The name of a text file containing an AST Region. The Region can be either 2D or 3D but must include celestial axes.
The path to a 2- or 3-D NDF holding a reduced map or cube. This need not necessarily hold JCMT data, but must have celestial axes in its current WCS Frame.
A group of raw JCMT data files.
A null (!) value, in which case a polygon region is used as defined by the parameters VERTEX_RA and VERTEX_DEC.
". If one or more NDFs are supplied for parameter IN, then a dynamic default is determined if possible from the first NDF. If this cannot be done, or if a Region is supplied for parameter IN, then no dynamic default is provided, and the user is prompted for a value if none was supplied on the command line. 
". The choice is made to minimise the possibility of a projection discontinuity falling within the sky area covered by the tiles.
The whole sky is covered by an HPX (HEALPix) projection containing 12 basic square facets, the
reference point of the projection is put at (RA,Dec)=(0,0) (except for facet six that has a reference point
of (12h,0)). The projection plane is rotated by 45 degrees so that the edges of each facet are parallel to X
and Y (as in Fig.3 of the A&A paper
" Mapping on the HEALPix grid
" by Calabretta and Roukema).
Each facet is then divided up into NxN tiles, where N is 64 for SCUBA-2 and 128 for ACSIS. Each
tile is then divided into PxP pixels, where P is 412 for ACSIS, 825 for SCUBA-2 850 um,
1650 for SCUBA-2 450 um. Facets are numbered from 0 to 11 as defined in the HEALPix
paper (Gorsky et. al. 2005 ApJ 622, 759) (note that the facet six is split equally into two
triangles, one at the bottom left and one at the top right of the projection plane). Within
a facet, tiles are indexed using the
" scheme described in the HEALPix paper.
This starts with pixel zero in the southern corner of the facet. The even bits number the
position in the north-east direction and the odd bits number the position in the north-west
direction. All the tiles in the first facet come first, followed by all the tiles in the second facet,
This is a fairly complex scheme. To help understanding, the SMURF:TILEINFO command can create an all-sky map in which each pixel corresponds to a single tile, and has a pixel value equal to the corresponding tile index. Displaying this map can help to visualise the indexing scheme described above.