Automatic Alignment for the first science run of the Virgo interferometer

During the past few years a network of large-scale laser interferometers, including the Virgo detector, has been developed with the aim of detecting gravitational waves. To properly operate the detectors, the longitudinal and angular positions of the suspended detector test masses, the interferometer mirrors, must be kept within a small range from the operating point.The design of the Virgo angular control system, called Automatic Alignment is based on a modified version of the Anderson–Giordano technique, a wave-front sensing scheme which uses the modulation–demodulation technique.This paper will present the theoretical background of the Virgo Automatic Alignment system, the implementation issues and the performances observed during the first Virgo science run (VSR1). A total RMS of 4 × 10−2 to 3 × 10−3 μrad for all angular degrees of freedom has been achieved.