Accurate filter positioning via HEIDENHAIN linear encoders
Observation of the Sun with hitherto unattainable accuracy
Already today solar researchers can see that there is activity on the solar surface. But current instrumentation does not enable them to see the causes. As of 2019 the DKIST on the Hawaiian island of Maui will make it possible to see them. With a mirror diameter of four meters it will be the largest solar telescope in the world and therefore provide a very detailed view of the Sun's surface.
The optical power of the DKIST is prerequisite for the instruments mounted on the telescope to be able to open up new insights into the processes on the Sun to scientists from all over the world. One of these instruments, the Visible Tunable Filter (VTF), is being developed by the Kiepenheuer Institute for Solar Physics (KIS) in Freiburg. The VTF will make it possible to examine precisely defined, very narrow wavelength bands of the light radiated from the solar surface. This permits the solar researchers to glean information among other things about plasma temperature, pressure ratios, magnetic field strengths and plasma movements on the solar surface and acquire data about changes in the magnetic field of the Sun.
The principle of the VTF is very simple. The sunlight is guided through an air gap between two coated, semitransparent glass plates. This causes interference of the light multiply reflected in the air gap. This leads to filtering of the wavelengths, the filtered spectral range results from the width of the air gap and thus from the distance between the glass plates. To be able to select one wavelength of sunlight to within a few picometers both plates have to be positioned absolutely parallel to each other with nanometer precision.
With the VTF, however, it is not just a matter of examining only one wavelength constantly. Much more interesting are the changes between the different wavelengths. Accordingly the glass plates will be moved permanently towards and away from each other in nanometer steps, in fact hundreds of times during the course of a two-hour measurement. For this the measuring system must be able to perform measuring steps of 20 pm. Furthermore, measurement errors over a period of one hour must not exceed a total of 100 pm. These are the dimensions of the diameters of atoms.
Tests are currently being performed on a scaled-down version of the VTF at the Kiepenheuer Institute for Solar Physics. Six HEIDENHAIN linear encoders of the LIP 382 model with standard scanning head and customized linear scale are mounted around the two glass plates. They determine the position of the plates—three for the top plate and three for the bottom plate. In the current test series the reliable and continuously repeatable accuracy of the setup has reached 0.17 nm per hour. The goal is 0.1 nm per hour. The KIS and HEIDENHAIN are going down that road hand in hand. Together the solar researchers and measuring technology specialists are achieving their goal step by step of having a completely new, high-precision view of the Sun's surface as of 2019.