title: Obtaining the integral of a healpy map
body: <p>I have a healpy map that is the deflection (gradient of the lensing potential), which was obtained during a particular lensing simulation of the CMB. I want to obtain a map of the lensing potential using healpy if possible. I notice that there is a healpy function <code>alm2map_der1()</code> which will give me a healpy map and its first derivative given the map's alms. I am assuming this first derivative is the gradient of the map - please correct me if I am wrong. Essentially I want to know if I can use healpy to do the backwards process of this. I want to remove the gradient and I just want the lensing potential.</p>
  
  <p>So far, my attempt has been to use the relation between the deflection and lensing potential power spectra; Cls of deflection = l(l+1) * Cls of lensing potential, and rearranging this to: Cls of lensing potential = Cls of deflection / l(l+1), then using synfast to convert this back into a map. I do not seem to be getting the correct map.</p>
  
  <p>Is there a better way to do what I am trying to do? Maybe even not using healpy?</p>