Choice of Reference Point

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:

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.