Scientists know surprisingly little about what happens on the Sun. Solar researchers want to change this with the new Daniel K. Inouye Solar Telescope (DKIST) on the island of Maui, Hawaii. The Kiepenheuer Institute for Solar Physics (KIS) in Freiburg, Germany is developing a Visible Tunable Filter (VTF) for this project. With a mirror diameter of four meters, it will be the largest solar telescope in the world, and therefore will provide a very detailed view of the Sun’s surface. The filter adjustment is controlled by HEIDENHAIN linear encoders with an accuracy of under one nanometer.
“To date, all we ever see is a sum of changes on the Sun. With the new telescope, we will be able to resolve structures on the Sun with a size of 20 km, and thus recognize individual changes,” said Professor Wolfgang Schmidt, Director of Observations at the Kiepenheuer Institute. This resolution is perfectly adequate to see in detail the processes happening on the Sun’s surface. According to the solar researchers, not much happens below this resolution on the Sun compared to Earth or other planets with cold or solid surfaces. This is because the gas structure of the Sun and the prevailing temperatures of more than 6000 °C cause the atoms to move too much.
Prof. Schmidt takes a simple example to demonstrate the enormous optical power required for this view of the solar surface. “To observe the Sun from Earth through a telescope and recognize structures with a size of 20 km is equivalent to reading a newspaper from a distance of 40 km — the actual text, not just the headlines.”
Instruments Analyze Solar Images
The optical power of the DKIST is prerequisite for the instruments mounted on the telescope to open new insights into the processes on the Sun. One of these instruments, the VTF, will make it possible to examine precisely defined, very-narrow-wavelength bands of light radiated from the solar surface. This permits researchers to glean information 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 results in interference of the light, which is repeatedly reflected in the air gap. Another result is the filtering of the wavelengths, and the filtered spectral range results from the width of the air gap and thus from the distance between the glass plates. The entire VTF, including the supporting structure, will be two stories high and weigh about four tons. One glass plate alone weighs more than 20 kg.
Accuracy in Dimensions of an Atom
In stark contrast to these dimensions are the requirements for the positioning of the glass plates. To be able to select one wavelength of sunlight to within a few picometers, both plates must be positioned absolutely parallel to each other, with nanometer precision. Here, 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, hundreds of times during the course of a two-hour measurement.
For this measuring system, that means a lot of high-precision work. In order to obtain the required positioning accuracy for each step over and over again, 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.
For the VTF to work properly, the construction is tested in a scaled-down version with a laser that functions as the Sun. Its beam is guided via prisms, mirrors, and lenses through the glass plates and back again to a high-speed camera that shoots 40,000 frames per second. These camera images are analyzed to draw conclusions about the accuracy attained in positioning the glass plates.
Six HEIDENHAIN LIP 382 linear encoders 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. The acquired position values go to the control electronics that adjust the position of the upper plate via piezos.
So why is the position of the lower plate measured when only the upper plate is moved? At first, the position of the upper plate was measured with the linear encoders. The team was not satisfied with the accuracy. The whole system reacts to the tiniest of changes — a temperature difference of just one hundredth of a degree affects the position of the lower plate. In order to capture such fluctuations, three additional linear encoders were implemented for the lower plate, which is permanently fixed. In the current test series, the reliable and continuously repeatable accuracy of the setup has now reached 0.17 nm per hour — the aim is 0.1 nm per hour.
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