The longitudinal polarimeter (LPOL) at HERA exploits the asymmetry in the Compton cross section by scattering circularly polarized photons off longitudinally polarized electrons. The interaction point is located 39 m downstream of the center of the HERMES target. The backscattered photons are detected in a calorimeter that measures the total energy of the photons. Due to their very large boost almost all photons are scattered into a very small cone around 180 degrees in the laboratory frame and travel along the electron beam. In order to separate the photons from the electron beam a dipole magnet is used to bend the electrons by 0.54 mrad. This is enough to extract the photons from the beam line 16 m downstream of this bend. The backscattered photons are detected using a crystal calorimeter made from four independent crystal detectors, which allow the simultaneous determination of the energy and position of the scattered photon on the face of the detector.

A view into the HERA tunnel with the laser beam guided above the HERA beam line for adjustment.
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