'Nano'-smooth glass plates for the Visible Tunable Filter (VTF)

0.000 000 000 5 metre (= 0.5 nanometres = 0.5 nm), this tiny distance is how the two plates made of quartz glass fit together. Over an extension of 25 cm in diameter, the distance between the two plates varies by less than the distance of two neighbouring atoms in a silicon crystal. These two glass plates make up the Fabry-Perot interferometer of the VTF.

The graphic shows the deviation of the distance between the inner surfaces of both glass plates. The colour scale ranges from -2 and 2.5 nanometres, with an average deviation of 0.5 nanometres.

For the measurements, both etalon plates were aligned in a special mount, horizontally and parallel to each other. The photo shows one of the plates (diameter of 350 mm, thickness of 100 mm) being lowered into the measuring device. (Measurement and illustrations: Zygo and A. Fischer)

This Fabry-Pérot interferometer is the centrepiece of a spectrometer (“Visible Tunable Filter”, VTF) that is currently being developed at KIS.

After its completion, the VTF will be installed at the US-based Daniel K. Inouye Solar Telescope (DKIST), currently under construction on Maui (Hawaii). DKIST will be by far the world’s largest solar telescope.

The superpolishing of the glass plates and the final adjustment of their inner surfaces with a (mean) precision of 0.5 nanometres were carried out by the US company Zygo-Ametek, on behalf of KIS. Some weeks ago, KIS engineers inspected and approved the finished plates on site. This is the first time that Fabry-Pérot plates of this size and quality have been produced. The next step will be the reflective coating which will have to be done without affecting the flatness and the parallel adjustment of the gap. This will be another technical challenge.

For more information on the Visible Tunable Filter, see: www.leibniz-kis.de/de/projekte/visible-tunable-filter/instrument-description/