoordSystem; may contain:
    1. One or more Generic Coordinate Frames
    2. One Time Frame
    3. One Space Frame
    4. One Spectral Frame
    5. One Redshift Frame
  3. PixelCoordSystem; may contain:
    1. One or more Generic Coordinate Frames
    2. One or more Pixel Coordinate Frames

1.1.1      Generic Coordinate Frame (CoordFrame)

Allows specifying metadata for non-STC coordinates.

A CoordFrame contains a Coordinate Flavor and may contain a Coordinate Reference Frame and/or a Coordinate Reference Position; it requires a frame_id.

 

1.1.1.1     Reference Position (CoordRefPos)

A CoordRefPos consists of a Coordinate.

 

1.1.1.2     Generic Coordinate Reference Frame (CoordRefFrame)

There are four types of Coordinate Reference Frame:

(All Transformation elements contain a projection attribute, using values from Table 4)

1.1.1.2.1   ScalarRefFrame

Contains:

  1. Scale
1.1.1.2.2   Cart2DRefFrame

Contains one of:

  1. Transform2
  2. Transform2Matrix
1.1.1.2.3   Cart3DRefFrame

Contains one of:

  1. Transform3
  2. Transform3Matrix
1.1.1.2.4   SphericalRefFrame

Contains:

  1. Frame
  2. Pole_Zaxis
  3. Xaxis

 

1.1.1.3     Coordinate Flavor (CoordFlavor)

CARTESIAN, SPHERICAL, UNITSPHERE, POLAR, CYLINDRICAL, HEALPIX, or STRING; contains: dimensionality.

1.1.2      Time Frame (TimeFrame)

A TimeFrame contains a Reference Position and a Time Scale, and may contain a Time Reference Direction.

 

1.1.2.1     Reference Position (ReferencePosition)

A Reference Position may be a Standard or a Custom one:

1.1.2.1.1   Standard Reference Position

TOPOCENTER, GEOCENTER, BARYCENTER, etc., as in Table 1, noting the exceptions.

All Solar System positions, except GEOCENTER, should include a Planetary Ephemeris.

1.1.2.1.1.1     Planetary Ephemeris (PlanetaryEphem; conditional)

JPL-DE200 or JPL-DE405; needed when such am ephemeris is used to transform times.

1.1.2.1.2   Custom Reference Position (CoordRefPos)

A Coordinate Reference Position consists of a Coordinate.

 

1.1.2.2     Time Scale

TT (TDT), ET, TAI (IAT), UTC, GPS, TDB (TEB), TCG, TCB, LST, LOCAL (Table 2).

 

1.1.2.3     Time Reference Direction (TimeRefDirection; conditional)

If Time Reference Position is not TOPOCENTER, this direction (a Coordinate element) needs to be provided.

1.1.3      Space Frame (SpaceFrame)

A SpaceFrame includes a Reference Position, Spatial Reference Frame, and Coordinate Flavor, and may include an Offset Center.

 

1.1.3.1     Reference Position (ReferencePosition)

A Reference Position may be a Standard or a Custom one:

1.1.3.1.1   Standard Reference Position

TOPOCENTER, GEOCENTER, BARYCENTER, etc., as in Table 1, noting the exceptions (but including RELOCATABLE).

All Solar System positions, except GEOCENTER, should include a Planetary Ephemeris.

1.1.3.1.1.1     Planetary Ephemeris (PlanetaryEphem; conditional)

JPL-DE200 or JPL-DE405; needed when such an ephemeris is used to transform times.

1.1.3.1.2   Custom Reference Position (CoordRefPos)

A Coordinate Reference Position consists of a Coordinate.

 

1.1.3.2     Space Reference Frame

A Space Reference Frame may be a Standard or a Custom one:

1.1.3.2.1   Standard Reference Frame

FK4, FK5, ICRS, ECLIPTIC, GALACTIC, etc., as in Table 3, including UNKNOWNFrame.

1.1.3.2.1.1     Equinox (conditional)

Bnnnn or Jnnnn, for FKi only

1.1.3.2.2    (Custom) Coordinate Reference Frame (CoordRefFrame)

There are four types of Coordinate Reference Frame:

(All Transformation elements contain a projection attribute, using values from Table 4)

1.1.3.2.2.1     ScalarRefFrame

Contains:

  1. Scale
1.1.3.2.2.2     Cart2DRefFrame

Contains one of:

  1. Transform2
  2. Transform2Matrix
1.1.3.2.2.3     Cart3DRefFrame

Contains one of:

  1. Transform3
  2. Transform3Matrix
1.1.3.2.2.4     SphericalRefFrame

Contains:

  1. Frame
  2. Pole_Zaxis
  3. Xaxis

 

1.1.3.3     Coordinate Flavor (CoordFlavor)

CARTESIAN, SPHERICAL, UNITSPHERE, POLAR, CYLINDRICAL, or HEALPIX; contains: dimensionality.

 

1.1.3.4     Offset Center (OffsetCenter; optional)

For offset coordinates; consists of a Coordinate.

1.1.4      Spectral Frame (SpectralFrame)

A SpectralFrame only contains a Reference Position

 

1.1.4.1     Reference Position

Note that the Spectral Reference Position requires position as well as a velocity vector. A Reference Position may be a Standard or a Custom one:

1.1.4.1.1   Standard Reference Position

TOPOCENTER, GEOCENTER, BARYCENTER, etc., as in Table 1, noting the exceptions (but including LSRx).

All Solar System positions, except GEOCENTER, should include a Planetary Ephemeris.

1.1.4.1.1.1     Planetary Ephemeris (PlanetaryEphem; conditional)

JPL-DE200 or JPL-DE405; needed when such am ephemeris is used to transform times.

1.1.4.1.2   Custom Reference Position (CoordRefPos)

A Coordinate Reference Position consists of a Coordinate.

1.1.5      Redshift Frame (RedshiftFrame)

A RedshiftFrame contains a Reference Position and a Doppler Definition.

It also contains an attribute that indicates whether the coordinate values represent redshifts or Doppler velocities.

 

1.1.5.1     Reference Position

Note that the Redshift Reference Position also requires position as well as a velocity vector. A Reference Position may be a Standard or a Custom one:

1.1.5.1.1   Standard Reference Position

TOPOCENTER, GEOCENTER, BARYCENTER, etc., as in Table 1, noting the exceptions (but including LSRx).

All Solar System positions, except GEOCENTER, should include a Planetary Ephemeris.

1.1.5.1.1.1     Planetary Ephemeris (PlanetaryEphem; conditional)

JPL-DE200 or JPL-DE405; needed when such am ephemeris is used to transform times.

1.1.5.1.2   Custom Reference Position (CoordRefPos)

A Coordinate Reference Position consists of a Coordinate.

 

1.1.5.2     Doppler Definition (DopplerDefinition)

Allowed values: OPTICAL, RADIO, RELATIVISTIC.

1.1.6      Pixel Frame (PixelFrame)

A PixelFrame contains a Coordinate Flavor and may contain a Coordinate Reference Frame, a Coordinate Reference Position, and/or a Reference Pixel. It contains one or more axis_order attributes.

 

1.1.6.1     Reference Position (CoordRefPos)

A CoordRefPos consists of a Coordinate.

 

1.1.6.2     Coordinate Reference Frame (CoordRefFrame)

There are four types of Coordinate Reference Frame:

(All Transformation elements contain a projection attribute, using values from Table 4)

1.1.6.2.1   ScalarRefFrame

Contains:

  1. Scale
1.1.6.2.2   Cart2DRefFrame

Contains one of:

  1. Transform2
  2. Transform2Matrix
1.1.6.2.3   Cart3DRefFrame

Contains one of:

  1. Transform3
  2. Transform3Matrix
1.1.6.2.4   SphericalRefFrame

Contains:

  1. Frame
  2. Pole_Zaxis
  3. Xaxis

 

1.1.6.3     Coordinate Flavor (CoordFlavor)

CARTESIAN, SPHERICAL, UNITSPHERE, POLAR, CYLINDRICAL, HEALPIX, or STRING; contains: dimensionality.

 

1.1.6.4     Reference Pixel (ReferencePixel)

ReferencePixel contains a 1, 2, or 3-dimensional Pixel.

1.2      Coordinates (Coords, AstroCoords, PixelCoords)

A Coordinates element consists of up to six Coordinate elements and contains a reference to a Coordinate System.  Each Coordinate has five components.  Each component may occur once (a definite or typical value) or twice (indicating a range of values):

  1. Value
  2. Error
  3. Resolution
  4. Size
  5. Pixel size

All components are optional; in addition, there may be a name.  One or more Coordinate elements may be provided through a binary FITS table.

A variety of error types may be defined; statements below on the data types of errors are to be considered preliminary, contingent on additional definitions.

There are nine types of Coordinate elements, each of which is a composite of the above-mentioned components, except where noted.

Each Coordinates object needs to refer to a Coordinate System through a coord_system_id attribute.

There are three types of Coordinates elements:

  1. (Generic) Coords; contains one or more Generic Coordinates and refers to a CoordSys; individual Coordinates need to refer to a specific Coordinate Frame through a frame_id
  2. AstroCoords; refers to an AstroCoordSystem and may contain:
    1. One or more Generic Coordinates; individual Coordinates need to refer to a specific Coordinate Frame through a frame_id
    2. One Time Coordinate
    3. One Spatial Position Coordinate
    4. One Spatial Velocity Coordinate
    5. One Orbit Coordinate
    6. One Spectral Coordinate
    7. One Redshift Coordinate
    8. One Coordinate File
  3. PixelCoords; refers to an PixelCoordSystem and may contain one or more:
    1. Generic Coordinate
    2. Pixel Coordinate

Individual Coordinates need to refer to a specific Coordinate Frame through a frame_id.

1.2.1      Generic Coordinate (GenCoordinate)

The Coordinate needs to refer to a specific Coordinate Frame through a frame_id. There are four types of GenCoordinate:

 

1.2.1.1     ScalarCoordinate

Optional units; all components are scalar doubles.

 

1.2.1.2     Vector2DCoordinate

Optional units; Error, Resolution, Size, PixSize may consist of:

  1. Two doubles
  2. 2x2 matrix
  3. Radius

 

1.2.1.3     Vector3DCoordinate

Optional units; Error, Resolution, Size, PixSize may consist of:

  1. Three doubles
  2. 3x3 matrix
  3. Radius

 

1.2.1.4     StringCoordinate

Only contains a (string) Value, no other components.

1.2.2      Time (Time)

The value (TimeInstant) is an instant of the AstronTime class; all other components (Error, Resolution, Size, PixSize) are doubles and need a unit (s, d, a or yr, cy).  AstronTime contains an Absolute Time element and an optional relative time element.  It also may have a TimeScale attribute.

AstronTime contains the following:

 

1.2.2.1     Absolute Time (AbsoluteTime)

Absolute Time may be of type ISOTime (ISO-8601 format), JDTime, or MJDTime. This should be a decimal for JD and MJD since a double is not guaranteed to have sufficient accuracy. In addition, for use with simulations, it may be of the type TimeOrigin which currently may only have the value RELOCATABLE.

 

1.2.2.2     Time Offset (TimeOffset; optional)

Decimal; offset with respect to Absolute Time; unit required.

1.2.3      Spatial Position (Position)

Units are required for all components, angular or linear.

There are three types of Position:

 

1.2.3.1     Position1D

All components are scalar doubles.

 

1.2.3.2     Position2D

CError2, CResolution2, CSize2, CPixSize2 may consist of:

  1. Two doubles
  2. 2x2 matrix
  3. Radius

 

1.2.3.3     Position3D

CError3, CResolution3, CSize3, CPixSize3 may consist of:

  1. Three doubles
  2. 3x3 matrix
  3. Radius

 

1.2.4      Spatial Velocity (Velocity)

Same model as Spatial Position; needs a spatial as well as a time unit.

Note that this item is for true space velocities, not for derived Doppler velocities.

There are three types of Velocity:

 

1.2.4.1     Velocity1D

All components are scalar doubles.

 

1.2.4.2     Velocity2D;

CError2, CResolution2, CSize2, CPixSize2 may consist of:

1.      Two doubles

2.      2x2 matrix

3.      Radius

 

1.2.4.3     Velocity3D;

CError3, CResolution3, CSize3, CPixSize3 may consist of:

  1. Three doubles
  2. 3x3 matrix
  3. Radius

1.2.5      Orbital Elements (Orbit; alternative)

Spatial Position and Velocity (values only, of course) may also be provided through orbital elements:

 

1.2.5.1     Semi-major axis (a; conditional)

Semi-major axis to be used for elliptical orbits (0 ≤ e < 1); for parabolic and hyperbolic orbits use periapsis distance q.

 

1.2.5.2     Periapsis Distance (q; conditional)

Required for open orbits (1 ≤ e); may be used instead of a for closed orbits.

 

1.2.5.3     Eccentricity (e)

 

 

1.2.5.4     Inclination (i)

 

 

1.2.5.5     Longitude of Ascending Node (Node)

 

 

1.2.5.6     Argument of Periapsis (Aop)

 

 

1.2.5.7     Mean Anomaly (M; optional)

Mean anomaly at time T; if absent, T will refer to pericenter.

 

1.2.5.8     Orbital Period (P; optional)

Redundant, but optional for closed orbits.

 

1.2.5.9     Epoch (T)

Epoch of mean anomaly, if M is present, or of periapsis if M is absent.

1.2.6      Spectral Coordinate (Spectral)

All components are scalar doubles; spectral unit required.

 

 

1.2.7      Redshift Coordinate (Redshift)

All components are scalar doubles; position and time unit required for Doppler velocities.

1.2.8      Coordinate File (CoordFile)

Astronomical coordinate components may be specified in FITS files; this element provides an unambiguous description and reference. It contains the following elements:

 

1.2.8.1     FITSFile

Provides a URI and, optionally, one or two attributes: hdu_num and hdu_name. The HDU that is referred to should be a FITS Binary (or ASCII) Table.

 

1.2.8.2     FITSTime (optional)

Contains the FITS TTYPE (name) of the column containing the data for each of the Time coordinate components (value, error, resolution, size, pixel size) present.

 

1.2.8.3     FITSPosition (optional)

Contains the FITS TTYPE (name) of the column containing the data for each of the Time coordinate components (value, error, resolution, size, pixel size) present. For vector coordinates FITSPosition contains the TTYPEs for the appropriate number of columns, separated by commas.

 

1.2.8.4     FITSVelocity (optional)

Contains the FITS TTYPE (name) of the column containing the data for each of the Time coordinate components (value, error, resolution, size, pixel size) present. For vector coordinates FITSVelocity contains the TTYPEs for the appropriate number of columns, separated by commas.

 

1.2.8.5     FITSSpectral (optional)

Contains the FITS TTYPE (name) of the column containing the data for each of the Time coordinate components (value, error, resolution, size, pixel size) present.

 

1.2.8.6     FITSRedshift (optional)

Contains the FITS TTYPE (name) of the column containing the data for each of the Time coordinate components (value, error, resolution, size, pixel size) present.

1.2.9      Pixel Coordinate (Pixel)

The  Pixel needs to refer to a specific Coordinate Frame through a frame_id.  The elements only contain a Value component and, optionally, a Name.

There are three types of Pixels:

 

1.2.9.1     Pixel1D

One double.

 

1.2.9.2     Pixel2D

Two doubles.

 

1.2.9.3     Pixel3D

Three doubles.

 

1.3      Coordinate Area (CoordArea, AstroCoordArea, PixelCoordArea)

Speci