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A dollop of delicious guacamole (II)

22/05/2003

Following stars across the sky... making friends with them, finding out what their lives have been like and letting them tell us what our own past and future hold... We will only be able to do all this if we can look at stars that are "fixed". This is where the Acquisition and Guiding devices (A&G) will come in.

Our A&G devices (discussed in the previous issue of “A Dollop of Delicious Guacamole”) will have two arms, on which various instruments will be mounted. Each will be fitted with a small mirror to deflect light from the guide star to the A&G instruments. The arms will be able to move to every point in the focal plane so that any star in the field of view can be used as a guide.

It will be like having our very own "spy in the sky", and in this issue we are going to look at how these A&G devices will work.

The all-seeing arm

On arm number one, two instruments will be mounted: an Acquisition and Slow Guide Camera (ASG) that will seek and track objects; and a Segment Figure Sensor that will monitor the shape of the segments that make up the telescope's primary mirror.

Once a guide star has been selected, the Acquisition and Slow Guide Camera will do what you would expect from an instrument of this kind: it will send the telescope the information it needs to track an object – and it will do it once every second. It will be a bit like a rifle sight , through which a marksman takes aim and fires at his target.

The Segment Figure Sensor will check that the 36 mirror segments are always aligned, reporting to the control system if any of them deviates from its optimal position. The control system will then make any adjustments needed to ensure that the primary mirror always behaves as a single surface.

This process, called 'phasing', will be performed by an unusual device: a matrix of microlenses. This is an array of tiny lenses (each half a millimetre in diameter) that are mounted in such a way as to mimic the shape of the primary mirror. They recreate the pattern of the mirror, determining the position of its segments to a margin of error of just thousandths of a millimetre. The whole process of monitoring and correcting errors in the shape and alignment of the mirror segments is called 'active optics'.

The innovations of arm 2

Arm 2 will be fitted with two instruments: the Fast Guide Camera (FGC), which will track at very high speeds, and the Segment Tilt Sensor (STS), which will monitor any optical errors in the telescope.

The difference between the Fast Guide Camera and the Slow Guide Camera is, obviously, speed. Whereas the slow camera will send information just once every second, the fast one will send it 200 times! The FGC’s main task will be to correct errors caused by vibration from the wind. It will do this by measuring the vibrations, sending information about them to the control system so that the secondary mirror can compensate using its chopping facility - a process that makes the mirroroscillate rapidly.

The last of the A&G device instruments, the Segment Tilt Sensor, will mainly be used to monitor any problems with focusing the telescope caused by misalignment of the primary and secondary mirrors. The sensor is so fast that it will also be used to monitor atmospheric turbulence and will play a vital part in setting up 'queued observation', a system that will determine what kind of observation is most suited to the prevailing atmospheric conditions at any given time.

What we have in our hands...

We have managed to integrate four different techniques to achieve one single objective: moving a massive structure with incredible ease and precision. Its urge to tell the story of the Universe will be unstoppable, its desire to pursue distant stars, galaxies and dreams endless. They were all beyond our reach until now. They will give science a new flavour, different from what has gone before: the flavour of GUACAMOLE.

FACTS ABOUT THE G&A DEVICE

Each focus of the GTC – in other words, both Nasmyth foci, the Cassegrain focus and the folded Cassegrain foci - will be fitted with a guiding and acquisition device. This will optimise image quality from whichever focus is being used for observation.

The "Advanced Mechanical and Optical Systems" (AMOS, S.A.) company, of Liège in Belgium, has been awarded the contract to build the guiding, acquisition and calibration systems for the GTC's Nasmyth foci. The company has experience in this field, having developed similar systems for the four VLT telescopes (Very Large Telescopes) at the European South Observatory (ESO) in Chile. The detectors are provided by E2V. The elements (CCD heads) where these detectors are lodged are being constructed by NTE and the detector controllers are being constructed by the IAC, under design of Greg Burley (Magellan Project) and modifications from the Project Office of the GTC.

Natalia R. Zelman

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