PRIMA Star Separator for VLTI telescopes

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 VLTI telescope array in Paranal
The VLTI telescope array in Paranal

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.

STS requirements

The main requirements for the Star Separator are:

  • Simultaneous observation of two objects in the Coude focal plane. This is the key function. The objects are located within 2 arcmin of each other.
  • Pointing and tracking of stars. The earth rotates which results in a rotating Coude focal plane and therefore rotating images of the two objects. The Star Separator must compensate for this movement with an accuracy 0.01 arcsec and 0.002 arcsec resolution.
  • Chopping on a star. Pointing at an object and its dark background with 1-5 Hz frequency provides the option to subtract the background from the object image thereby greatly enhancing the result of the observation.
  • Tip-tilt control. The output should be controlled in tip-tilt to align it with the delay lines and subsequently the detectors.
  • Pupil alignment. The output pupil must be carefully aligned with the input pupil, to avoid vignetting and to ensure a proper overlap for the metrology system.

STS design description

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:

  • A field separator consisting of two concave spherical mirrors.
  • Field Selector (FS)-mirrors for pointing/tracking.
  • A Beam Compressor (BC)-mirror for re-imaging the Coude focus.
  • A Pupil Re-imager (PR)-mirror used to re-image the pupil at the specified position.
  • An off-axis parabola to produce a horizontal collimated output beam of 80 mm diameter.

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