...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.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

...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.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

...case

Note that the methods applied to a FrameSet may be used equally well with a Plot.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

...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. To allow for this possibility, you might prefer to use a parameter constant to represent the value zero when testing for errors.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

...not.

Most of the Mappings provided by the AST library work in both directions, although the LutMap can behave otherwise.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

...specifier,

This is a consequence of implementing the AST library in C.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

...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?

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

...axis

For instance, a line of constant Declination is not a geodesic

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

...(``-'').

If both component Frames have blank Domains, then the default Domain for the CmpFrame is the string ``CMP''.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

...``Compound''.

Any attempt to change the System value of a CmpFrame is ignored.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

...digits.

The leading digit is zero and is therefore not seen in this particular example.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

...Frame.

Although this is reversed if the FrameSet's Invert attribute is non-zero.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

...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.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

...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.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

...AST_NULL

Note that AST_NULL (one underscore) is a routine name and is distinct from AST__NULL (two underscores) which is a null Object pointer. Since we are passing the name of one routine to another routine, AST_NULL would normally have to appear in a Fortran EXTERNAL statement. In this example, however, a suitable statement is already present in the AST_PAR include file.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

...HREF="node409.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.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

...1988.

Available by ftp from fits.cv.nrao.edu /fits/documents/wcs/wcs88.ps.Z

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

...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.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

...1994).

Available by ftp from fits.cv.nrao.edu /fits/documents/wcs/aips27.ps.Z

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

...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.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

...encodings

In practice, this means you should avoid mixing FITS-IRAF, FITS-WCS, FITS-AIPS and FITS-PC encodings since they share many keywords.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

...coordinate.

Remember that IN holds the original ``output'' coordinates when applying the inverse transformation and OUT holds the original ``input'' coordinates.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

...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).

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

...point.

Remember, the physical coordinate space need not necessarily be 2-dimensional, even if the plotting surface is.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

...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.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

...PATH.

If you have not installed AST in the usual location, then substitute the appropriate directory in place of ``/star'' wherever it occurs.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

...follows:

Use the ``-pgp'' option instead if you wish to use the Starlink version of PGPLOT which uses GKS to generate its output.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

...``/star/bin''

Or the equivalent directory if AST is installed in a non-standard location.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.