The Visible Tunable Filter (VTF), which has been designed and built at KIS for a good 10 years in cooperation with NSO (Boulder, USA), MPS (Göttingen, Germany) and IRSOL (Locarno, Switzerland), is nearing completion.

The heart of the VTF, a Fabry-Perot interferometer, colloquially known as an etalon, has been built, measured, tested and optimized over the last five years in a separate laboratory under controlled, clean conditions. It consists of two superimposed flat glass plates, 35 cm in diameter and 10 cm thick, whose spacing is controlled by three piezo actuators. The plates, made of the purest optical glass, were coated on both sides by contractors in the USA and France and then measured and further processed alternately by the two partners in order to obtain surfaces that are as smooth and flat as possible. Both the small-scale surface roughness and the large-scale variations of the plate spacing are in the range of a few atoms (less than 2 nanometers) over the entire effective area of the etalon. The plate spacing is measured by means of optical sensors, whose measurement accuracy has been significantly improved by the VTF team in collaboration with the manufacturer especially for this purpose. The plate spacing is measured several thousand times per second and the voltages on the piezo actuators are readjusted accordingly via a powerful computer. Sufficient stability of the plate spacing, less than 150 picometers variation over periods of several hours, also requires good temperature stabilization of the etalon. The spectral resolution of the etalon is greatly influenced by how accurately and stably the plate spacing can be measured and maintained.

All in all, the etalon is a very sensitive optical component in operation and must be protected against external influences in order to function within specifications, but even when switched off it must not be subjected to large shocks in order to prevent damage. A few weeks ago, the etalon had to be moved from the in-house lab to the VTF lab and installed so that the entire instrument could be configured and tested for the first time. This took place in preparation for the official Lab Acceptance Test (LAT) in the presence of an NSO team, which is being performed this week with the goal of verifying the correct operation and performance of the instrument before it is dismantled and shipped to the Daniel K. Inouye Solar Telescope (DKIST) on Maui, Hawaii. The relocation of the etalon was meticulously planned, all steps of the process defined, and practiced several times with a dummy.

The pictures give an impression of the move. After the move and first functional tests, all covers were mounted on the VTF and the temperature control was switched on. All test measurements within the LAT were controlled via network so that the VTF laboratory did not have to be entered.