The HEALPix Software Package

Contents and purposes

We have developed a package of HEALPix based mathematical software, consisting of C, C++, Fortran90, IDL/GDL, Java and Python source codes as well as documentation and examples. Successful installation produces a set of facilities using standardised FITS I/O interfaces (https://heasarc.gsfc.nasa.gov/docs/software/fitsio) as well as libraries which users can link to their own applications. Among the tasks performed by the components of the HEALPix package are the following:

• Simulation of the full sky CMB temperature and polarisation maps as realisations of random Gaussian fields, with an option to constrain the realisation by prior information.
• Analysis of the full sky CMB temperature and polarisation maps resulting in power spectra and/or spherical harmonic coefficients. Relevant conventions are given in Appendix A. Note that the convention used for polarization has been changed in release 1.2!.
• Global smoothing of whole sky maps with a Gaussian kernel.
• Degradation and upgrade of the resolution of discrete maps.
• Global searches on the maps for nearest-neighbours and the maxima/minima of the discretised functions.
• Algebraic conversion of the maps between the RING and NESTED numbering schemes, and mapping back and forth between positions on the sphere and discrete pixel index space.
• Pixel queries for various geometrical shapes (discs, triangles, polygons ...)
• Visualisation of the HEALPix formatted sky maps in the Mollweide, orthographic, cartesian cylindrical and gnomonic projections of the whole sky or small areas of it.

The package includes documents which describe the installation process, the facilities available and a large number of subroutines contained in the various libraries. It is available to the scientific community at https://healpix.sourceforge.io.

HEALPix was the format chosen by the WMAP collaboration for the production of sky maps (see https://map.gsfc.nasa.gov/news/index.html) from the mission data.

HEALPix software and format have been used by the HFI and LFI consortia of Planck collaboration for the simulation, analysis and data release of Planck data (see https://www.cosmos.esa.int/web/planck).

HEALPix software has been selected by the GAIA satellite mission currently surveying our Galaxy (see https://www.esa.int/Our_Activities/Space_Science/Gaia_overview)

Licensing

The full-fledged HEALPix software package described above and in the accompanying documentation can be downloaded from https://sourceforge.net/projects/healpix/files/latest/download under version 2 of the GNU General Public License (GPLv2) which is described at length in https://www.gnu.org/licenses/old-licenses/gpl-2.0-faq.html.

Beside this, a subset of C routines called healpix_bare, and providing support for back and forth conversion between pixel indexes and angular or vector coordinates on the sphere, and between RING and NESTED pixel indexes, is available from https://sourceforge.net/projects/healpix/files/healpix_bare_1.0 under a more permissive 3-clause BSD license. It was developed with a focus on accuracy, reliability and clarity, with good timing performances, and is intended for developers of BSD-licensed HEALPix related codes, so that they won't have to reimplement some central algorithms.
However, for ultimate optimization, and to get all the features of HEALPix, the GPLv2-licensed package must be used.