May 27, 2016

By Jake Parks

SALT below the night sky in Sutherland, South Africa. Photo Credit: The University of Texas McDonald.Anyone who wears glasses is familiar with the annoying role humidity can play in our ability to see clearly. Now, just imagine how annoying it would be if changing humidity did not only fog up your glasses, but it also warped the lenses themselves—screwing with your focus. Well, the latter is exactly the problem the Southern African Large Telescope (SALT) has faced for the past decade.

SALT—which is located at the South African Astronomical Observatory (SAAO) near Sutherland, about 400 km from Cape Town—is the largest single optical telescope in the southern hemisphere. SALT’s primary mirror is actually composed of 91 individual, 1-meter wide hexagonal mirrors. When all 91 of SALT’s mirror segments are properly aligned and pointing towards a common focus, they effectively act as one enormous, 10-meter spherical mirror.

Unfortunately, because SALT is located in the Karoo desert—where the temperature and humidity can vary drastically throughout the night—maintaining the necessary mirror alignment requires constant vigilance. As the temperature changes throughout the night, the steel truss that supports the primary mirror expands and contracts, which can throw off the focus of the entire telescope

SALT has a system in place to initially set the proper mirror alignment called the Center of Curvature Alignment System (CCAS). This system relies on a laser which shines down on SALT’s primary mirror. The reflected light spots from the CCAS laser are then measured by a sensor, which determines the tip and tilt required for each individual mirror segment to properly align the mirror.

After initial alignment using CCAS, an additional system called the Segment Alignment Maintenance System (SAMS) maintains the mirror alignment over the course of the night. However, when SALT was originally commissioned in 2006, the sensors responsible for measuring the separation and height of each mirror segment relied on capacitors—which are very sensitive to humidity. Though the capacitive sensors should theoretically be able to correct for changes in humidity throughout the night, they eventually proved impractical and were abandoned in 2008.

According to Marsha Wolf, Senior Scientist at the UW-Madison Department of Astronomy, “historically, the 91 mirror segments had to be aligned once to multiple times during the night,” with each alignment costing approximately 20 minutes of precious observing time. Though 20 minutes may not seem like much, this time really adds up.

In order to eliminate the time needed to realign the mirror segments because of changes in temperature, SALT recently began testing a new SAMS. The upgraded SAMS relies on a collection of inductive, rather than capacitive, sensors mounted underneath the individual mirror segments of SALT, which are designed to maintain alignment across humidity swings. Ultimately, the goal of SAMS will be to maintain proper mirror alignment for up to five nights of observing in a row.

Though SAMS is still in the final testing phases, SALT observers were recently treated to their first night of observing which did not require initial mirror alignment. And with SAMS up and running, researchers expect SALT to soon see huge improvements in both data quality and telescope efficiency.

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