How to convert and save healpy map to different coordinate system?

Question

I have a healpix map with galactic coordinate. I would like to convert that map into equatorial coordinate system. I know that I could plot map in equatorial coordinate using mollview() function. Do you have any way to save such transformed map?

Thanks Vinu

Answer 1

The following function changes the coordinates system of a map.

def change_coord(m, coord):
    """ Change coordinates of a HEALPIX map

    Parameters
    ----------
    m : map or array of maps
      map(s) to be rotated
    coord : sequence of two character
      First character is the coordinate system of m, second character
      is the coordinate system of the output map. As in HEALPIX, allowed
      coordinate systems are 'G' (galactic), 'E' (ecliptic) or 'C' (equatorial)

    Example
    -------
    The following rotate m from galactic to equatorial coordinates.
    Notice that m can contain both temperature and polarization.
    >>>> change_coord(m, ['G', 'C'])
    """
    # Basic HEALPix parameters
    npix = m.shape[-1]
    nside = hp.npix2nside(npix)
    ang = hp.pix2ang(nside, np.arange(npix))

    # Select the coordinate transformation
    rot = hp.Rotator(coord=reversed(coord))

    # Convert the coordinates
    new_ang = rot(*ang)
    new_pix = hp.ang2pix(nside, *new_ang)

    return m[..., new_pix]

The example is your case