code: def run(self,field=None,simple=False,force=False):
          
          if field is None: fields = [1,2]
          else:             fields = [field]
          for filenames in self.filenames.compress(~self.filenames.mask['catalog']).data:
              infile = filenames['catalog']
              for f in fields:
                  outfile = filenames['mask_%i'%f]
                  if os.path.exists(outfile) and not force:
                      logger.info("Found %s; skipping..."%outfile)
                      continue
                  
                  pixels,maglims=self.calculate(infile,f,simple)
                  logger.info("Creating %s"%outfile)
                  outdir = mkdir(os.path.dirname(outfile))
                  data = odict()
                  data['PIXEL']=pixels
                  data['MAGLIM']=maglims.astype('f4')
                  ugali.utils.healpix.write_partial_map(outfile,data,
                                                        self.nside_pixel)
docs: Loop through pixels containing catalog objects and calculate
          the magnitude limit. This gets a bit convoluted due to all
          the different pixel resolutions...