makeHpxWcs

lsst.afw.geom.makeHpxWcs(hips_order, pixel, shift_order=9)

Make a SkyWcs object with HEALPix grid projection (HPX).

The SkyWcs generated by this function is suitable to be used with a Hierarchical Progressive Survey (HiPS) FITS image as described in https://www.ivoa.net/documents/HiPS/20170519/REC-HIPS-1.0-20170519.pdf

A HiPS image covers one HEALPix cell, with the HEALPix nside equal to 2**hips_order. Each cell is ‘shift_order’ orders deeper than the HEALPix cell, with 2**shift_order x 2**shift_order sub-pixels on a side, which defines the target resolution of the HiPS image. The IVOA recommends shift_order=9, for 2**9=512 pixels on a side. See Notes below to convert from hips_order to image resolution.

Parameters:

hips_orderint

HiPS order, such that HEALPix nside=2**hips_order. Must be a positive integer.

pixelint

Pixel number in the nest ordering scheme.

shift_orderint, optional

Shift order for subpixels, such that there are 2**shift_order sub-pixels on a side of the HiPS cell. Must be a positive integer.

Returns:

wcslsst.geom.SkyWcs

Raises:

ValueError: Raise if hips_order is <=0, or if shift_order is <=0, or

if pixel number is out of range for the given hips_order (0 <= pixel < 12*nside*nside).

Notes

Table 5 from https://www.ivoa.net/documents/HiPS/20170519/REC-HIPS-1.0-20170519.pdf shows the relationship between hips_order, number of tiles (full sky coverage), cell size, and sub-pixel size/image resolution (with the default shift_order=9):

hips_order	Number of Tiles	Cell Size	Image Resolution
0 1 2 3 4 5 6 7 8 9 10 11 12 13	12 48 192 768 3072 12288 49152 196608 786432 3145728 12582912 50331648 201326592 805306368	58.63 deg 29.32 deg 14.66 deg 7.329 deg 3.665 deg 1.832 deg 54.97 arcmin 27.48 arcmin 13.74 arcmin 6.871 arcmin 3.435 arcmin 1.718 arcmin 51.53 arcsec 25.77 arcsec	6.871 arcmin 3.435 arcmin 1.718 arcmin 51.53 arcsec 25.77 arcsec 12.88 arcsec 6.442 arcsec 3.221 arcsec 1.61 arcsec 805.2mas 402.6mas 201.3mas 100.6mas 50.32mas