List the sky tiles that overlap a given set of data files or an AST Region


This routine returns a list containing the indices of the sky tiles (for a named JCMT instrument) that overlap a supplied AST Region, map, cube or group of raw data files.


Specifies the region of sky for which tiles should be listed. It may be:
  • 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.

The JCMT instrument (different instruments have different tiling schemes and pixel sizes). The following instrument names are recognised (unambiguous abbreviations may be supplied): " SCUBA-2(450)" , " SCUBA-2(850)" , " ACSIS" , " DAS" . 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 type of JSA projection that should be used to describe the area of sky covered by the returned lost of tiles. Will be one of " HPX" , " HPX12" , " XPHN" or " XPHS" . The choice is made to minimise the possibility of a projection discontinuity falling within the sky area covered by the tiles.
TILES() = _INTEGER (Write)
An output parameter to which is written the list of integer tile indices.
The ICRA Dec value at each vertex of a polygon, in degrees. Only used if IN is null.
The ICRA RA value at each vertex of a polygon, in degrees. Only used if IN is null.

Tile Definitions

It should never be necessary to know the specific details of the tiling scheme used by SMURF. But for reference, it works as follows:

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 " nested" 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, etc.

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.

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