This is a project to build a near infrared capability into the Robert Stobie Spectrograph (RSS) for the Southern African Large Telescope (SALT) 11 meter telescope. With this capability, the RSS will be able to simultaneously record spectroscopic data in the visible and the near infrared. This capability will open a new window for the discovery and study of the most distant and earliest galaxies in the universe by measuring the onset of star-formation and the luminosity function of the first star-forming galaxies.
The project will contribute to the training of a new generation of astronomical instrumental scientists including postdoctoral researchers, graduate students and undergraduate students.
Many of the fundamental breakthroughs and advances in astrophysics are enabled by advances in instrumentation. New discoveries or better understanding of the universe always follow technology that opens new observational areas. One such area is the infrared spectrum of light – light too red to be visible to humans – that comes from astronomical objects. Although infrared imagers have been in use on telescopes for quite some time, spectrographs that split the light into a spectrum of separate wavelengths have been slower in coming. Efficient infrared spectrographs that are able to observe multiple objects at once on the world’s largest telescopes are only now becoming operational. With such instruments, astronomers can survey extremely faint objects across the universe. Infrared wavelengths are longer than visible light, permitting the light to pass through the dust surrounding otherwise hidden, supermassive black holes at the centers of galaxies or newly forming stars inside dense gas clouds. Stellar features only seen in the infrared can be used to characterize the final stages of dying massive stars as they generate powerful winds by expelling their outer layers and finally explode as supernovas. In short, the infrared universe reveals unique answers to many questions of star and galaxy formation and evolution. The University of Wisconsin-Madison (UW) is building an infrared astronomical instrument called the Robert Stobie Spectrograph Near Infrared Arm (RSS-NIR). Its home will be the 11-meter diameter Southern African Large Telescope (SALT) in the Karoo Desert of South Africa. The RSS-NIR will operate at near infrared wavelengths out to 1700 nanometers. Its capabilities include a "swiss-army-knife" suite of different observing modes. It takes images, diffracts the light into infrared spectra, captures images in single shades of infrared colors via a tunable Fabry-Perot etalon and narrow band filters, and measures the degree of light polarization caused by reflecting off of or passing through different media like dust grains or magnetic fields in deep space. One of the challenges with astronomical observations in infrared light is the fact that all objects, including the instrument itself, emit infrared radiation. The amount of infrared emission goes up with the object’s temperature, like a heating element on a hot electric stove. This thermal radiation shows up in an astronomical image as contaminating background light that did not come from the source object. For faint objects that astronomers want to observe, the background from even a room temperature instrument can be larger than the signal from the target object, rendering it undetectable. To decrease the instrument thermal background contamination, the whole RSS-NIR has to be cooled to very low temperatures. Components of the RSS-NIR exist in compartments that have three different operating temperatures. The initial optical components operate at ambient observatory temperatures (-10oC to +25oC). After a dichroic, or two-color, element splits the visible and infrared light, the remaining parts of the RSS-NIR are enclosed by an insulated box called a pre-dewar and maintained at -40oC. Within that enclosure is a cryogenic dewar operating at -150oC containing the infrared detector that records the images. The -40oC pre-dewar enclosure houses many moving mechanisms that configure the instrument for different observation modes. For imaging, different optical filters are inserted into the beam. For spectroscopy, a diffraction grating is inserted into the beam and rotated to angles ranging from 10 to 100 degrees to observe different wavelength ranges. The angle of the diffracted beam through the grating must rotate as well. This requires that the camera and detector dewar also follow this angular motion around curved rails. The RSS-NIR is currently in its integration and testing phase in the laboratory at UW. When installed on the top of the telescope RSS-NIR will sit at a 37 degree angle and will rotate through 120 degrees. This requires that the mechanisms work through a range of varying gravity vectors, which means they must be tested under similar conditions on a simulator in the lab. After room temperature checkout is complete in early 2013, the pre-dewar enclosure will be installed and the entire instrument will be cooled to -40oC and run through the same series of tests. Only after passing all of these tests will it be ready for shipment to South Africa and integration onto the telescope. The SALT with the combined RSS-NIR and RSS-VIS is expected to be fully operational by 2015. Its sensitivity to infrared wavelengths will allow the telescope to observe more distant galaxies in the universe, to better peer down into dust enshrouded regions, and to observe diagnostics of stellar winds and supernovas that are not present in visible wavelengths.
