On May 11th, I drove 14 hours deep into the South African Karoo, a region similar to western New Mexico to visit the largest astronomical observatory in the Southern hemisphere. The whole area is very remote, and there are only a few places to stay. The Karoo is semi-desert, with low rainfall, and the darkest skies in the country, easily ranking in at number one on the Bortle sky brightness scale (with #1 is the darkest skies on earth). In the photo below, I drove our Land Rover high up on a hill to overlook the Karoo landscape.
Overlooking South Africa's Karoo
After driving over 20 km from the small town of Sutherland, we were in the middle of no-where, and had another 8 km to go. This is where we stayed for 3 days, at a the Blesfontein Guest Farm, a delightful cozy, yet isolated place which contained over 1,000 sheep on a 28,000 acre farm. Talk about isolation.. and dark! This is an astronomer’s paradise.
Driving across the Karoo
We stayed in a guest cottage near the main farmhouse, which included an indoor braai (grill), and a sunken tub! The cottage, called the “Cow Shed”, also had a full kitchen and the beds were lined with electric blankets as the temperature approached freezing this weekend. In winter (June-August) they get snow, one of the few places in South Africa to do so. Being an isolated sheep farm, you can probably guess what our meals consisted of – lots of mutton and good ol’ South African wine. The price was about $22 a night.
The Cowshed guest house
The town of Sutherland was built in the mid 1800’s and has one stop-sign on it’s main street, and about 10 Bed & Breakfasts with names like ‘Galileo”, “Galaxy”, “Andromeda”, “Kosmos”, “Jupiter” and others so you can tell who they cater to! Most places even have telescopes for guests to use. Our farmhouse had two, mounted in a roll-off roof observatory.
Southerland is home to the South African Astronomical Observatory (SAAO), situated high atop a hill about 15 kilometers from town. The SAAO is now home to the largest single telescope in the Southern Hemisphere, the Southern African Large Telescope (S.A.L.T.). The telescope was completed in 2005 and first light obtained in November of that year. Although completed, the scope is now being used mainly for on-going testing, calibration, and fixing numerous mechanical and optical problems that are not unusual for a telescope of this size and complexity.
While daytime public tours are available, I was granted special permission to spend a night working with the SALT astronomers and engineers and have access to the whole telescope and control rooms. This once-in-a-lifetime opportunity was the main reason I drove the 16 hour distance to get here. Upon arrival I was met by Russian astronomer Alexei Kniazev who gave me a brief tour of the living quarters and then invited me to dine with him and all the other staff astronomers and engineers before heading up to the observatory.
S.A.L.T. Observatory
During dinner Mr. Kniazev gave me an briefing on the overall history and function of the S.A.L.T. obsevatory, followed by an indepth tour of the complete facilities.
The S.A.L.T. primary mirror measures in at 11 meters across, and is composed of 91 individual hexagonal mirrors. Each mirror weighs 500 kilograms and were made in Russia from a material called Astrosital, a glass-ceramic material with very low thermal coefficient, which means it does not expand or contract within normal operating temperatures. The glass was then shipped to Eastman Kodak for cutting and polishing, and finally shipped to the observatory for mirroring with a 100 nm layer of aluminum.
S.A.L.T. Primary Mirror
The primary mirror’s 91 mirrors are arranged and collimated into a sphere, and use a Center of Curvature Alignment Sensor (CCAS) laser system to measure and align the individual mirrors into a perfect sphere. The laser is situated within a high tower outside the observatory and is prominent in the photo above. Every few days this laser is used to align the mirrors and 8 sensors on each mirror segment constantly measure each position and track that information back in the control room. A computer makes tiny adjustments as needed through 3 small servos attached to the back of each mirror to keep the mirrors aligned.
Primary mirror supports, sensors and servos
The telescope’s most unusual property is that it is fixed in altitude at 37 degrees. This altitude was chosen as it is centered on the Magellanic clouds, a key target to study in the Southern skies. To track the sky, the telescope’s optical system payload, which sits 13 meters above the primary at focus, tracks across the mirror. Being able to use any part of the mirror is possible as the mirror is a sphere, so there is no real center. The payload tracking gimbal is able to move 3 meters in any direction, which allows the telescope to track an object for 6 degrees at a time. This gives astronomers from 1 to 3 hours of track-time depending on the location of the target in the sky. The gimbal uses a laser to measure the distance to the primary mirror as it tracks, and moves the payload inward or outward as needed to keep the optics in focus. To prevent field-rotation, the platform also can rotate as the sky revolves overhead. The payload, and tracking platform combined weigh just over 5 tons but can track to accuracy of 5 microns!
The camera and gimbal for the S.A.L.T.
This photo on the right shows the payload system, containing a 50-filter cartridge, and a $600,000 CCD camera called the SALTICAM.
The payload can also carry the Robert Stobie Spectograph (RSS) which is the star of the S.A.L.T. This instrument, designed by a joint effort of the University of Wisconsin and Rutgers University is designed mainly to study objects in the ultra violet wavelength down to 320 nm. Some of the SALT project scientists, including my guide for the night, will use the RSS to make detailed studies of planetary nebulas in very distant galaxies to help understand the composition of very early stars. Planetary nebulas are useful in this regard as they show the physical composition of stars at the last phase of a stars life. You get to see what stars are made of, at the moment of their deaths, when they have virtually stopped creating new elements.
Since the telescope can rotate 360 degrees in azimuth, it can cover 71% of the sky at some point during a year. Astronomers queue up targets for observation well in advance, and computers are used to schedule observations based on when a target will be observable, taking into consideration the moon and obstruction by the tall CCAS tower.
The observatory itself contains the telescope, control room, mirror coating room, and a payload workshop. They also have room located under the primary mirror to house a future spectroscopic instrument currently under development. The main observatory bay, with the telescope, has many temperature and humidity sensors. The entire environment is controlled by computers to regulate the interior observatory temperature and humidity one full day in advance. Meteorological instruments and computer modeling predict the nights observing conditions and use air conditioning and dozens of louvers to match the observatory dome’s environment. As a side-note, the louvers are computer controlled in such a way as to prevent moonlight from shining on the primary. As the moon moves across the sky (or as the dome rotates) louvers that open towards the moon close and those on the opposite side on the dome open. Very clever! You can see all the louvers in my photo of the outside of the observatory above.
The telescope’s massive mechanical tracking system, primary mirrors, complex environmental controls, and optical imaging systems are all controlled from one control room. The optical-fiber feeds from these components are kept at the same temperature by keeping them, and their computers at the same temperature with an ethylene glycol cooling system. The final results of these computers are then fed into the warm control room.
Me (far left) in the control room
Only two people, an equipment operator and a project astronomer, can operate the S.A.L.T.. This gives you some idea of the computing power of the whole system.
Computer workstation
The SALT team is based out of Cape Town, South Africa, which is a 4-hour drive. An astronomer and engineer take turns spending a week at the observatory, lodging in a small hostel located below the telescope hill. The hostel includes a decent astronomical library, computers, pool table and a staffed kitchen. I had dinner with the night’s astronomers and the kitchen staff made me a midnight lunch-box to take up to the SALT. There was plenty of coffee, tea, and caffeinated cold drinks.
Like any new complicated system, there are bound to be bugs. Many of the environmental defects and system bugs have been worked out, but two main problems are preventing the SALT from being completely active at this time. The RSS (spectroscopic imager) which was designed to record ultra violet had a major flaw; it’s main lens absorbed ultraviolet light instead of passing it! This was due to a manufacturing error in which a plastic mixing container was used to hold the glue that held the lens layers together. This glue (which was designed to pass ultraviolet light) absorbed traces of the plastic mixing cup. After a few months, the plastic molecules changed (probably due to ultraviolet exposure) and began to absorb ultraviolet light, thus blocking the very light the astronomers needed to record! The SALT team sent the camera back to Wisconsin where a new lens was built. It took almost a year to solve the problem, but they finally created a perfect lens that had no contamination. Then they broke it! A technician indivertibly used the wrong kind of lens cleaning spray, a Freon based solvent, which froze the lens and cracked it. It’s taken more than 6 months to build a new one, which should be shipped to South Africa any day now.
A mirror fragment from the S.A.L.T
A few weeks ago, while changing out one of the mirror segments they dropped it at the only part of the operation that had no safety controls to prevent a mirror from falling. It smashed to the floor and shattered. They now have 3 spare mirror segments left. I was given a piece of the broken one as a souvenir!
The other problem is that the SALT design team hired a non-astronomer to design the collimation laser, the CCAS. Remember the laser shines continuously at the primary during tracking, to keep the optical payload at precise focus. This engineer installed a laser that worked at 673 nm, the same spectrum as ionized sulfur, a key element when analyzing star composition! Now, when they look at a star, that entire region of light is awash in laser light. It’s back to drawing board on that one.
So while much of the work at SALT is still faultfinding and correcting design flaws, here’s one of the first photos taken by SALT of 47 Tucanae, before the scope’s adaptive optics are operational.
http://www.salt.ac.za/fileadmin/files/telescope/first_light/PR47tuc.jpg
The hill that the S.A.L.T. sits on a barren rocky hill. In the following photo you can see the small buildings that house the astronomer’s living quarters and the large S.A.L.T. observatory to the left. The hill is also home to several other telescopes.
SAAO Site
The oldest telescope is the 1.9 meter (almost 6 feet) diameter built in the 1930′s.
1.9 meter SAAO telescope
SAAO-1-9 meter scope counterweights and drive system
This beautiful scope, now retrofitted with the most advanced digital cameras and spectroscopic imagers still get a lot of use, but astronomy students from all over South Africa and beyond.
Sunset at the S.A.L.T.
So I drove 16 hours in pristine clear skies to a once-in-a-lifetime opportunity to visit the Southern African Large telescope and spend a few days under the most incredible skies in the world with my own telescope. I arrived at 7PM and by 8PM the entire sky clouded over and winds picked up to 30 kph. This stayed the same until the night I went up to the SALT, where the winds rose to 48 kph with gusts to 60. The whole sky was clouded over the whole night, and all the next day and night too.
Finally at 0430AM when I was packing up, the sky cleared and the winds stopped completely. As I drove across the Karoo on the way home, the whole sky was filled with stars and its inky black background. I was in awe at the incredible beauty of the night sky, but also terribly disappointed at not being able to do any observing with my scope and especially the S.A.L.T.
One note… if you ever go to a location with a darkness rating of Bortel 1, on a moonless night and the sky completely overcast, you will find yourself in total darkness like being in a deep cave. It’s an incredible experience as I felt my way across the parking lot to find my car and could not see anything. Not even my hand 2 inches from my face. I literally walked into my car, bumping my shin on its bumper. I had to search with my hands to locate the door and finally get some light from the interior. Wow.
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By the way – as I headed into the city, I realized that it’s impossible to think of “Paddington Station” without thinking of a bear in a large hat. It just can’t be done.
