High Resolution Spectropolarimetry

High resolution sunpot image acquired with GREGOR: Spatial scales of 0.07 arcsec are resolved. This corresponds to the diffraction limit of a 1.5 m aperture. Hence, the demanding specifications of the GREGOR telescope are fullfilled. For the quality of this image adaptive optics system of GREGOR is essential, as it has to correct for internal as well as atmospheric turbulences in the air. This image was reconstructed from a burst 100 images, taking advantage of the speckle code 'KISIP'.

The goal of this research focus is to develop and deploy forefront instrumentation and data analysis methods for high-precision spectropolarimetry with a spatial resolution which approaches the diffraction limit of the new generation of solar telescopes whose apertures exceed one meter. To achieve this goal we undertake the development in the following areas:

  • Wave front control, with the emphasis on image stabilization, adaptive optics, multi-conjugate adaptive optics, and control algorithms
  • High precision spectrographs, including FPI spectrometer development, grating spectrometer development and modeling of advanced optical components and systems
  • Detectors, with the focus on camera readout and preprocessing systems, high throughput detectors and special detectors for spectropolarimetry
  • Image restoration methods, including statistical image processing, and wave front estimation and deconvolution

Main activities of KIS in the area of wave front control was the development of the Multi-Conjugate AO system for Gregor, which started in the optics lab of the institute in Freiburg and continued at GREGOR. The system includes three deformable mirrors which compensate different layers of atmospheric turbulence along the line of sight. The control software has been nearly completely rewritten and integrated into a new user interface based on Qt.

The development of the “high order ground layer adaptive optics system” for Gregor took place in parallel to these efforts. Heart of this system is a deformable mirror with 256 actuators, which permits a higher precision wavefront control compared to the first-light AO for Gregor where the deformable mirror has only 80 actuators. This system is expected to control at least 120 modes of wavefront deformation. The system was installed at the telescope early 2012.
In order to support the operation of MCAO, a method was developed to sense the height distribution of turbulence in the Earth’s atmosphere by observing angle-of-arrival fluctuations at several positions on the solar limb. The results compare well with the results obtained with stellar scintillometers. This technique will be developed as a permanent facility at the observatory.

A major development in spectroscopy is the laser frequency comb, which is integrated into the echelle spectrograph of the VTT. With this system we measure the solar spectrum with a highly accurate, high stability frequency scale. This project is funded by the Leibniz society in collaboration with scientists from the Max-Planck-Institut für Quantenoptik in Garching. It will be so far the only LFC at a solar telescope in the world.

The characteristics of TESOS are being studied in detail to understand better the precise calibration of spectro-polarimetric observations with etalon-based spectrographs. This work is done in view of the development of the Visible Tunable Filtergraph for ATST. In this context, an additional CMOS camera was integrated into TESOS, which provides 10 ms exposures with a rate of 100 frames/s. It is now possible to generate a high speckle reconstructed image for each TESOS exposure which can be used as a deconvolution key.

The Kiepenheuer Institute Speckle Imaging Package (KISIP) continues to be highly in demand as a standard for solar speckle imaging worldwide, and its maintenance a resource issue. The software package includes the methods for statistical PSF estimation, extended to partial AO correction. The software package is available to the community (https://forge.leibniz-kis.de/kisip/).