A near-infrared spectrograph—an instrument that splits light into its individual colors, or wavelengths, to form a spectrum like a rainbow—will soon be shipped to South Africa from UW-Madison. The next-generation instrumentation, custom-built by staff in UW–Madison’s Washburn Astronomical Laboratories, just passed pre-shipping review and is on schedule to be disassembled and soon shipped for installation and use at the Southern African Large Telescope (SALT) in the remote, desert highland of the Karoo near Sutherland, South Africa.
The unit will be ready to use by August at SALT.
Jeff Percival, a senior scientist, worked on computer programming and software control systems. Mike Smith, an instrumental engineer, designed and built the hardware.
The principal investigator, Senior Scientist Marsha Wolf, illuminated some key points for us.
Why does a telescope need a spectrograph?
A spectrograph is essentially a big digital camera with an extra optical component called a diffraction grating that splits light into colors. A spectrum contains features that are like fingerprints from different chemical elements that allow us to learn more about the objects we observe. For example, the general color of a galaxy can tell us something about its age. A blue color tells us that it contains populations of young stars, while a red color indicates that it contains older stars that are passively aging. However, if we have a spectrum of the galaxy’s light, we can get a much more detailed estimate of its chemical makeup, its specific age, and how many episodes of star formation it has had over the last billion years.
Why are we sending this to SALT?
SALT is the largest single optical telescope in the southern hemisphere and among the largest telescopes in the world. It is funded by the South African government and a consortium of institutions worldwide, including UW-Madison. We contribute 16 percent of the telescope’s budget annually (read more about our partnership with SALT here). A primary mission of Washburn Labs is to develop new instrumentation for our observing facilities that will enhance the department’s research.
Has the astronomy department ever built a telescope before?
The UW Astronomy Department’s Washburn Astronomical Laboratories has built instruments for telescopes before (not the telescopes themselves, though we have done some telescope subsystems). An existing spectrograph on the SALT telescope that operates at visible wavelengths, the Robert Stobie Spectrograph (RSS-VIS), was led by UW Astronomy Emeritus Professor Ken Nordsieck and completed in 2005. This spectrograph is a workhorse instrument at the telescope for observing galaxies, faint extended gaseous nebula, supernovae, stars, and more.
More recently, Washburn Labs built a part of the NEID precision radial velocity spectrograph on the 3.5-meter WIYN Telescope in Arizona. This instrument pushes the state-of-the-art for observing exoplanets, or planets orbiting other stars outside our solar system. The part we built was the telescope port adapter that precisely places the star image on the optical fiber that feeds the spectrograph in the same way every time (to within 3 micrometers of the center of the 60-micrometer diameter fiber) and maintains its alignment on the fiber with an active guiding system that updates every 20-thousandths of a second during the observations.
How will this spectrograph be used at the South African Astronomical Observatory?
Currently, SALT’s instruments all operate at visible wavelengths. UW’s new near infrared spectrograph will open a new observing regime at near infrared (NIR) wavelengths. The NIR waveband of the electromagnetic spectrum is important for a couple reasons. First, it allows us to observe more distant galaxies. Because the universe is so large, and is expanding, light from distant galaxies gets redshifted (shifted to longer wavelengths) on its way to us over the millions or billions of years of transit time to reach us. Light that was emitted from the galaxy at visible wavelengths is detected on Earth as infrared wavelengths. By observing the distant galaxy with a NIR spectrograph, we can study the same spectral features that we would study in the visible spectrum of a nearby galaxy.
Secondly, there are some features in the spectra of nearby objects that only show up in the infrared. For example, when stars explode as supernovae, emission lines in the NIR tell us how powerful the explosion was. Another example occurs when the supermassive black hole at the center of a galaxy begins actively accreting new material (gas and dust) from its host galaxy and becomes an active galactic nucleus, or AGN. During this phase the black hole can form giant jets of plasma that punch through the interstellar medium (gas between the stars) of the galaxy and out into surrounding space. As the jets shoot through the galaxy, they can form shocks in the galaxy’s gas that show up as features in the NIR spectrum. We can piece together a story of how the AGN may have regulated the evolution of the galaxy.
Why is this important for UW?
This new instrument is important for astronomers at UW because it will allow us to observe an entire galaxy at once, rather than needing to make 212 separate observations, which greatly increases the observing efficiency.
Another benefit of providing a new instrument is that we get more observing time on the telescope. We plan to conduct a large galaxy survey with the extra time and make the data public, as a lasting legacy to the field of astronomy.
