After
leaving the Dove prism, the rotating line image needed
to be precisely positioned on the photographic film
containing the bubble chamber tracks. Two mirrors were
used to steer the beam, with one controlling the horizontal
and the other the vertical position.
The
exact position of the line image on the film could be
calculated if the exact positions of the mirrors were
known. Each mirror held two small prisms designed to
reflect laser light straight back in any direction.
This laser light could interfere with a reference
signal from the same laser, and by counting the number
of fringes produced by the interfering beams the movement
of the mirror could be calculated to within a wavelength
of laser light. This method of determining very short
distances is called interferometry.
The
line image could be positioned at any point on the film
to an accuracy of about 0.001 millimetres.
