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User Defined Fiducial Points

When reading a FrameSet from a FitsChan, projection parameters PVi_0, PVi_1 and PVi_2 (for longitude axis ``i'') are used to indicate a user-defined fiducial point as described in section 2.5 of paper II. This results in a shift of IWC origin being applied before the WcsMap which converts IWC into native spherical coordinates. The values of these projection parameters, if supplied, are stored as the corresponding PVi_m attributes of the WcsMap.

When writing a FrameSet to a FitsChan, the PV attributes of the WcsMap determine the native coordinates of the fiducial point (the fixed defaults for each projection described in paper II are used if the PV attributes of the WcsMap have not been assigned a value). The corresponding celestial coordinates are used as the CRVALi keywords and the corresponding pixel coordinates as the CRPIXj keywords.



next up previous 264
Next: Common Non-Standard Features
Up: Paper II - Celestial Coordinates
Previous: Choice of LONPOLE/LATPOLE

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