title: Plot a FITS image over a grid of the entire sky
body: <p>I would like to plot a FITS image over a sketch of the entire sky.</p>
  
  <p>I got this far in plotting the entire sky:</p>
  
  <pre><code>import matplotlib.pyplot as plt
  from astropy.utils.data import get_pkg_data_filename
  from astropy.io import fits
  
  image_file = get_pkg_data_filename('tutorials/FITS-images/HorseHead.fits')
  image_data = fits.getdata(image_file, ext=0)
  
  plt.subplot(111, projection='aitoff')
  plt.grid(True)
  plt.imshow(image_data, cmap='gray')
  plt.show()
  </code></pre>
  
  <p>But I can't seem to get the FITS image properly aligned with the grid</p>
  
  <hr>
  
  <p>The above code results in the following
  <a href="https://i.stack.imgur.com/rMEWY.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/rMEWY.png" alt="actual result"></a>
  But I'm trying to get something more like
  <a href="https://i.stack.imgur.com/p9UOo.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/p9UOo.png" alt="expected result"></a>
  where the blue square is the actual position of the image in <code>image_file</code></p>