When writing a FrameSetFrameSet to a
FitsChanFitsChan, the pixel coordinates of the
reference point for linear axes (i.e. the CRPIXj values) are
chosen as follows:
- If the FrameSet is being written to a FitsChan which previously
contained a set of axis descriptions with the same identifying letter,
then the previous CRVALjvalues are converted into the coordinate system
of the FrameFrame being written (if possible). These values are then
transformed into the pixel Frame, and the closest integer pixel values
are used as the CRPIX keywords.
- If the above step could not be performed for any reason, the
central pixel is used as the reference point. This requires the image
dimensions to be present in the FitsChan in the form of a set of
NAXISj keyword values.
- If both the above two steps failed for any axis, then the pixel
reference position is set to a value of 1.0 on the pixel axis.
The pixel to WCS MappingMapping is then used to find the corresponding
CRVALjvalues.
Again, the above procedure is over-ridden if the Frame is a
SkyFrameSkyFrame or a SpecFrameSpecFrame, in which
case the conventions of papers II and III are used as described below.