Basic unit type Time units may be second (s), hour (h: 3600 s), day (d: 86400 s), Julian year (a, yr: 365.25 d), Julian century (cy: 36525 d), or empty (i.e., dimensionless) for ISO-8601 format Coordinate units may be angular (degrees, radians, hours, arcmins, arcsecs), linear (meters, km, mm, AUs, parsecs, kpc, Mpc, lightyears), or empty (i.e., dimensionless) Angular coordinate units may be degrees, radians, hours, arcmins, arcsecs Velocity units are a posUnitType / velTimeType; the latter may be second, day, hour, year (a or yr), century, but not empty Spectral units may be frequency (Hz, kHz, MHz, GHz), wavelength (meters, mm, micron, nm, Angstrom), or energy (eV, keV, MeV, GeV, TeV) The actual time scale used: TT, TAI, UTC, TDB, etc. Terrestrial Time; the basis for ephmerides Obsolete synonym for TT Ephemeris Time; predecessor of, and continuous with, TT Barycentric Dynamic Time: time at the solar system barycenter synchronous with TT on an annual basis Terrestrial Coordinate Time Barycentric Coordinate Time; runs slower than TDB but is consistent with physical constants International Atomic Time; runs 32.184 s behind TT Synonym for TAI Coordinated Universal Time; currently (2004) runs 34 leapseconds behind TAI Local Siderial Time; only for ground-based observations; note that the second is shorter Only to be used for simulations in conjunction with a relocatable spatial frame Substitution group for absoluteTimeType component of astronTimeType ISO8601 time; note: only a limited subset of ISO 8601 is allowed: yyyy-mm-ddThh:mm:ss.sss... IDREF to an ISO8601 time JD time IDREF to a JD time MJD time (=JD - 2400000.5) IDREF to an MJD time A relocatable time origin for simulations Substitution group: Relative time is elapsed time relative to the absolute reference time component Actual elapsed time offset IDREF to the elapsed relative time part of a AstronTime astronTime is the generalized astronomical time type and consists of two or three elements: TimeScale, optional relative time offset, and an absolute time (ISO8601 or a decimal JD or MJD; or it may be an IDREF to one of those three) For vector coordinates we need arrays of doubles A vector of 2 doubles A vector of 3 doubles A vector of 4 doubles A vector of 9 doubles The head element for the CoordinateValue substitution group Substitution group for a scalar coordinate value double IDREF to a Value Substitution group for a scalar coordinate error double IDREF to an Error Substitution group for a scalar coordinate resolution double IDREF to a Resolution Substitution group for a scalar coordinate size double IDREF to a Size Substitution group for a scalar coordinate pixel size double IDREF to a Pixel Size Abstract coordinate type; a concrete Coordinate consists of a Value, Error, Resolution, Size, and PixSize Generic scalar coordinate type Single CError, CResolution, CSize, CPixSize elements indicate definite values; pairs indicate ranges Time coordinate type Single CError, CResolution, CSize, CPixSize elements indicate definite values; pairs indicate ranges 2 doubles with optional position angle Substitution group for a 2-D coordinate value double vector IDREF to a Value2 Substitution group for a 2-D coordinate error size2Type Error element 2x2 matrix Error element IDREF to an Error2 Substitution group for a 2-D coordinate resolution size2Type Resolution element 2x2 matrix Resolution element IDREF to a Resolution2 Substitution group for a 2-D coordinate size size2Type Size element 2x2 matrix Size element IDREF to a Size2 Substitution group for a 2-D coordinate pixel size size2Type PixSize element 2x2 matrix PixSize element IDREF to a PixSize2 Generic 2-D coordinate type Single CError2, CResolution2, CSize2, CPixSize2 elements indicate definite values; pairs indicate ranges 3 doubles with optional position angles Substitution group for a 3-D coordinate value Vector of 3 doubles IDREF to a Value3 Substitution group for a 3-D coordinate error size3Type Error element 3x3 matrix Error element IDREF to an Error3 Substitution group for a 3-D coordinate resolution size3Type Resolution element 3x3 matrix Resoltuion element IDREF to a Resolution3 Substitution group for a 3-D coordinate size size3Type Size element 3x3 matrix Size element IDREF to a Size3 Substitution group for a 3-D coordinate pixel size size3Type PixSize element 3x3 matrix PixSize element IDREF to a PixSize3 Generic 3-D coordinate type Single CError3, CResolution3, CSize3, CPixSize3 elements indicate definite values; pairs indicate ranges Coordinate substitution group Generic scalar coordinate element Generic string coordinate element Generic coordinate element for pixels Position coordinate substitution group 1-D Position coordinate 2-D Position coordinate 3-D Position coordinate Velocity coordinate substitution group 1-D Velocity coordinate 2-D Velocity coordinate 3-D Velocity coordinate Specifies a FITS file and optionally a specific HDU by HDU number or HDU name Refers coordinate components to specific columns in the FITS file HDU The column name for the coordinate value; comma-separated if multi-dimensional The column name for the coordinate error; comma-separated if multi-dimensional The column name for the coordinate resolution; comma-separated if multi-dimensional The column name for the coordinate size; comma-separated if multi-dimensional The column name for the coordinate pixel size; comma-separated if multi-dimensional Coordinate references to a specific FITS file The generic coordsType The astronomical (STC) coordsType Some or all coordinate values may be given in file The pixel coordinates type Head element of Coords group (not abstract) The coordinate element consists either of a coordinate file or a sequence of time element, spatial coordinate element, velocity element, and redshift element; the spatial and velocity elements may be scalar or vector; it needs to refer to a coordinate system Contains pixel coordinates