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A collection of common non-standard features are supported when reading a
FrameSet from a FitsChan, in addition
to those embodied within the
available encodings of the FitsChan class. These are translated into the
equivalent standard features before being used to create a FrameSet.
Note, the reverse operation is never performed: it is not possible to
produce non-standard features when writing a FrameSet to a FitsChan
(other than those embodied in the available encodings of the FitsChan
class). The supported non-standard features include:
- EQUINOX keywords with string values equal to a date preceded
by the letter B or J (e.g. ``B1995.0'').
- EQUINOX or EPOCH keywords with value zero (these are converted to
B1950).
- The IRAF ``ZPX'' projection is represented by a
WcsMap with type of
AST__ZPN. Projection parameter values are read from any WATi_nnn
keywords, and corresponding PVi_m attributes are set in the
WcsMap. The WATi_nnn keywords may specify corrections to the basic
ZPN projection by including ``lngcor'' or ``latcor'' terms. These are
supported if they use half cross-terms, in either simple or Chebyshev
representation.
- The IRAF ``TNX'' projection is represented by a WcsMap with type of
AST__TPN (a distorted TAN projection retained within the WcsMap class
from an early draft of the FITS-WCS paper II). Projection parameter values
are read from any WATi_nnn keywords, and corresponding PV
attributes are set in the WcsMap. If the TNX projection cannot be
converted exactly into an AST__TPN projection, ASTWARN keywords are
added to the FitsChan containing a warning message (but only if the
Warnings attribute of the FitsChan is set appropriately). Currently,
TNX projections that use half cross-terms, in either simple or Chebyshev
representation, are supported.
- ``QV'' parameters for TAN projections (as produced by
AUTOASTROM
are renamed to the equivalent ``PV'' parameters.
- TAN projections that have associated ``PV'' parameters on the
latitude axis are converted to the corresponding TPN (distorted TAN)
projections. This conversion can be controlled using the PolyTan attribute
of the FitsChan class.
Next: Paper III - Spectral Coordinates
Up: Paper II - Celestial Coordinates
Previous: User Defined Fiducial Points
AST A Library for Handling World Coordinate Systems in Astronomy
Starlink User Note 210
R.F. Warren-Smith & D.S. Berry
24th May 2011
E-mail:ussc@star.rl.ac.uk
Copyright (C) 2009 Science \& Technology Facilities Council