The Beam Trajectory Monitor for the TTF-FEL at DESY

The design, construction and first tests of a prototype of the beam trajectory monitor at the TTF-FEL are presented. With this monitor the off-axis spontaneous undulator radiation is imaged through a set of pinholes onto silicon pixel detectors. This allows to measure simultaneously 12 points of the transverse beam position with an accuracy of better than 10 $\mu$m over a length of 5 m. The prototype consists of a pinhole mask and the pixel detectors accurately placed to each other inside a vacuum chamber which is held under beam vacuum. We first present the design and the production technology of the silicon pixel detector. Then the optimization of the pinhole mask and its realization using lithographic methods are discussed. To determine its spatial accuracy the detector is scanned using a laser test stand and a low-intensity electron beam of an electron microscope. The symmetry axis of the detector defines a straight line within 0.4 $\mu$m. Additionally, the electron microscope is used to study the radiation hardness of the detector. The sensitivity of the detector to low energy X-rays is measured using a vacuum ultraviolet beam at the synchrotron light source HASYLAB. Finally first calibration measurementsof the BTM prototype detectors are shown