docs: def _setup_subpix(self,nside=2**16):
          """
          Subpixels for random position generation.
          """
          # Only setup once...
          if hasattr(self,'subpix'): return
  
          # Simulate over full ROI
          self.roi_radius  = self.config['coords']['roi_radius']
  
          # Setup background spatial stuff
          logger.info("Setup subpixels...")
          self.nside_pixel = self.config['coords']['nside_pixel']
          self.nside_subpixel = self.nside_pixel * 2**4 # Could be config parameter
          epsilon = np.degrees(hp.max_pixrad(self.nside_pixel)) # Pad roi radius to cover edge healpix
          subpix = ugali.utils.healpix.query_disc(self.nside_subpixel,self.roi.vec,self.roi_radius+epsilon)
          superpix = ugali.utils.healpix.superpixel(subpix,self.nside_subpixel,self.nside_pixel)
          self.subpix = subpix[np.in1d(superpix,self.roi.pixels)]