Improved Adaptive Optics Mirror Delivered

In December 2011, the French company Safran–Reosc delivered their first thin shell Zerodur mirror to ESO (ann12015). The company has now topped this by delivering a second shell mirror [1] of even higher quality than the first. Both mirrors are a mere 2 millimetres thin but 1.12 metres in diameter, and will be crucial for the upgrade of Unit Telescope 4 of ESO’s Very Large Telescope (VLT) to a fully adaptive system [2]. A thin shell mirror is deformed several thousand times per second to compensate for turbulence in the Earth’s atmosphere. The result is a much sharper image, allowing astronomers to study the Universe in greater detail than possible before.

The new shell features many improvements. It has a surface accuracy that is better by a factor of five than the older one and very small forces are needed to correct the shape of the mirror’s surface, half that needed  for the first shell. This is of particular importance as it leaves plenty of force available to correct for image degradation produced by the atmospheric turbulence. The uniformity in the thickness of the shell has also been improved by a factor of three, and the finish of the mirror are also improved on both faces.

A second shell mirror is needed as these systems are unavoidably rather fragile. If the first mirror shell were to break, it would render one of the VLT Unit Telescopes inoperable for years as it awaited the manufacture of a replacement. The second shell is an insurance against such an unlikely, but catastrophic, scenario.

Although the second mirror was originally intended to be a spare it is so much better than the first, that it will eventually become the operational model while the first shell will become the spare.

The suppliers of the complete deformable secondary mirror system supporting the thin shell, Microgate and ADS International (Italy), have developed dozens of new procedures, handling tools, crates and transport boxes to ensure that every step of the way for this fragile mirror shell is safe. Paradoxically, the safest place for the shell is when it is mounted on the telescope and securely held by the magnetic fields provided by the 1170 voice coils.

The successful conclusion of this project shows a mastering of the technique by Safran–Reosc, and is a very positive outcome for the E-ELT telescope project. The application of these processes shows that this technology can be extended to the very large deformable mirror envisioned for the E-ELT which will have the same thickness, but with a much larger diameter of 2.5 metres.

Notes

[1] This second thin shell mirror will be finished with special coatings and 1170 magnets glued onto its concave rear face. When installed as part of the deformable secondary mirror system the magnets are controlled by voice coils that, when active, will produce a magnetic field that pulls on the fixed magnets and deforms the shape of the shell.

[2] With the installation of the new mirror, the adaptive correction is done by the telescope’s secondary mirror and all the telescope’s instruments benefit from vastly improved image quality. In earlier systems the correction was performed within the individual instruments.