Specifying a Co-ordinate Frame

Several applications (WCSFRAME, CURSOR, LISTMAKE, etc.) have parameters that are used to select a co-ordinate Frame. These parameters are usually called FRAME. When selecting a Frame from an existing FrameSet  (e.g. read from the WCS component  of an NDF), the Frame may be specified in one of the following ways:

• As an integer Frame index within the specified FrameSet starting at 1 for the first Frame.

• As a `Domain' name (e.g. PIXEL, AXIS, SKY), selected from those available in the FrameSet.

• As a Sky Co-ordinate System (SCS) specification. This notation has been inherited from the IRAS90 package (see SUN/163), and can be used to indicate a specific celestial co-ordinate system. Simply specifying the Domain name SKY will select any SkyFrame  that is present, irrespective of the particular celestial co-ordinate system that the SkyFrame represents. If you want to request a particular celestial co-ordinate system (e.g. galactic, equatorial, ecliptic, etc.), then use an SCS specification. If the requested system is not present, but a related SkyFrame can be found for a different system, then the SkyFrame will be modified so that it represents the requested system (the Mappings between the SkyFrame and the other Frames in the FrameSet will also be modified appropriately).

  An SCS specification is made up of two parts; a co-ordinate system name, followed by an optional epoch giving the reference equinox. Any of the three system names EQUATORIAL, ECLIPTIC and GALACTIC can be used. Case is insignificant, and abbreviations may be given.

  Ecliptic and equatorial co-ordinates are referred to the mean equinox of a given epoch. This epoch is specified by appending it to the end of the name of the sky co-ordinate system, in parentheses; for instance EQUATORIAL(1983.5) (only the four most significant decimal places are used). The epoch may be preceded by a single character, B or J, indicating if the epoch is a Besselian epoch (B) or a Julian epoch (J). If this character is missing (as in the above example), then the epoch is assumed to be Besselian if it less than 1984.0, and Julian otherwise. If no equinox is specified in this way, then a default of B1950.0 is used.

  If a Julian epoch is used to specify the reference equinox for an equatorial co-ordinate system, then the equatorial co-ordinates are assumed to be in the IAU 1976, FK5, Fricke system. If the equinox is specified using a Besselian epoch, then the co-ordinates are assumed to be in the FK4, Bessel-Newcomb system.

  When a Frame is specified using an SCS specification, it will usually also be necessary to specify the epoch at which the positions were determined. This will be done using the separate Parameter EPOCH. This epoch is required because some celestial co-ordinate systems are non-inertial and rotate slowly with respect to other celestial co-ordinate systems, introducing fictitious proper motions. Knowing the date at which the positions were determined allows the effect of this fictitious proper motion to be eliminated when converting between different systems.

When specifying a new Frame (rather than selecting an existing Frame from a FrameSet), you can either give a Domain name, or an SCS specification, but you cannot give a Frame index. Any string may be used as a Domain name, and you will usually be required to specify the number of axes in the Frame. The exception to this is if you specify one of the Domain names SKY, GRAPHICS, NDC, CURPIC or BASEPIC, in which case a two-dimensional Frame is always created.

The nature of the current Frame can also be changed using WCSATTRIB, which allows new values to be assigned to specified Frame attributes. For instance, assigning a new value to the System attribute will change the co-ordinate system used to describe positions within the domain covered by the Frame. The main attributes relevant to Frames are described here.