The RASNIK real-time relative alignment monitor for the CDF inner tracking detectors

We describe the design and operation of the RASNIK optical relative alignment system designed for and installed on the CDF inner tracking detectors. The system provides low-cost minute-by-minute alignment monitoring with submicron precision. To reduce ambiguities, we modified the original three-element RASNIK design to a two-element one. Since the RASNIKs are located within 10–40 cm of the beam line, the systems were built from low-mass and radiation-hard components and are operated in a mode which reduces damage from radiation. We describe the data-acquisition system, which has been running without interruption since before the CDF detector was rolled into its collision hall in March 2001. We evaluate what has been learned about sources of detector motion from almost 2 years of RASNIK data and discuss possible improvements to the system.