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Parts of a whole!

13/11/2003

This August, construction of the GTC's primary mirror reached an important milestone: the achieving of the master segment. This first segment will help to refine the polishing process - it will be used for calibration to ensure that the other mirrors come up to the required specifications.

The 36 segments of the Gran Telescopio CANARIAS’ (GTC’s) primary mirror, together with 6 spares, will have to go through a series of exhaustive tests before they meet the required specifications. The Zerodur™ blanks will then be polished, a technologically complex process that must be properly carried out if the mirrors are to give a faithful reflection of the Universe.

The first segment, known as the “master segment” contains all the information needed to work out the details of the process that the other segments will go through. This information is vital for optimising the process and for working out how long it will take for all the mirror segments to be polished.

THE PACE-SETTING SEGMENT

The German “Schott” company produced the blanks for the GTC’s primary mirror segments, 42 hexagonal blocks that are 1.9 metres in diameter, 8 centimetres thick, weigh 470 kilos and made of Zerodur™, a vitreous ceramic material extraordinarily uniform in composition and undergoes virtually no change as the temperature fluctuates.

When completed, the blanks were sent to the French company SAGEM for polishing.

One of the difficulties inherent in producing segments like the GTC’s is their shape: rather than being spherical, the segments will form a hyperboloid which means that each of them will be slightly different from the others and makes the whole process is much more complicated.

During the first stage in the polishing process, called “spherical planing”, some of the surface of the Zerodur™ block will be removed to make it ready for polishing. Next will come an aspherical “lapping” followed by the first stage in a pre-polishing process that will cause the Zerodur™’s surface to change from matt to shiny. From this point on the segment will be precisely measured using optical techniques (interferometry) and a repetitive computer controlled polishing process will correct any surface defects. As the segment comes close to its final shape it will be placed into its permanent support cradle before being polished with ions to remove any remaining tiny defects, to bring it up to the optical quality that the GTC demands.

To test whether the polishing process has completed correctly, the mirror will be put to work. Light will be shone at it and the reflection analysed to find out if the information sent back from the surface of the mirror is accurate. However, because the surface is not spherical, normal measuring methods - directing light at the centre of curvature and expecting it to be reflected back to the same point - cannot be used. All we would see if we used this method would be a big blot.

Instead, an instrument known as a “null corrector” is used. This grasps the light reflected back by the hyperboloid mirror and concentrates it (“artificially”) at a single point. The light reflected by the segment will then be analysed by interferometry, mapping the topography of the mirror surface to decide whether it is good enough or if it will have to be re-polished.

This first segment, which has been given the name “La Palma” and the technical name “OP-M1-SG-002-003”, is known as the master segment. The segments that follow it will have the same radius of curvature and will be shaped for the position they will occupy in relation to the master segment. This is so that the segments will function as a single mirror when they are all put together. To achieve this we will have to take “associative” measurements - 5 segments will be placed next to the master and measurements will be taken simultaneously from each of them. This segment, which was the first to be produced, will therefore be the last to be handed over for use at the telescope.

Now that this milestone has been reached, the SAGEM plant can start the massive task of polishing the GTC mirror segments.

Natalia R. Zelman

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