Scientists at the Laser Zentrum Hannover (LZH) have developed and produced narrowband filters which blocks interfering signals for the European Extremely Large Telescope (E-ELT). In contrast to telescopes stationed in outer space, Earth-based telescopes inevitably measure interference signals from the atmosphere. In order to filter out this static, scientists at the LZH are developing special coatings for the optical components of the E-ELT.
The work group has already developed its first filters with a thickness of 60 to 100µm on a substrate with a diameter of 100mm. The narrowband filters block out interference and transmit signals into space in narrow bands in the infrared range (IR). The coating, made using ion beam sputtering, must have a homogeneity of 0.1 per cent or less.
Production of complex filters is a challenge, since a compromise between the coating thickness and the quality must always be found. Because of this, Dr Stefan Günster, head of the coatings group at LZH, has worked closely with the astronomers of the Max Planck Institute for Extraterrestrial Physics and the University Observatory of the Ludwig-Maximilian-Universität München. The filter coating should improve the signal-to-noise ratio which means the measurement time can be shortened, and thus more exact measurements are possible.
The relatively high physical thickness of the coatings places mechanical stress on the optics, meaning they can be deformed. Günster said: ‘We are presently working on processes, which can be used to monitor and control the form accuracy of the optics.’
As soon as the coating process has become established, the narrowband filters can also be used in other areas such as for biological and medical applications that need high quality measurements in the IR range.