The tip-tilt servo
Wavefront tilts due to atmospheric turbulence cause motion of the star image, which is removed by a tip-tilt servo system. The tip-tilt mirrors are mounted just behind the BRT. The tip-tilt detectors and their associated optics are located on the optical table with the beam combining optical system. Polarising beamsplitter cubes direct hlaf the total light to quadrant detectors which produce signals proportional to the angular misalignment of the beam in the vertical and horizontal directions. The detected errors are used to generate the actuating signals which drive the tip-tilt mirrors, with the low-frequency components used to correct for siderostat guiding and tracking errors.
The four tip-tilt actuating signals (horizontal and vertical for both beams) are connected to a real-time seeing monitor which measures the RMS fluctuations in the actuating signals during observation. The signals are sampled at up to 1kHz rate and the fluctuations are calculated over an integrating period chosen to coincide with the integrating time used for fringe visibility measurement. Much of the power at low frequencies is not due to seeing but arises from tracking errors in the siderostats. The true tip-tilt fluctuations are Gaussian, and are used to calculate the effective value of ro during an observation and this information is then used to partially correct the fringe visibility for losses due to seeing.
The atmospheric refraction correctors (ARC)
As with many astronomical instruments, SUSI is adversely affected by atmospheric refraction which elongates the star image in elevation, the effect worsening with increasing distance from the zenith. A set of Risley prisms in each beam is used to correct for this effect. Two pairs of prisms are used in each beam and are located near the output end of the BRT. The prism pairs consist of prisms of different glass and apex angle. The prism pairs are counter-rotated in order to produce the correct amount of refraction for actual zenith distance of the star being observed.
The star acquisition system
The field of view of the tip-tilt wavefront sensors is ~3 arc seconds, and a wide-field acquisition system is needed to acquire the star and align it to the main optical axis of the interferometer.
The SUSI acquisition system uses a pellicle beamsplitter located near the output of the BRT which can be moved into either the north or south beam. It reflects ~8% of the incident light to an intensified CCD camera. The field of view is ~8 arc minutes for the inner siderostats but this decreases to ~1.5 arc minutes for the outermost siderostats due to vignetting by the vacuum pipes.
The intensified video camera of the Star Acquisition
System. Behind it are the mirrors forming the top part of the
periscopes that lift the light from the outside (in the grey pipes) to
the level of the beam Reducing Telescope.