... software

It comes with bundled copies of the ERFA and Starlink PAL librariessun268 which are built at the same time as the other AST internal libraries. Alternatively, external PAL and ERFA libraries may be used by specifying the ``-with-external_pal'' option when configuring AST

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... coordinates.

A pair of Mappings can be combined in a third way using a TranMapTranMap. A TranMap allows the forward transformation of one Mapping to be combined with the inverse transformation of another to produce a single Mapping.

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...

XML documents which use only the subset of the STC schema supported by AST can be read by the XmlChan class to produce corresponding AST objects (subclasses of the StcStc class). However, the reverse is not possible. That is, AST objects can not currently be written out in the form of STC documents.

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... Collection

The Starlink Software Collection can be downloaded from http://www.starlink.ac.uk/Download/.

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... 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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...ss:howtowritewcs).

If you are writing the WCS calibration to a FITS file you obviously have the choice of storing the FITS-WCS cards directly.

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... package.

An interface is provided with AST that allows it to use PGPLOT (SUN/15sun15) 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 FrameSetFrameSet 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 LutMapLutMap can behave otherwise.

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... value

The exception to this rule is that if the Format value includes a precision of ``$.*$'', then Digits will be used to determine the actual precision used.

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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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... attribute

The units used for the internal floating-point representation of an axis value can be determined by examining the InternalUnit attribute of the Frame. For most Frames, the Unit and InternalUnit attributes will be equal, but InternalUnit is always set to ``rad'' for SkyFrames.

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... frequencies

Note, this simple relationship only applies if all frequencies are topocentric.

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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 InvertInvert 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 MappingMapping.

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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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...SkyFrame,

More probably, you would want to write a FrameSetFrameSet, 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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... yourself.

Storing values in the FitsChan for FITS headers NAXIS1, NAXIS2, etc. (the grid dimensions in pixels), before invoking astWrite can sometimes help to produce a successful write.

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... encodings

In practice, this means you should avoid mixing FITS-IRAF, FITS-WCS, FITS-AIPS, 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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...

http://www.astro.gla.ac.uk/users/norman/star/autoastrom/

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... met.

If the -TAB algorithm is used, the positive value of the TabOK attribute is used as the table version number (the EXTVER header) in the associated FITS binary table.

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