Telescope and Optics

Telescope structure. The first four mirrors are marked. The optical path is drawn in red.

As the name implies, the GREGOR telescope is a Gregorian telescope, a design invented by James Gregory in the 17th century. Gregorian telescopes consist of two concave mirrors (M1 & M2). The unique feature of the design is a focus before the secondary mirror (F1). For solar telescopes, usually a (cooled) field stop is placed at F1, which reflects a large part of the sun light out of the telescope, because only a small part of the solar surface can be observed in high resolution.

 

The GREGOR telescope is located on top of a building with a fully retractable dome. This setup minimizes internal seeing, which influences the image quality. The telescope structure is open and stiff with an alt-azimuth mount. The eigenfrequency of the structure is around 12 Hz. Its surface is designed to keep its temperature within 0.2° K of the air temperature. The tracking precision is around 0.2 to 0.5 arcseconds. The absolute pointing accuracy of the telescope is around 1 arcsecond. These properties remain also during strong wind conditions (up to 20 m/s). The telescope structure was installed in 2004.

 

 

GREGOR control system

Graphische Benutzerschnittstelle des GREGOR Kontrollsystems.

The control system of GREGOR consists of different modules. Guiding, control of the motors, and cooling of M1 are driven by a system made by the telescope manufacturer.  This system is controlled by the GREGOR control system (GCS), which is the system that the observer uses to control the telescope. GCS calculates the coordinates of the Sun in the sky and sends commands for positioning and guiding to the telescope. GCS also controls the motion of the image derotator, which counteracts the image rotation induced by the alt-az telescope mount, and of other optical components in the beam.

 

The adaptive optics and the scientific instruments have their own graphical user interfaces and can communicate with the GCS through standardized commands. The telescope and the instruments can be operated from a control room on the 3rd floor in this way, thus minimizing any disturbance near the optical elements.

Main optical characteristics

  • 150 cm free aperture
  • Gregory configuration with additional tertiary mirror (M3)
  • light weight optics
  • integrated adaptive optics
  • Image de-rotator
  • nominal field of view 150" (max. 300")
  • effective focal length: 55.6m (F/38) 
  • low instrumental polarisation
  • polarisation and calibration unit in symmetric beam
  • wavelength range from 350 nm to several µm
  • night time observations possible
  • primary mirror (D=1,5m) actively thermally controlled 
  • M2 (D=0.43m) and M3 (D=0,36m) passively cooled

The telescope uses a 3-mirror Gregorian configuration with three active mirrors. The primary mirror (f/1.7) is thermally controlled (with 200 W absorbed power the temperature difference to ambient air shall not exceed 0.5°C). A cooled field stop at the prime focus F1 provides a field of view of nominal 150 arcsec (maximum 300 arcsec) and reflects the unused light outside the telescope. The elliptical secondary mirror M2 (F1/1.29) magnifies the primary image and generates the secondary focus (F2) 200 mm above of the elevation axis. A polarimetry package is located near the secondary focal plane F2, at the center of the tube. An elliptical tertiary mirror M3 (F/3.97) reimages the secondary focus via M4 and through the coudé train (M5, M6, M7) into the laboratory. M3 is supported by an axial drive stage which is used for focusing at the tertiary focus.

The image rotation induced by the alt-azimuthal mount is compensated by a rotating image de-rotator with three mirrors. The de-rotator can be removed from the beam.

A flat mirror M11 redirects the beam horizontally into the laboratory feeding the adaptive optics (AO) system. M11 and the following parts rest on an optical table in the observing room. For use of the telescope without AO M11 can be removed.

Mirrors

Originally it was planned to manufacture the first three mirrors of the GREGOR telescope from the light-weight silicon carbide material Cesic.  For the primary this would have resulted in a weight of only 90 kg.  Cesic has a very high thermal conductivity - more than 100 times better than the glass ceramic materials with low thermal expansion that are normally used in astrophysics.  This results in a very homogeneous mirror temperature, thus facilitating the cooling of the reflecting surface.  When the primary is directed towards the Sun, it absorbs some 200 W, heating up the surrounding air unless the mirror surface is efficiently cooled.  This would cause dramatic losses in image quality.  Using an appropriate cooling system, the temperature of the GREGOR primary is therefore stabilized to temperature differences smaller than 0.5°C w.r.t. the ambient air. 
Unfortunately, technological problems prevented the manufacture of the 1.5 m primary from Cesic material.  Therefore, a lightweight main mirror from Zerodur was manufactured.  The mass of the mirror is about 215 kg. Cooling and mounting of the mirror were adjusted to the new material in order to achieve a comparable performance. 
For testing purposes, a 1 m Cesic mirror was used temporally. The secondary mirror was replaced in July 2018 by one made from Zerodur to improve its optical properties, which significantly improved the contrast.

Adaptive Optics

The GREGOR telescope is equipped with a high order adaptive optics system (HOAO). It allows diffraction limited resolution (0.08‘‘) for seeing above r>= 10 cm. The wavefront sensor has 156 subapertures to measure the wavefront deformation. The deformable mirror has 256 actuators and is able to correct for 140 degrees of freedom with a control loop frequency of 2100 Hz.

In the near future an extension called multi-conjugate adaptive optics will extend the corrected field by a factor of more than 10.