System - Co-ordinate 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 co-ordinate systems. For instance, the SkyFrame  class can use galactic co-ordinates, equatorial co-ordinates 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 co-ordinate 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.

SkyFrame

The SkyFrame class  supports the following System values and associated celestial co-ordinate systems.

• "FK4": The old FK4 (barycentric) equatorial co-ordinate 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 co-ordinate 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 co-ordinate system should also be qualified by both an Equinox and an Epoch value.

• "FK5" or "EQUATORIAL": The modern FK5 (barycentric) equatorial co-ordinate system. This should be qualified by an Equinox  value.

• "GAPPT", "GEOCENTRIC" or "APPARENT": The geocentric apparent equatorial co-ordinate system, which gives the apparent positions of sources relative to the true plane of the Earth's equator and the equinox (the co-ordinate origin) at a time specified by the qualifying Epoch value. (Note that no Equinox is needed to qualify this co-ordinate system because no model `mean equinox' is involved.) These co-ordinates 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 co-ordinates are derived from the standard FK5 (J2000.0) barycentric co-ordinates 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 co-ordinates (IAU 1980), referred to the ecliptic and mean equinox specified by the qualifying Equinox value.

• "GALACTIC": Galactic co-ordinates (IAU 1958).

• "SUPERGALACTIC": De Vaucouleurs Supergalactic co-ordinates.

• "UNKNOWN": Any other general spherical co-ordinate 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".

SpecFrame

The SpecFrame  DSBSpecFrame  classes supports the following System values and associated spectral co-ordinate systems (the default is "WAVE" -- wavelength):

• "FREQ": Frequency (Hz)

• "ENER" or "ENERGY": Energy (J)

• "WAVN" or "WAVENUM": Wave-number (1/m)

• "WAVE" or "WAVELEN": Vacuum wave-length (m)

• "AWAV" or "AIRWAVE": Wave-length in air (m)

• "VRAD" or "VRADIO": Radio velocity (m/s)

• "VOPT" or "VOPTICAL": Optical velocity (m/s)

• "ZOPT" or "REDSHIFT": Redshift (dimensionless)

• "BETA": Beta factor (dimensionless)

• "VELO" or "VREL": Relativistic velocity (m/s)

The default value for the Unit  attribute for each system is shown in parentheses. Note, changes to the Unit attribute for a SpecFrame will result in the Mapping from pixel to spectral co-ordinates being modified in order to reflect the change in units.

TimeFrame

The TimeFrame class supports the following System values and associated coordinate systems (the default is "MJD"):

• "MJD": Modified Julian Date (d)

• "JD": Julian Date (d)

• "JEPOCH": Julian epoch (yr)

• "BEPOCH": Besselian (yr)

The default value for the Unit attribute for each system is shown in parentheses. Strictly, these systems should not allow changes to be made to the units. For instance, the usual definition of "MJD" and "JD" include the statement that the values will be in units of days. However, AST does allow the use of other units with all the above supported systems (except BEPOCH), on the understanding that conversion to the "correct" units involves nothing more than a simple scaling (1 yr = 365.25 d, 1 d = 24 h, 1 h = 60 min, 1 min = 60 s). Besselian epoch values are defined in terms of tropical years of 365.2422 days, rather than the usual Julian year of 365.25 days. Therefore, to avoid any confusion, the Unit attribute is automatically cleared to "yr" when a System value of BEPOCH System is selected, and an error is reported if any attempt is subsequently made to change the Unit attribute.