code: def get_expstart(header,primary_hdr):
      
  
      if 'expstart' in primary_hdr:
          exphdr = primary_hdr
      else:
          exphdr = header
  
      if 'EXPSTART' in exphdr:
          expstart = float(exphdr['EXPSTART'])
          expend = float(exphdr['EXPEND'])
      else:
          expstart = 0.
          expend = 0.0
  
      return (expstart,expend)
docs: shouldn't this just be defined in the instrument subclass of imageobject?