A wavefront tilt correction servo for the Sydney University Stellar Interferometer

The tilt correction servo for the Sydney University Stellar Interferometer (SUSI) consists of a 'pyramid' detector and piezo-electrically controlled tilt mirrors. The system measures image position and re-centers it with a sample frequency of 1000 Hz thereby holding the two beams of the interferometer parallel with a standard deviation of 0.164 ± 0.025 arcseconds. With an aperture size of 0.06 metres this implies less than a 2% loss in the visibility measurements made by SUSI. The servo has been used to magnitude 6.5 stars and is predicted to have a limiting magnitude of 7.5 and possibly as high as 8.5. The system not only corrects for the tilt introduced by the atmosphere but will supply a good estimate of seeing conditions using the same optical path through the atmosphere as the visibility measurements of the interferometer.