- ...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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- ...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.
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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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- ...specifier,
- This is a consequence of
implementing the AST library in C.
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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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- ...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.
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- ...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.
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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 IN holds the original ``output''
coordinates when applying the inverse transformation and OUT holds 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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