...coordinates.
By pixel coordinates, we mean a coordinate system in which the first pixel in the image is centred on (1,1) and each pixel is a unit square. Note that the world coordinates will not necessarily be celestial coordinates, but if they are, then they will be in radians.
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...package.
An interface is provided with AST that allows it to use PGPLOT (SUN/15) for its graphics, although interfaces to other graphics systems may also be written.
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...case
Note that the methods applied to a FrameSet may be used equally well with a Plot.
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...them.
About the only difference is that you cannot store a pointer to an AST ``function'' in a variable and use the variable's value to invoke that function again later.
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...zero
We will assume throughout that the ``OK'' value is zero, as it currently is. However, a different value could, in principle, be used if the environment in which AST is running requires it. This is why a simple interface is provided to isolate you from the actual value of the error status.
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...not.
Most of the Mappings provided by the AST library work in both directions, although the LutMap can behave otherwise.
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...read.
Anyone who seriously uses the C run time library ``scanf'' function will know about the need for this check!
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...once.
It will probably not be obvious why this restriction is necessary, but consider creating a Frame with one longitude axis and two latitude axes. Which latitude axis should be associated with the longitude axis?
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...axis
For instance, a line of constant Declination is not a geodesic
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...(``-'').
If both component Frames have blank Domains, then the default Domain for the CmpFrame is the string ``CMP''.
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...``Compound''.
Any attempt to change the System value of a CmpFrame is ignored.
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...digits.
The leading digit is zero and is therefore not seen in this particular example.
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...Frame.
Although this is reversed if the FrameSet's Invert attribute is non-zero.
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...order).
If you find that how this ambiguity is resolved actually makes a difference to the conversion that results, then you have probably constructed a FrameSet which lacks internal self-consistency. For example, you might have two Frames representing indistinguishable coordinate systems but inter-related by a non-null Mapping.
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...present.
This is because any transformation to or from this new Frame must go via the base Frame representing the original data grid coordinate system, which we assume was correctly related to all the other Frames present.
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...HREF="node415.html#SkyFrame">SkyFrame,
More probably, you would want to write a FrameSet, but for purposes of illustration a SkyFrame contains a more manageable amount of data.
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...1988.
Available by ftp from fits.cv.nrao.edu /fits/documents/wcs/wcs88.ps.Z
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...instead.
There are many other differences between the previous and the current FITS-WCS encodings. The keywords to describe axis rotation and scaling is used purely as a label to identify the scheme.
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...1994).
Available by ftp from fits.cv.nrao.edu /fits/documents/wcs/aips27.ps.Z
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...written.
This can happen if a particular keyword is present in the input header but is not used in the output header (whether particular keywords are used can depend on the WCS information being stored). In such a case, the original value would not be over-written by a new output value, so would remain erroneously present.
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...encodings
In practice, this means you should avoid mixing FITS-IRAF, FITS-WCS, FITS-AIPS and FITS-PC encodings since they share many keywords.
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...coordinate.
Remember that ``ptr_in'' identifies the original ``output'' coordinates when applying the inverse transformation and ``ptr_out'' identifies the original ``input'' coordinates.
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...surface.
Like any FrameSet, a Plot can be used as a Mapping. In this case it is the inverse transformation which is used when plotting (i.e. that which transforms between the current and base Frames).
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...point.
Remember, the physical coordinate space need not necessarily be 2-dimensional, even if the plotting surface is.
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...curve
normalized so that the start of the curve is at offset 0.0 and the end of the curve is at offset 1.0 - offset need not be linearly related to distance.
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...PATH.
If you have not installed AST in the usual location, then substitute the appropriate directory in place of ``/star'' wherever it occurs.
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...follows:
Use the ``-pgp'' option instead if you wish to use the Starlink version of PGPLOT which uses GKS to generate its output.
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...``/star/bin''
Or the equivalent directory if AST is installed in a non-standard location.
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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