### Common Non-Standard Features

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.