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System - Coordinate system used to describe positions within the domain

Description:
In general it is possible for positions within a given physical domain to be described using one of several different coordinate systems. For instance, the SkyFrame class can use galactic coordinates, equatorial coordinates, etc, to describe positions on the sky. As another example, the SpecFrame class can use frequency, wavelength, velocity, etc, to describe a position within an electromagnetic spectrum. The System attribute identifies the particular coordinate system represented by a Frame. Each class of Frame defines a set of acceptable values for this attribute, as listed below (all are case insensitive). Where more than one alternative System value is shown, the first of will be returned when an enquiry is made.

Type:
String.

Class Applicability:
Frame
The System attribute for a basic Frame always equals "Cartesian", and may not be altered.
CmpFrame
The System attribute for a CmpFrame always equals "Compound", and may not be altered. In addition, the CmpFrame class allows the System attribute to be referenced for a component Frame by including the index of an axis within the required component Frame. For instance, "System(3)" refers to the System attribute of the component Frame which includes axis 3 of the CmpFrame.
FrameSet
The System attribute of a FrameSet is the same as that of its current Frame (as specified by the Current attribute).
SkyFrame
The SkyFrame class supports the following System values and associated celestial coordinate systems:

  • "FK4": The old FK4 (barycentric) equatorial coordinate system, which should be qualified by an Equinox value. The underlying model on which this is based is non-inertial and rotates slowly with time, so for accurate work FK4 coordinate systems should also be qualified by an Epoch value.
  • "FK4-NO-E" or "FK4_NO_E": The old FK4 (barycentric) equatorial system but without the "E-terms of aberration" (e.g. some radio catalogues). This coordinate system should also be qualified by both an Equinox and an Epoch value.
  • "FK5" or "EQUATORIAL": The modern FK5 (barycentric) equatorial coordinate system. This should be qualified by an Equinox value.
  • "GAPPT", "GEOCENTRIC" or "APPARENT": The geocentric apparent equatorial coordinate system, which gives the apparent positions of sources relative to the true plane of the Earth's equator and the equinox (the coordinate origin) at a time specified by the qualifying Epoch value. (Note that no Equinox is needed to qualify this coordinate system because no model "mean equinox" is involved.) These coordinates give the apparent right ascension and declination of a source for a specified date of observation, and therefore form an approximate basis for pointing a telescope. Note, however, that they are applicable to a fictitious observer at the Earth's centre, and therefore ignore such effects as atmospheric refraction and the (normally much smaller) aberration of light due to the rotational velocity of the Earth's surface. Geocentric apparent coordinates are derived from the standard FK5 (J2000.0) barycentric coordinates by taking account of the gravitational deflection of light by the Sun (usually small), the aberration of light caused by the motion of the Earth's centre with respect to the barycentre (larger), and the precession and nutation of the Earth's spin axis (normally larger still).
  • "ECLIPTIC": Ecliptic coordinates (IAU 1980), referred to the ecliptic and mean equinox specified by the qualifying Equinox value.
  • "GALACTIC": Galactic coordinates (IAU 1958).
  • "SUPERGALACTIC": De Vaucouleurs Supergalactic coordinates.
  • "UNKNOWN": Any other general spherical coordinate system. No Mapping can be created between a pair of SkyFrames if either of the SkyFrames has System set to "Unknown".

Currently, the default System value is "FK5". However, this default may change in future as new astrometric standards evolve. The intention is to track the most modern appropriate standard. For this reason, you should use the default only if this is what you intend (and can tolerate any associated slight change in future). If you intend to use the FK5 system indefinitely, then you should specify it explicitly.

SpecFrame
The SpecFrame class supports the following System values and associated spectral coordinate systems (the default is "WAVE" - wavelength):

  • "FREQ": Frequency (GHz)
  • "ENER" or "ENERGY": Energy (J)
  • "WAVN" or "WAVENUM": Wave-number (1/m)
  • "WAVE" or "WAVELEN": Vacuum wave-length (Angstrom)
  • "AWAV" or "AIRWAVE": Wave-length in air (Angstrom)
  • "VRAD" or "VRADIO": Radio velocity (km/s)
  • "VOPT" or "VOPTICAL": Optical velocity (km/s)
  • "ZOPT" or "REDSHIFT": Reshift (dimensionless)
  • "BETA": Beta factor (dimensionless)
  • "VELO" or "VREL": Relativistic velocity (km/s)

The default value for the Unit attribute for each system is shown in parentheses. Note that the default value for the ActiveUnit flag is non-zero for a SpecFrame, meaning that changes to the Unit attribute for a SpecFrame will result in the SpecFrame being re-mapped within its enclosing FrameSet in order to reflect the change in units (see astSetActiveUnit function for further information).



next up previous
Next: TextLab(axis) - Draw descriptive axis labels for a Plot?
Up: AST Attribute Descriptions
Previous: Symbol(axis) - Axis symbol

AST A Library for Handling World Coordinate Systems in Astronomy
Starlink User Note 211
R.F. Warren-Smith & D.S. Berry
30th April 2003
E-mail:ussc@star.rl.ac.uk

Copyright (C) 2003 Central Laboratory of the Research Councils