SFL projection

Since the dec07 GILDAS release, the SFL projection was implemented as:

$\displaystyle x$	$\displaystyle =$	$\displaystyle (\alpha-\alpha_0) \times cos(\delta-\delta_0)$	(11)
$\displaystyle y$	$\displaystyle =$	$\displaystyle \delta-\delta_0$	(12)

This convention is very near to the radio/GLS one, except $x$

depends on the cosine of $\delta-\delta_0$ , not $\delta$ itself. Unfortunately, this convention does not match the GC02 one, in particular the stage 2 described in section 1 where the native sphere is Euler-rotated to the celestial sphere. This is not as simple as the simple $\delta$ shift in the above formula.

Choice is made to discard the above implementation and go for the CG02 one, without changing the GILDAS name SFL and the associated internal identifier for this projection. This assumes this projection was seldom used on the GILDAS side (import/export from/to FITS was anyway not available). This may break the interpretation of old GILDAS cubes using this projection, if any, but introduces full compatibility with FITS-SFL files. The implementation is detailed in Appendix A and is done since GILDAS release oct23.