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Using Default SkyFrame Attributes

The default values supplied for many SkyFrame attributes will depend on the value of the SkyFrame's System attribute. In practice, this means that there is usually little need to specify many of these attributes explicitly unless you have some special requirement. This can be illustrated by using astShow to examine a SkyFrame, as follows:

astShow( astSkyFrame( "System=FK4-NO-E, Epoch=1958" ) );

The output from this might look like the following:

 Begin SkyFrame         # Description of celestial coordinate system
#   Title = "FK4 equatorial coordinates; no E-terms; mean equinox B1950.0; 
epoch B1958.0"   # Title of coordinate system
    Naxes = 2   # Number of coordinate axes
#   Domain = "SKY"      # Coordinate system domain
    Epoch = 1958        # Besselian epoch of observation
#   Lbl1 = "Right ascension"    # Label for axis 1
#   Lbl2 = "Declination"        # Label for axis 2
    System = "FK4-NO-E"         # Coordinate system type
#   Uni1 = "hh:mm:ss.s"         # Units for axis 1
#   Uni2 = "ddd:mm:ss"  # Units for axis 2
#   Dir1 = 0    # Plot axis 1 in reverse direction
#   Bot2 = -1.5707963267949     # Lowest legal axis value
#   Top2 = 1.5707963267949      # Highest legal axis value
    Ax1 =       # Axis number 1
       Begin SkyAxis    # Celestial coordinate axis
       End SkyAxis
    Ax2 =       # Axis number 2
       Begin SkyAxis    # Celestial coordinate axis
       End SkyAxis
 IsA Frame      # Coordinate system description
#   Eqnox = 1950        # Besselian epoch of mean equinox
 End SkyFrame

Note that the defaults (indicated by the ``#'' comment character at the start of the line) for attributes such as the Title, axis Labels and Format specifiers are all set to values appropriate for the particular equatorial coordinate system that the SkyFrame represents.

This means, for example, that if we were to use this SkyFrame to format a right ascension value stored in radians using astFormat ([*]), it would automatically result in a string in sexagesimal notation (such as ``12:14:35.7'') suitable for display. If we changed the value of the SkyFrame's Digits attribute (which is inherited from the Frame class), the number of digits appearing would also change accordingly.

These choices would be appropriate for a System value of ``FK4-NO-E'', but if a different System value were set, the defaults would be correspondingly different. For example, ecliptic longitude is traditionally expressed in degrees, so setting ``System=ecliptic'' would result in coordinate values being formatted as degrees by default.

Of course, if you do not like any of these defaults, you may always over-ride them by setting explicit attribute values yourself.



next up previous
Next: Formatting Celestial Coordinates
Up: Celestial Coordinate Systems (SkyFrames)
Previous: Attributes which Qualify Celestial Coordinate Systems

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