Add reverse dict for flats indices

nabu/preproc/ccd.py

@@ -171,6 +171,7 @@ class FlatField(CCDProcessing):
         for flat_idx, flat_url in self.flats.items():
             self.flats_arr[flat_idx] = self.flats_reader.get_data(flat_url)
         self._sorted_flat_indices = sorted(self.flats.keys())
+        self._flat2arrayidx = {k: v for k, v in zip(self._sorted_flat_indices, np.arange(self.n_flats))}
 
     def _load_darks(self):
         self.n_darks = len(self.darks)