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pixell is a library for loading, manipulating and analyzing maps stored in rectangular pixelization. It is mainly targeted for use with maps of the sky (e.g. CMB intensity and polarization maps, stacks of 21 cm intensity maps, binned galaxy positions or shear) in cylindrical projection, but its core functionality is more general. It extends numpy’s ndarray to an ndmap class that associates a World Coordinate System (WCS) with a numpy array. It includes tools for Fourier transforms (through numpy or pyfft) and spherical harmonic transforms (through libsharp) of such maps and tools for visualization (through the Python Image Library).


  • Python>=2.7 or Python>=3.4
  • gcc/gfortran or Intel compilers (clang might not work out of the box)
  • libsharp (downloaded and installed)
  • automake (for libsharp compilation)
  • healpy, Cython, astropy, numpy, scipy, matplotlib, pyyaml, h5py, Pillow (Python Image Library)


For installation instructions specific to NERSC/cori, see NERSC.

For installation instructions specific to MacOS X, see MACOSX (h/t Thibaut Louis).

For all other, below are general instructions.

To install, clone this repository and run:

$ python setup.py install --user

To test the installation, you can run:

$ py.test

You may need to install pytest for the above to work (with pip install pytest –user).

Existing libsharp installation (optional)

Libsharp is installed automatically by setup.py. If instead you want to use an existing libsharp installation, you can do so by symlinking the libsharp directory into a directory called _deps in the root directory, such that the file pixell/_deps/libsharp/libsharp/sharp.c exists.

Intel compilers

Intel compilers might require a two step installation as follows

$ python setup.py build_ext -i --fcompiler=intelem --compiler=intelem
$ python setup.py install --user


If you have write access to this repository, please:

  1. create a new branch
  2. push your changes to that branch
  3. merge or rebase to get in sync with master
  4. submit a pull request on github

If you do not have write access, create a fork of this repository and proceed as described above. For more details, see Contributing.