A Night with the Southern African Large Telescope

You can’t look through the giant telescopes of today; they are just not made that way.   The business ends of professional telescopes are engineered to take electronic detecting instruments not eyepieces.   Nor can you steer them around they sky.   Instead astronomers email in what they want a telescope to look at and a computer works out when the instrument will do the job and, when the time arrives, steers the leviathan around the sky.    The new Southern African Large Telescope (SALT) is one such instrument.  It sits on the high plateau of the Great Karoo arid region in the Northern Cape of South Africa, shiny and new.   And that’s were we found it in August 2005 when I took a group of 12^th graders out to South Africa to make teaching resources about the telescope for SA and UK schools.   SALT is due to be officially opened in November, and the “first light” declaration is imminent, so engineers and scientists were busy tweaking it to make sure that it wowed its audience.

We met SALT’s Systems Engineer Gerhard Swart and Project Scientist David Buckley in the warm and cosy control room, in front of a bank of computer screens.   They were both dressed in arctic gear and festooned with mountaineering accessories.   David also sported an enormous fur hat.   The Karoo is the coldest place in South Africa and this was the middle of winter. The night temperature had plunged to -16^oC the previous week.  They explained that they were getting the giant telescope ready to use one of its detecting instruments for the first time; a monstrous digital camera delightfully named SALTICAM.     We followed them into the dome.  Everything about this beast is big.   I apologise in advance for running the gamut of superlatives.    What we saw was a cross between the inside of a Medieval cathedral and the Death Star from Star Wars.  The cavernous interior contained a huge metallic framework supporting a curved surface that looked like a giant honey comb. It was cold outside already and dropping fast but the dome’s air conditioning was working overtime to equal the chill of the night sky and stop temperature differences spoiling its view.   Gerhard showed us how the telescope is moved it azimuth – it’s a hover craft!    The underside of its mount is studded with meter wide circular rubber skirts which, when inflated, glide over a surface that is (according to Gerhard) “the smoothest piece of concrete in Africa ”.  

Suddenly we really were in a scene from star wars.   The sounds were incredible.   The enormous telescope rose up and pirouetted around at a surprisingly sprightly rate.    At the same time the colossal roof of the dome started to rotate the other way and the huge cover to the outside world started to open.   Outside, backed by a starry sky, a giant stared down into the mouth of the telescope like some cosmic dentist. 

SALT’s primary mirror is made up from 91 hexagonal sections, each a metre wide, giving it a light grasp of up to 11 metres in diameter.   The mirrors talk to each other.   Their edges are covered in transmitters and sensors so that they can detect the positions of their neighbours.   Yet these are only members of an orchestra; the conductor is inside a 34m tower next to the telescope.    At the top of the tower, a laser fires down onto SALT’s mirrors, detecting their position and angle and feeding signals to them to move into alignment at the start of the night.

Gerhard took the lift to the dome’s upper gallery.   He would use the mountaineering ropes and attachment to attach himself securely to the business end of the telescope 13m above the segmented mirror.   SALT was built for only one tenth of the cost of a conventional giant telescope.   The saving was in the mount.   To be able to precisely position a mirror array in line with an astronomical object takes fiendishly expensive engineering.   So SALT doesn’t bother.   Its mirror is fixed at 37^o to the vertical and the hovercraft azimuth movement only sets the telescope in the direction of the target and then drops it gently into place.   After that all steering is done by the tracker that Gerhard had strapped himself to.   This rides on two sets of rails at right angles to ach other, giving 12^o of movement to the telescopes field of view.   At any one time the telescope is able to view objects in a band around the sky between 31^o and 43^o to the vertical.   But as the Earth rotates 70% of the Karoo ’s night sky (approximately between declination -75 and +10) slips into SALT’s viewing window at some time. OK, so 30% can never be seen but is it worth spending ten times the cost of SALT to get that remaining bit?  There’s an awful lot of Cosmos that SALT can explore in 70% of the Southern sky.  

Inside the control room David stared at his monitors.   We waited for the computer to be engaged and start on its automatic observing tasks.   Instead David said “Right, where shall we go?”  It was at this point I realised I was at a star party with the world’s newest giant telescope.  

All those researchers around the world who have paid for SALT to do observing for them expect the telescope to operate flawlessly, much as an amateur expects a newly bought instrument to work perfectly out of the box.   To achieve this, “first light” is not really first light; engineers have to spend months setting up the scope and tweaking it until it is perfect.   This is what David and Gerhard were doing.   They spent the day making adjustments to SALT, not knowing how long it would take.   Here we were at 11.30pm .    It would have been a waste of time putting together a schedule that started at 8!   So as we looked over David’s shoulder, he selected interesting objects from a list that SALT’s computers said were visible in its viewing window.   Then at the click of a mouse, the enormous telescope picked itself up, slewed to that part of the sky, set itself down and aligned it’s tracker to a globular cluster deep in the sky over the heart of the Milky Way.    Here the giant eye could stare at the 10 billion year old group of stars for just over an hour.  Any object in SALT’s “window” can be observed for a maximum of between 50 minutes to 3 hours, depending on its distance from the celestial pole.  The further from the pole, the quicker the Earth’s rotation would move the object through SALT’s 12^o field of view. As the photons of light from the globular cluster fell on the CCDs of SALTICAM after their thirty thousand year journey across the galaxy, the computer monitors resolved it into a swarm of stars

By now it was very late and I had to get the students three hundred miles to the shores of the Indian Ocean by the next evening.    So with great reluctance I had to leave this amazing star party with David slewing from one vista to the next and Gerhard up and down high above the mirror adjusting and readjusting to get things just perfect.    As we drove through the early morning darkness back to our beds in the nearby town of Sutherland, I thought about how lucky we were to be at the birth of an instrument as powerful and as beautiful as SALT.  Whenever I hear that its observations have helped to solve another cosmic mystery, I will remember the thrill of hearing the words “Right, where shall we go?”

M. Cripps 16 08 05