The European Southern Observatory (ESO) is developing the PRIMA facility for the Very Large Telescope Interferometer (VLTI) in Chile. PRIMA will enable interferometric imaging of very faint objects and performing high precision astrometry. Key science objectives are detection of exoplanets and exploration of nuclear regions of galaxies. TNO has built 8 PRIMA Star Separators (STS) for both the Unit Telescopes (UT) and Auxiliary Telescopes (AT).
The PRIMA facility is designed to enable simultaneous interferometric observations of two objects. Depending on the mode of operation, PRIMA can be used either to produce images of the fainter of the two objects using a phase reference technique (imaging mode) or to measure the angular separation between the two objects (astrometry mode). In the astrometry mode, PRIMA will be able to detect exoplanets and brown dwarfs. The Star Separator (STS) is an opto-mechanical system, designed to separate the light of the two astronomical objects and feed it into the long stroke delay line. The STS compensates for field rotation, stabilises the beam tip tilt and adjust the lateral and axial alignment of the pupil. Chopping and/or counter-chopping on the science object or the guide star has also been implemented. The STS system is a unique combination of precision optics, advanced mechanical design and sophisticated control engineering.
The main requirements for the Star Separator are:
The design of both UT and AT Star Separators is athermal, i.e. the system only scales for any drift in temperature expected at the telescope focus during a night. Hence temperature does not influence the performance. The mechanisms are based on flexures, resulting in a very stiff system without backlash and hysteresis. The STS first eigenfrequency is above 100Hz. The chopping frequency is 5 Hz with an amplitude of 1.5". The rise time is 20ms and there is neither over-shooting nor noticeable ripples on the plateau. The angular resolution of the mechanisms is 15 nanorad with a repeatability of 40 nanorad. The STS consists of the following elements: