A collection of common non-standard features are supported when reading a
FrameSetFrameSet from a FitsChanFitsChan, 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
WcsMapWcsMap with type of
AST__ZPN. ProjectionProjection parameter values are read from any WATi_nnn
keywords, and corresponding PVi_mPVi_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
WarningsWarnings 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
AUTOASTROMsun242
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 PolyTanPolyTan attribute
of the FitsChan class.