Beam Combining System

Brief outline

The beam-combining optics for SUSI utilise dielectric coatings throughout. There are practical difficulties in producing coatings which perform satisfactorily over the full spectral range of 400 - 900nm, and accordingly SUSI has been designed to have two independent beam-combining systems to cover the spectral ranges of 400 - 550nm and 550 - 900nm, respectively.


The layout of the SUSI "blue" optical table.

* PBS polarizing beam splitters
* QD quadrant detectors (for tip-tilt correction)
* RQD reference quadrant detector
* BS main beamsplitter
* P dispersing prisms
* S passband-selection slits
* PMT signal photomultipliers

The layout of the "blue" beam-combining and detector optics is shown above. Dielectric polarising beamsplitter cubes (PBS) transmit primarily horizontally polarised light which is used for the interferometer. The vertically polarised light is reflected and is used by tip-tilt quadrant detectors (QD) for the tip-tilt servo system. The position of these two quadrant detectors is regularly adjusted to coincide with the reference quadrant detector (RQD). This ensures that the beams from the two arms of the interferometer overlap correctly at the dielectric beamsplitter plate (BS), where interference occurs.

Two monochromators, each consisting of a 60 degree prism (P), a camera lens and a slit (S), are used to select the wavelength and bandwidth of the light from the beamsplitter BS. Aperture wheels are located between the beamsplitter and the monochromators to define the actual effective aperture of the interferometer and allow it to be varied to suit the seeing conditions. The light is detected by photomultiplier tubes the interference fringe visibility is estimated from the number of photons detected by each photomultiplier.

Work has recently progressed on a 'red' beam combiner using a substantially different configuration, promising dramatically increased sensitivity. The new beam combiner operates between 600 and 900 nm and scans through the entire fringe packet 5 times per second. First fringes were acquired on Canopus on Tuesday 19 Nov 2002 and were held on the Red Table system for about an hour.

The alignment of the optical beams in a long-baseline interferometer is critical. Small angular errors can cause significant loss of signal. The beam combining optical table consequently incorporates additional alignment optics not shown on the diagram above. Light can be injected into the interferometer from an 'artificial star' and directed outwards through the beamsplitter BS all the way to the selected siderostats and then back again. The artificial star system is also used to establish the optical centre of the interferometer and to determine the vacuum OPD offsets due to the asymmetric distribution of the siderostats about the centre of the interferometer.


The main optical table in SUSI contains the optics to combine the "blue" light beams and detect the interference signal. It also has the optics and detectors to use half the light to remove image motion (tip-tilt correction), plus optics to inject an alignment beam into the system from the "back end". The other aperture wheels and shutters visible in this (very old) picture allow remote operation of all these systems from the control room.