GREGOR is a new solar telescope with a 1.5 m aperture which is presently being assembled at the Teide Observatory, Tenerife, Spain. The project is undertaken by a predominantly German consortium, consisting of the Kiepenheuer Institut für Sonnenphysik, the Astrophysikalisches Institut Potsdam, the Universitäts-Sternwarte Göttingen, and other national and international Partners. GREGOR is designed for high-precision measurements of the magnetic field and the gas motion in the solar photosphere and chromosphere with a resolution of 70 km on the Sun, and for high resolution stellar spectroscopy.HISTORY
The German solar community operated two telescopes at Tenerife since the mid-Eighties, the 70cm Vacuum Tower Telescope (VTT), a new facility, and the 45cm Gregory Coudé Telescope (GCT) which was relocated from the Locarno Observatory of the Göttinger Universitätssternwarte. The need to adequately address new scientific questions in solar physics can only be met by large aperture, high resolution telescopes. It was therefore decided to replace the older GCT with a modern telescope, using the existing infrastructure of a known, excellent site. The open design concept with a retractable dome, allowing free airflow through the telescope tube, was adopted because the structure and entrance window of an evacuated telescope of the size of GREGOR would have become prohibitively heavy.
The old GCT and its dome were dismantled in May 2002, the construction work on the upper part of the building started at the same time. In June 2004 the new dome was assembled and in September of the same year the telescope structure was erected. The main mirror is planned to be integrated in 2005 to start the first observations in 2006.
The optical configuration is a Gregory coudé using 3 powered mirrors including an f/1.75 light-weighted primary mirror with 1.5 m aperture. A cooled field stop at the prime focus provides a field of view of 300 arcsec. A polarimetry package is located near the secondary focal plane, before the first oblique reflection. A derotator and an adaptive optics system are located close to the tertiary focus, delivering a stabilized science image to several post focus instruments. The effective focal length is about 60 m and the final focal plane can be placed in two laboratories.
Open Telescope GREGOR is designed as a completely open telescope with a retractable dome, to avoid internal seeing. It is designed to perform fully under conditions of strong wind. The telescope structure has an Alt-Azimuth mount and a direct pointing capability accuracy of 0.25 arcsec.
Lightweight Technology The first three mirrors are made of light weighted carbon reinforced silicon carbide (Cesic), resulting in a weight of only 170 kg for the 1.5 meter primary. The Cesic material has high thermal conductivity more than 50 times that of Zerodur which results in a very homogeneous mirror temperature and allows back side air cooling the telescope points to the sun. Active cooling of the mirror is necessary to quench seeing resulting from sunlight absorbed at the mirror surface.
Adaptive Optics The main components of the Adaptive Optics (AO) system include a tip-tilt mirror, two 68 actuator bimorph deformable mirrors (DM) and a Shack-Hartmann wavefront sensor (WFS) with 78 subapertures. This configuration achieves the diffration limit (0.08" at 500 nm) for seeing better than 0.65". The system bandwidth will be about 100 Hz.
Postfocus instruments There will be a longslit spectrograph and a two-dimensional spectrometer as first generation post focus instruments. The two dimensional spectrometer is the refurbished "Göttinger Spectrometer" with two Fabry-Pérot Interferometers in the collimated beam. The spectral resolution will be ?? < 5 pm, the field of view will be about 50". The wavelength coverage is from 530 nm 870 nm. The longslit spectrograph is a new design using the existing grating of the GCT and is located below the main observing room. The spectrograph will be used first in the near infrared wavelength range, and will be extended to visible wavelengths later. Both spectrometers can be used as Stokes vector polarimeters.
There is room for additional post focus instruments in the main observation room. A stellar high resolution spectrograph will be added to the spectrpgraph room below the main observing room. The POLIS spectrometer, which is currently operated at the VTT, will also be relocated to GREGOR.
The magnetic activity of the sun plays a dominant role in virtually all processes in the solar atmosphere. It is responsible for the energy balance of the outer atmosphere, it causes the activity cycle and the concomitant variability of the solar luminosity, and it produces most of the sometimes spectacular phenomena like sunspots, prominences, flares and coronal mass ejections. Theoretical studies and numerical computations suggest that much of the interaction between the solar plasma and the magnetic field occurs on very small spatial scales of about 70 km on the Sun, corresponding to an angle of 0.1 arcsec. It is therefore important to have a large enough telescope which can resolve such small details. In addition, a large aperture is needed to achieve the photometric accuracy and sensitivity needed for a quantitative physical understanding of the solar magnetic field.