Refactor to simplify DataFrame access

Author: Jack3690

src/pista/imaging.py

@@ -357,8 +357,9 @@ def xy_to_radec(self):
                              [s,  c]])
 
         # Extract pixel coordinates
-        x = self.df['x'].values
-        y = self.df['y'].values
+        df = self.df
+        x = df['x'].values
+        y = df['y'].values
 
         # Convert to sky
         ra, dec = w.pixel_to_world_values(x, y)