Over the coming months, the world’s largest Fabry-Pérot interferometer (FPI) will be assembled and tested at the Kiepenheuer Institute for Solar Physics. Among its main components are two high-purity quartz glass disks, each coated on one side with a highly reflective layer. The coated surface has a diameter of 28 cm and its mean evenness does not exceed the incredible value of 0.000 000 001 m. By way of comparison, if lake Titisee in the Black Forest had this evenness, this would imply that the height of its waves remains below 5 micrometers (1/10 of a hair). On a smaller scale, the roughness of the coated surface is even 10 times better, reaching the size of individual atoms.

A few weeks ago, KIS delivered the flight unit of the image stabilization unit for the Photospheric and Helioseismic Imager (PHI) of the Solar Orbiter Mission. This was an important milestone for the project, and at the same time it was the first space-qualified instrument that had been developed, built and tested at KIS. In the meantime, and after detailed tests and calibration measurements, PHI was delivered to ESA for integration to the spacecraft.

In order to compensate image blurring caused by turbulence in Earth's atmosphere, present ground-based solar telescopes are equipped with a technology called Adaptive Optics (AO): It uses a single deformable mirror which can change its shape very rapidly to compensate the image blurring in realtime.

The solar atmosphere shows a wide variety of wave phenomena. Among them are the bouyancy-driven internal gravity waves (IGWs) - a phenomenon common in the terrestrial atmosphere and oceans. We use realistic numerical simulations to study how these waves emanate from the solar surface and propagate through an unmagnetized or a magnetized atmosphere.

The energy from the solar interior is transported by convection to the solar surface. When observing the lowest layer of the solar atmosphere with a high-resolution solar telescope the granular structure of the hot up-flowing gas cells becomes recognizable. Recently, scientists from the Kiepenheuer Institut for Solar Physics have performed unprecedented spectroscopic observations with the Vacuum Tower Telescope (VTT) on Tenerife to systematically investigate the large-scale convective motion in the photosphere. The measurement of accurate absolute velocities was enabled by the scientific instrument LARS (Laser Absolute Reference Spectrograph) which employs a state-of-the-art Laser Frequency Comb as a calibration ruler for the solar spectrum.

The solar 11-year cycle affects many of the observable values of the Sun. In the course of the solar cycle, the oscillation frequencies of the Sun and the amplitudes of these oscillations change marginally but measurably. Solar oscillations are standing sound waves inside the Sun. Similarly to sound waves here on Earth, the pressure within the medium is key to the way in these waves travel. This is why these oscillations are also referred to as p modes (p for pressure). Changes in the p-mode characteristics throughout the solar cycle are due to the varying strength of the magnetic field over time at different depths inside the Sun.

The solar telescope GREGOR enables scientists to measure magnetic fields and flows on the Sun with unprecedented accuracy. The first results are now available.

At times of strong magnetic activity, the Sun offers spectacular sights of violent eruptions, evolving sunspots and strong magnetic fields. But there are also times at which it seems as though nothing is happening on the Sun surface with its almost regular pattern of grains referred to as granules. These ‘quiet’ regions also contain magnetic fields but they are very weak and therefore difficult to measure. Scientists at Kiepenheuer Institute for Solar Physics work with the GREGOR solar telescope, inaugurated in 2012, to analyse sunspots and their finely chased structure with an accuracy never seen before.

This and other first GREGOR results will be presented in nine articles of a special edition of Astronomy & Astrophysics.

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 VTF (Visbile Tuneable Filter) is a two-dimensional high-resolution spectropolarimeter and is currently developed at the Kiepenheuer Institute. As one of the state-of-the-art scientific instruments at the future 4-m-class telescope DKIST on Hawaii it will play a major role in the next decade of solar observations.

The setup of the VTF consists of several Fabry-Pérot interferometers (FPI), a wavelength-dependend narrow pre-filter and a polarization modulator. Due to the requested accuracy of the physical measurements, an investigation of the instrumental impacts on the physical data acquisition was required in advance. Based on the inquired simulations, the requirements for manufacturing were inferred and strategies for the data calibration developed.

LARS, the Laser Absolute Reference Spectrograph of the Kiepenheuer-Institute is a scientific instrument for the ultramodern observation of the Sun at the Vacuum Tower Telescope (VTT) at the Observatorio del Teide on Tenerife. LARS enables the measurement of the solar spectrum of a selected field-of-view with the high-resolution Echelle-spectrograph of the VTT. Additionally, the emission spectrum of the newly installed Laser Frequency Comb is superimposed with the solar spectrum. Since each emission peak of the comb spectrum represents one well-defined frequency, the solar spectrum can be calibrated on an absolute wavelength scale. Figuratively, the comb serves as a ruler for the spectral lines. The accuracy is of the order of m/s or below and is therefore a multiple better than former devices. The successful upgrade of the Laser Frequency Comb in May 2016 enables the continuous operation of this worldwide unique spectroscopic observation of the Sun.