Clear view for the Extremely Large Telescope – LZH develops IR narrowband filter

Image of the planned E-ELT, in which the IR narrowband filter of the LZH can be found / Source: Swinburne Astronomy Productions/ESO

When telescopes look into space, they usually have only a limited view, even on star-bright nights – molecules in the atmosphere „block“ their view. Scientists at the Laser Zentrum Hannover e.V. (LZH) have developed and produced narrowband filters for the European Extremely Large Telescope (E-ELT), which block interfering signals.

In contrast to telescopes stationed in outer space, earth based telescopes inevitably measure interference signals in 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 Coatings has already developed first filters with a thickness of 60 to 100 µm on a substrate with a diameter of 100 mm. 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 homogeneity of 0.1% 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, works 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. This means the measurement time can be shortened, and thus more exact measurements are possible.

The scientists working with Dr. Günster use a special wideband filter monitor developed by the LZH, which covers a very wide wavelength range of 380 to 1700 nm. “With this instrument we can monitor the coating during production, and optimize it if necessary”, explains Stefan Günster. “Since coating the filter is very sophisticated, due to its thickness, we can save considerable development time doing so.”

The relatively high physical thickness of the coatings places mechanical stress on the optics: the filters are deformed and influence the measurements. “We are presently working on processes, which can be used to monitor and control the form accuracy of the optics,” adds Dr. Günster.

As soon as the coating process has become established, the narrowband filters can also be used in other areas: for example for biological and medical applications that need high quality measurements in the IR range.

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