title: Healpy: changing coordinates to a map and saving the new one
body: <p>I have a map in galactic coordinates and I need to save it in equatorial coordinates in another file .  I know i can use:</p>
  
  <pre><code>import healpy as hp
  map=hp.read_map('file.fits')
  map_rot=hp.mollview(map, coord=['G','C'], return_projected_map=True)
  </code></pre>
  
  <p>and this should return a 2D <code>numpy</code> array stored in <code>map_rot</code>. But when I read <code>map_rot</code>, I found out it is a masked_array filled ONLY with <code>-inf</code> values, and <code>mask=False</code> , <code>fill_value=-1.6735e+30</code>  (so, apparently, <code>-inf</code> is not a mask).   Moreover, the total number of elements of <code>map_rot</code> do not match with the number of pixels I would expect for a map (<code>npix=12*nside**2</code>). For example if <code>nside=256</code> I would expect to obtain <code>npix=786432</code>, while <code>map_rot</code> has <code>400*800=320000</code> elements. What's going on? </p>
  
  <p>(I have already seen <a href="https://stackoverflow.com/questions/24636372/apply-rotation-to-healpix-map-in-healpy">this post</a>, but I have a map in polarization, so I need to rotate Stokes' parameters. Since <code>mollview</code> knows how to do that, I was trying to obtain the new map directly from <code>mollview</code>. )</p>