Español | English
February 5, 2023

GTCdigital

News

Full list of news

The birth of the secondary mirror

11/08/2004

The secondary mirror of the Gran Telescopio CANARIAS (GTC) will receive light from the primary mirror and reflect it on to the Cassegrain focus or the tertiary mirror (from where it will travel to the Nasmyth or folded Cassegrain foci). It is an innovative system used on the latest large telescopes which can work in the optical and infrared ranges.

It is innovative because the secondary mirror on some telescopes has to be swapped when different wavelengths of light are studied, resulting in the loss of a whole day’s observation time or even longer.

In a meticulous manufacturing process, the mirror blank was produced, machined, cladded and polished and it passed through a number of processes including thermal treatment to give it long-term stability.

WHAT GOES INTO THE MAKING OF A SECONDARY MIRROR?

The mirror is made from beryllium, which is much stronger and lighter than glass, steel or aluminium. It needs to be made from a strong, light material because it has to be able to both move rapidly to correct rapid perturbations, and perform a technique known as automatic chopping, without being subject to the slightest oscillation within its own structure.

To perform as required, the mirror will sometimes need to move at a speed of 5hz – in other words it will have to oscillate 5 times per second. It will also need to be capable of rapid changes in direction, so that it can take two different images together – one of the object being observed and a second of the dark background. By contrasting these two images, diffuse background light can be eliminated so that accurate information about the object can be recorded.

The beryllium block was produced from beryllium dust, which is toxic and highly explosive and therefore required sensitive handling by highly skilled specialists.

In a process called “Hot Isostatic Pressure”, or HIP, a container of beryllium dust was subjected to high temperatures and pressures to compact it into a solid block. Steel, titanium and aluminium can also be consolidated using HIP.

Compacting results in a linear reduction of 30% - in other words, the material loses 2/3 of its volume. This method of obtaining the substrate was developed by the American company Brushwellman, the only company in the world that produces beryllium and beryllium products.

The solid beryllium was then machined by the American company Axsys. Its job was to lighten the material by removing up to 85% of the original substrate, leaving a piece between 3.5 and 7 millimetres thick. The beryllium was then coated with nickel. As well as protecting the beryllium from exposure, this coating allows the mirror to be polished as highly as is required. The optical surface side of the mirror starts off 125 microns thick (1 micron = 0.001 mm).

Next came the polishing process, which gave rise to the final optical shape by removing between 50 and 70 microns of the nickel coating. Cladding was wrapped around the mirror for this process, to ensure that the surface was polished to the same degree all over, even at its edges. Thermic treatments were also interspersed in the process to progressively relax the internal tensions produced in the beryllium substrate during the manufacturing, machining and nickel coating processes, which involved changes in temperature between –30 and 70 ºC.

Polishing this mirror is a complex process, due first to the fact that its surface is hyperboloid, rather than a sphere which would be easy to polish to a thickness of 32 microns; and second, because of the need to continue the optical shape right up to the quasi-hexagonal irregular edge which mimics the pattern of the segments in the primary mirror.

It was originally hoped that the mirror would weigh 55kg, but the specifications have been tightened since then: the mirror, which is 1.2m in diameter, will weigh no more than 38 kg.

Natalia R. Zelman

Full list of news