The Collaborative International Dictionary of English v.0.48:

Photochronograph \Pho`to*chron"o*graph\, n. [Photo- +
   chronograph.]
   1. (Physics) An instrument for recording minute intervals of
      time. The record is made by the power of a magnetic field,
      due to an electric signaling current, to turn the plane of
      polarization of light. A flash, coinciding in time and
      duration with the signal, is thus produced and is
      photographed on a moving plate.
      [Webster 1913 Suppl.]

   2. (Astron.) An instrument for the photographic recording of
      star transits.
      [Webster 1913 Suppl.]