SRON’s ‘immersed gratings’ are a candidate for use in the European Extremely Large Telescope. An SRON-led Dutch consortium – that also includes TNO, NOVA/Astron and Philips – will study if the technology can also be used for METIS, one of the eight planned instruments behind the telescope’s enormous mirror. The grating for METIS must be much larger and flatter than the current ultramodern gratings, and realising this will be an enormous technological challenge.
The European Extremely Large Telescope (E-ELT) will be constructed on CerroArmazones in Chile, a 3064 metre high mountain in the Atacama desert. The telescope will have a 42-metre diameter mirror, which will allow us to study the universe in even greater detail than we can with current telescopes on earth and in space. Special optical techniques will correct for distortions caused by the earth’s atmosphere so that the telescope will be able to make photos with a resolution almost 20 times greater than those of the Hubble space telescope. The E-ELT – designed by the European Southern Observatory (ESO) – will be able to observe objects that are about 15 times weaker than what we can see with current telescopes. The greatest scientific discoveries are expected in areas such as exoplanets, the first objects in the universe, the evolution of galaxies and hopefully in the area of dark energy and dark matter. The telescope is expected to make its first observations somewhere between 2020 and 2022.
METIS – full name The Mid-infrared E-ELT Imager and Spectrograph – is one of the eight proposed instruments for the E-ELT. It is the only instrument that can measure infrared radiation at wavelengths greater than 3 micrometres, which also makes it easier to realise corrections for image distortion. A recently available technology allows the radiation captured to be separated according to wavelength by using a specially developed combination of a prism and an optical grating. These so-called immersed gratings make it possible to drastically reduce the size of the instrument without reducing its performance. Immersed gratings are illuminated from an optically transparent medium, as a result of which the resolution and the dispersion of the grating increase with the refractive index of the medium.
Research into immersed gratings has been carried out in the United States since the 1990s. SRON and TNO have developed the technology further since 2006 for TROPOMI, an environmental satellite of ESA that will be launched in 2014-2015. If the utility of the immersed grating can be demonstrated with a prototype then that could mean a big boost for METIS. An immersed grating would allow a highly compact design for METIS, making it far easier to produce the necessary optical components. Due to the exceptionally stringent specifications for the light-capturing surface, spectral resolution, image quality and stability, E-ELT-METIS is expected to result in groundbreaking science.
Work on the immersed grating for METIS is being realised by SRON, NOVA/ASTRON, TNO and Philips Innovation Services.