next up previous 264
Next: PreserveAxes - Preserve axes?
Up: AST Attribute Descriptions
Previous: PolarLong - The longitude value to assign to either pole


PolyTan - Use PVi_m keywords to define distorted TAN projection?

Description:
This attribute is a boolean value which specifies how FITS "TAN" projections should be treated when reading a FrameSet from a foreign encoded FITS header. If zero, the projection is assumed to conform to the published FITS-WCS standard. If positive, the convention for a distorted TAN projection included in an early draft version of FITS-WCS paper II are assumed. In this convention the coefficients of a polynomial distortion to be applied to intermediate world coordinates are specified by the PVi_m keywords. This convention was removed from the paper before publication and so does not form part of the standard. Indeed, it is incompatible with the published standard because it re-defines the meaning of the first five PVi_m keywords on the longitude axis, which are reserved by the published standard for other purposes. However, headers that use this convention are still to be found, for instance the SCAMP utility (http://www.astromatic.net/software/scamp) creates them.

The default value for the PolyTan attribute is -1. A negative values causes the used convention to depend on the contents of the FitsChan. If the FitsChan contains any PVi_m keywords for the latitude axis, or if it contains PVi_m keywords for the longitude axis with "m" greater than 4, then the distorted TAN convention is used. Otherwise, the standard convention is used.

Type:
Integer.
Class Applicability:
FitsChan
All FitsChans have this attribute.


next up previous 264
Next: PreserveAxes - Preserve axes?
Up: AST Attribute Descriptions
Previous: PolarLong - The longitude value to assign to either pole

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