The optical path length compensation (OPLC) system

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optics_layout

SUSI input optics

  • OPLC optical path length compensator carriage
  • LDC longitudinal dipersion corrector
  • OT optical table

In any interferometer it is necessary to keep the optical path difference (OPD) between the two arms very much less than the "coherence length" of the light. In SUSI the OPD arises principally from

  • the asymmetric distribution of siderostat stations about the centre of the instrument, and
  • the "astrometric" or external OPD governed by the position of the target star in the sky.

This second contribution can amount to several hundred metres at the longest SUSI baselines and changes continuously as the Earth rotates. Consequently the astrometric OPD is time-dependent.

The OPLC carriage system

The (OPLC) consists of two "cat's eye" retroreflecting telescopes, each consisting of an f/3 paraboloid with a small flat mirror at the focus. The flat allows small changes in OPD to be introduced by movement of the flat along the axis of the system. The retroreflecting telescopes mounted back to back on a kinematically located carriage which moves along a 70m precision rail track in the long tunnel section of the main building. Light from the two siderostats is directed towards the carriage from each end of the track. As the carriage moves it differentially adds path to one arm and removes it from the other, and consequently the total OPD range that can be compensated by the OPLC is ~140m. This range of compensation is sufficient for baselines up to ~200m. A second carriage that will provide an additional +/-280m of compensation will be installed when the longest baselines are commissioned.

The OPLC carriage is connected to a drive carriage via a spring coupling that minimises the transfer of any vibration from the drive carriage to the optics. A third "cable-laying" carriage connects signal and power cables to the OPLC without causing drag.

 large susi image

Not a very good view of the fine compensation carriage. The
two horizontally spaced holes allow the star light in and out of the
retro-reflecting optics on the "near" side of the carriage. The
vertically spaced holes do the same for the infrared metrology
beams. The retro-reflector is duplicated on the "far" side of the
carriage.

The laser metrology system

The laser metrology system measures the total OPD introduced by the OPLC and monitors the deviations from smooth tracking of the carriage. The system is a double laser homodyne interferometer designed and built for SUSI by the National Measurements Laboratory (NML) of the CSIRO Division of Telecommunications and Industrial Physics. Infrared lasers are used so as to be outside the sensitive range of the starlight detectors. OPD change is detected via the Doppler shift in the beat between a reference beam and a probe beam. Changes in the beat frequency correspond to movement of the compensator carriage. All positions are measured relative to a fiducial mark provided by an inductive transducer mounted next to the track.
oplc The 70m long path compensation system has carriages that move to equalise the light paths in the two arms of the interferometer. This view shows the fine compenation carriage on the righthand rails. The optics in the foreground is mostly part of the infrared pathlength monitoring interferometer which keeps precise track of where the carriage is along the rails.

The longitudinal dispersion corrector (LDC)

The astrometric OPD is actually a path difference in vacuum (the atmospheric paths above each siderostat are equal, apart from small path inequalities caused by turbulence and the Earth's curvature). In principle, the internal compensating path should also be in vacuum and for astrometric interferometers such as NPOI this is essential. For interferometers like SUSI that are designed specifically to measure angular dimensions or for imaging, it is not necessary and the internal path compensation is done in air. However since a difference in air path is introduced, the optical dispersion in the two arms of the interferometer will be different. This effect is usually referred to as (differential) longitudinal dispersion. The effects can be reduced to acceptable levels by introducing compensating amounts of suitably chosen glasses into the interferometer. The SUSI longitudinal dispersion corrector (LDC) uses a combination of BK7 and F7 glass blocks added to one or the other beam to change the glass paths by fixed amounts.

Beam combining system
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