Beam position monitor calibration at the FLASH linac at DESY

In the FLASH (free electron LASer in Hamburg) facility at DESY more than 60 BPMs (beam position monitors) with single bunch resolution are currently installed, and more are planned for future installation. Their calibration has been initially made by measuring each electronics board in the RF laboratory. However the ultimate calibration of each monitor is made by measuring its response to beam movement. This is a time- consuming procedure depending on the availability and accuracy of other components of the machine such as steering magnets. On the other hand it has the advantage of getting in one measurement the answer of the monitor with all its components and of being independent of the monitor type. The calibration procedure and particularities for various types of BPMs in various parts of the linac will be discussed. A procedure based on orbit response measurements is also now under study. This would significantly speed up the calibration procedure, which is particularly important in larger accelerators such as the European XFEL (X-ray free electron laser), to be built at DESY.