The function astNorm is provided to cope with the fact that some coordinate systems do not extend indefinitely in all directions. Some may have boundaries, outside which coordinates are meaningless, while others wrap around on themselves, so that after a certain distance you return to the beginning again (coordinate systems based on circles and spheres, for instance). A basic Frame has no such complications, but other more specialised Frames (such as SkyFrames, representing the celestial sphere--) do.
The role played by astNorm is to normalise any arbitrary set of coordinates by converting them into a set which is ``within bounds'', interpreted according to the particular Frame in question. For example, on the celestial sphere, a right ascension value of 24 hours or more can have a suitable multiple of 24 hours subtracted without affecting its meaning and astNorm would perform this task. Similarly, negative values of right ascension would have a multiple of 24 hours added, so that the result lies in the range zero to 24 hours. The coordinates in question are modified in place by astNorm, as follows:
double point[ 2 ]; ... astNorm( frame, point );
If the coordinates supplied are initially OK, as they would always be with a basic Frame, then they are returned unchanged.
Because the main purpose of astNorm is to convert coordinates into the
preferred range for human consumption, its use is almost always
appropriate immediately before formatting coordinate values for
display using astFormat (). Even if
the Frame in question does not restrict the range of coordinates, so
that astNorm does nothing, using it will allow you to process other
more specialised Frames, where normalisation is important, without
changing your software.
AST A Library for Handling World Coordinate Systems in Astronomy