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A dollop of delicious guacamole (I)

09/05/2003

GUACAMOLE is the name of the Gran Telescopio CANARIAS' (GTC's) Acquisition, Guiding and Calibration Module, which will be used to track stars during observation.

Picture a giant 350-tonne metal structure that moves along perfectly smoothly, coordinating the various collections of steel supports, motors, mirrors, cables and instruments. They will all glide along, virtually unhindered by friction and with microscopic precision, locating and tracking objects billions of light years away across the Universe.

What you are picturing is the Gran Telescopio CANARIAS, a telescope currently under construction that will have the largest primary mirror in the world when it enters service in 2005. The mirror will be made up of 36 hexagonal segments, each 1.9 metres from vertex to vertex, giving a total surface area equivalent to that of a circular mirror 10.4 metres in diameter.

The larger a telescope’s primary mirror, the greater its capacity for viewing distant objects - in other words we will be able to see things that were previously out of view because we could not ‘reach’ them. However, the larger the primary mirror the more complex the systems, components and instruments that the telescope needs become. One of these instruments is the Acquisition and Guiding Device (A&G).

What does a Acquisition and Guiding Device do?

From our point of view, the sky moves. In reality, though, it is the earth that turns and this means that to observe a star we must follow it as it travels across the sky.

To do this, telescopes have advanced acquisition, guiding, and calibration systems to track objects for observation: they identify a star to serve as a guide, in the vicinity of the object being observed, and then take readings from it every few seconds. The information thus obtained is used to adjust the telescope’s position so that the object always remains at the same point within the focal plane. Without this the image would move and it would be impossible to study the features of an object.

The GTC’s A&G system is called GUACAMOLE (GUiding, Acquisition and CAlibration MOduLE) and it has been designed by the Project Office of GRANTECAN, the public company responsible for managing the telescope’s construction phase. Nicholas Devaney, who oversaw preliminary designs for GUACAMOLE, says that the system is “much more sophisticated” than any previously seen: “as well as guiding the telescope, it will control the active optics system and be fitted with wave front sensors developed by the project office.”

In other words, whereas A&G systems were used on previous large telescopes to locate and track objects, on the next generation they will deliver many other services. They will be sophisticated instruments acting as the telescope’s “quality controller,” with the job of continually optimising its performance.

All present and correct, Sir!

The GTC will have one A&G device per focus, but we will concentrate on those developed for the Nasmyth foci. They will be installed on the instrument rotators (which we have discussed in “Life's twists and turns…”). Each device will have two arms, both of them 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 any point in the focal plane so that any star in the field of view can be used as a guide.

To see how the arms will work and the instruments they will handle, you will have to wait for the next dollop of our delicious GUACAMOLE.

Natalia R. Zelman

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