hpctran: Convert between HEALPix row and spherical coordinate

Package: imcoords

Usage

hpctran lng=xxx lat=xxx
hpctran row=xxx

Parameters

row: HEALPix table row (1 indexed). This is used as input if the direction is "row2ang" or is used to store the value if the direction is "ang2row".

lng, lat: Spherical coordinate consisting of a longitude and latitude. These are used as input if the direction is "ang2row" or is used to store the value if the direction is "row2ang". The units are interpreted as selected by the cunits parameter. The type of coordinates appropriate for a particular map is defined by the map provider.

nside = 512: The number of pixels per face side.

cunits = "degrees" (degrees|hourdegree|radians): The units of the longitude and latitude. The "hourdegree" is for longitude in hours and latitude in degrees.

maptype = "nest" (nest|ring): The map pixelization type which may be "nest" or "ring".

direction = "ang2row" (ang2row|row2ang): The conversion direction. "ang2row" converts a spherical coordinate to a map row or pixel number. "row2ang" converts a map row or pixel number to a spherical coordinate.

Description

HEALPix is an acronym for Hierarchical Equal Area isoLatitude Pixelization of a sphere. See the reference section for a technical description of the pixelization and mathematics. As suggested in the name, this pixelization, or tiling, produces a subdivision of a spherical surface in which each "pixel" covers the same surface area as every other pixel. A HEALPix FITS "map" is a table where each row contains "pixel" data for a region on the sphere. It is a table because the pixels don't form a raster as in an image.

The pixelization is defined by a resolution parameter which may be expressed in various ways. This task uses the number of pixels along a side of one of the 12 basic faces. The number of pixels/rows is 12 * nside * nside. The pixelization has two forms supported by this task. These are called "nested" and "ring".

The HEALPix WCS task, hpctran, provides a translation between the table row number and a spherical coordinate. It is up to the creator of the table to choose the spherical coordinate system. This might be an equatorial, galactic, or super-galactic system. There may be a keyword specifying the system. This is the case with WMAP data.

This task only provides the conversion. Access to the "pixel" data requires other tools. For binary tables the tables may be used.

This task allows the spherical coordinates to be input and output in three forms, as hours and degrees (e.g. RA/DEC), as degrees (e.g. l/b), and as radians. On input one may use sexagesimal since IRAF automatically converts this to decimal. On output the values are produced in decimal form.

The output is provide in two ways to provide flexibility in scripting. One is writing the results to the task parameters. Note that it is recommended that tasks which write to there parameter be "cached" with the cache command to avoid problems with background submission or multiple scripts running in parallel. The other output is printed to the standard output. Regardless of the direction of conversion the printed output is in the same order of row number, longitude, and latitude.

Examples

A CMB WMAP file is obtained and one wants the temperature at a particular point on the sky. Note that the WMAP format is "nested" and coordinate system is galactic.

cl> hpctran lng=50.12 lat=-33.45
2298092 50.12 -33.45000000000001
cl> = hpctran.row
2298092
cl> tdump wmap_iqusmap_r9_5yr_K1_v3.fits col=TEMPERATURE row=2298092
cl> tdump ("wmap_iqusmap_r9_5yr_K1_v3.fits", col="TEMPERATURE",
>>> row=hpctran.row)

Reference

HEALPIX - a Framework for High Resolution Discretization, and Fast Analysis of Data Distributed on the Sphere, by K.M. Gorski, Eric Hivon, A.J. Banday, B.D. Wandelt, F.K. Hansen, M. Reinecke, M. Bartelmann, 2005, ApJ 622, 759.

Credit

Some code from the HEALPix distribution at http://healpix.jpl.nasa.gov was translated to SPP for use in this routine.