SUSI input optics
The fringe visibility measured by a stellar interferometer is reduced by atmospheric turbulence. To minimize the visibility loss, the aperture size should be no larger than ~ro ("r nought") unless higher order adaptive optics is employed. In good seeing (~1 arc second image size), ro ranges from ~8 to 15cm across the spectrum from blue to red. As a result, the effective maximum aperture size in SUSI was chosen to be 14cm.
The siderostats and siderostat stations
Following the design of the prototype interferometer, it was also decided to use siderostats consisting of plane, alt-azimuth mounted mirrors which direct starlight in a fixed output direction. Because siderostats are not used at normal incidence their effective aperture is reduced by a cosine factor. The SUSI siderostats have been designed with a mirror diameter of 20cm in order to achieve the desired clear aperture of 14cm.
A siderostat station, showing the 20cm mirror in its alt-az mounting (the siderostat), which feeds starlight into one of the north or south periscopes mounted on either side of it.
The siderostats reflect the incident starlight to one of two relay mirrors which are located directly north and south of the siderostat. The relay mirror is chosen which minimises the angle of incidence used for the particular observation. The relay mirrors direct light vertically downward into the vacuum system through optical windows. Movable relay mirrors, located in chambers within the siderostat pier, then direct the light from the station to the centre of the baseline and thence into the main optical laboratory. These lower mirrors can be moved in or out of the main beam path and are used to select the siderostat configuration to be used for a particular observation. The two relay mirrors together form a periscope.
In order to avoid additional seeing degradation, the optical paths from the siderostats to the main beam combining laboratory are contained within a system of pipes in which a modest static vacuum is maintained during observations.
The siderostats housings are insulated and have manually operated roll-off roofs which allow the housing to be completely open while observing. This minimises any local seeing effects. As well, the electronics which service the stations are enclosed in weatherproof cabinets located ~3m west of the main housings.
In order to achieve the desired wavelength and obtain high reflectivity, all the mirrors in SUSI use enhanced silver coatings, with the exception of the siderostat mirrors which are somewhat exposed to the weather.