docs: def mjd_obsc(self):
          """Given a CFHT Megaprime image header compute the center of exposure.
  
          This routine uses the calibration provide by Richard Wainscoat:
  
          From: Richard Wainscoat <rjw@ifa.hawaii.edu>
          Subject: Fwd: timing for MegaCam
          Date: April 24, 2015 at 7:23:09 PM EDT
          To: David Tholen <tholen@ifa.hawaii.edu>
  
          Looks like the midpoint of the exposure is now UTCEND - 0.73 sec - (exposure time  / 2  )
          Wobble on the 0.73 sec is +/- 0.15 sec.
  
          @rtype : float
          """
          # TODO Check if this exposure was taken afer or before correction needed.
  
          try:
              utc_end = self['UTCEND']
              exposure_time = float(self['EXPTIME'])
              date_obs = self['DATE-OBS']
          except KeyError as ke:
              raise KeyError("Header missing keyword: {}, required for MJD-OBSC computation".format(ke.args[0]))
  
          utc_end = Time(date_obs + "T" + utc_end)
          utc_cen = utc_end - TimeDelta(0.73, format='sec') - TimeDelta(exposure_time / 2.0, format='sec')
          return round(utc_cen.mjd, 7)