code: def get_exptime(self, img):
          
          header = self.get_header(img)
          if 'EXPTIME' in header.keys():
              etime = header['EXPTIME']
  
          elif 'EXPOSED' in header.keys():
              etime = header['EXPOSED']
          else:
              etime = 1.0
          return etime
docs: Obtain EXPTIME