GREGOR (Gregorian type) ground-based Solar TelescopeBackground Optical Characteristics Instrument complement Project Status References
GREGOR is a 1.5 m solar telescope installed at the Observatorio del Teide on Tenerife island (Canary Islands, Spain) and built by a German consortium led by the Kiepenheuer-Institut für Sonnenphysik (KIS, Freiburg), with the Leibniz-Institut für Astrophysik Potsdam (AIP), the Max-Planck-Institut für Sonnensystemforschung (MPS), Lindau and the Institut für Astrophysik Göttingen (until 2008) as partners. The IAC (Instituto de Astrofísica de Canarias) and the Astronomical Institute of the Academy of Sciences of the Czech Republic contributed to the telescope or the instrumentation. 1)
The name is not an acronym; the telescope is named after James Gregory (1638-1675). During the 17th century, the Scottish mathematician and astronomer developed a telescope in which a secondary concave mirror directed the reflected light from the primary parabolic mirror through a tiny hole in the primary mirror on to the eye-lens and thus into the eye. This optical principle is also used in the new telescope in Tenerife. The Teide Observatory is located at an altitude of 2390 m, while the Teide volcano has an altitude of 3718 m. 2)
Scientific objectives: The prime scientific goal of GREGOR is high precision measurements of the solar magnetic field. 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 visible phenomena, like sunspots, prominences, flares and coronal mass ejections.
From theoretical and numerical computations it is known 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.
Specific science objectives:
• Emergence, evolution and disappearance of magnetic flux: Magnetic flux appears at the solar surface as dipoles with a variety of sizes, from large spots to small magnetic elements. The total flux of the Sun is replaced within 2 or 3 days. Since the magnetic flux does not constantly increase, a mechanism for flux disappearance must exist. The corresponding processes occur on the scale of the smallest magnetic elements.
• Energy budget of sunspots: The strong magnetic field of a sunspot blocks the convective energy transport to the solar surface. This blocking effect qualitatively explains the presence of cool sunspots, but the sunspot temperature is much higher as one would expect from complete suppression of convective energy transport. Small-scale phenomena, like umbral dots or penumbral grains are likely to provide the observed heat flux in a sunspot.
• Chromosphere: The bright points at the boundaries and the interior of the supergranulation cells play a key role for the heating and the dynamics of the chromosphere. The size of and the wave motion in these structures need to be measured with high photometric precision and sufficient spatial resolution.
Some background: The development of GREGOR started around the turn of the millennium and marked an important paradigm change. Since the 1970s, large solar telescopes were constructed as evacuated systems in order to eliminate internal seeing and to improve the imaging quality. With an aperture of 1.5 meters, an evacuated telescope with an entrance window was no longer an option, so GREGOR is an open telescope with active cooling of the primary mirror. A retractable dome allows flushing of the telescope with ambient air. Thanks to its mature adaptive optics and a suite of spectroscopic, polarimetric, and imaging instruments, it is now one of world's most powerful solar telescopes.
GREGOR was formally inaugurated in 2012, and since then, the commissioning and science verification of the telescope and its instruments has been completed. A few remaining major elements, such as the image derotator and a slit scanning system, have been added to the system in the meantime. GREGOR received a lot of attention from the European solar community during the first two access campaigns supported by the SOLARNET solar physics network in 2015 and 2016 (Ref. 14). 3)
Main optical characteristics:
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), light weighted and thermally controlled primary mirror (M1). A cooled field stop at the prime focus F1 provides a field of view of nominally 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 could 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. The scientific focus will be distributed by a rotating mirror to the different post focus instruments.
Figure 1: Solar telescope layout (image credit: GREGOR consortium)
Originally it was planned to manufacture the first three mirrors of the GREGOR telescope from the light-weight silicon carbide material CESIC® (Carbon-fiber reinforced Silicon Carbide). 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 will therefore be 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 lightweighted 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 could be used temporally. The secondary and tertiary mirrors are made from CESIC and ready for active cooling.
AO (Adaptive Optics):
The GREGOR solar telescope is equipped with a high order adaptive optics (HOAO). It will allow diffraction limited resolution (0.08 arcsec) for seeing above r0 ≥ 10 cm. The wavefront sensor has156 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 2000 Hz.
In the near future an extension called multi-conjugate adaptive optics will extend the corrected field by a factor of more than 10.
- 1500 mm free aperture
- Gregory configuration with additional tertiary mirror (M3)
- light weighted optics
- integrated adaptive optics
- Image de-rotator nominal field of view: 150 arcsec (max. 300 arcsec)
- effective focal length: 55.6 m (F/38)
- low instrumental polarization
- polarization and calibration unit in symmetric beam
- wavelength range from 350 nm to several µm
- night time observations possible
- mirrors made from silicon carbide (CESIC)
- primary mirror (D=1.5 m) active thermally controlled
- M2 (D=0.43 m) and M3 (D=0.36m) passive cooled.
GREGOR Solar Telescope
GREGOR is the largest solar telescope in Europe. It is the third-largest solar telescope in the world, after the Big Bear Observatory in California and the McMath-Pierce solar telescope at Kitt Peak National Observatory in Arizona, USA. GREGORis designed for observations of the solar photosphere and chromosphere in the visible and near infrared. Presently it is equipped with the following post-focus instruments for solar observations.
Figure 2: Left: Photo of the GREGOR telescope structure; Right: Photo of the GREGOR telescope inauguration at Teide Observatory on 21, May 2012 (image credit: GREGOR consortium)
Figure 3: Photo of the Teide Observatory with GREGOR on top of the building on inauguration day (image credit: IAP)
Instrument complement (GRIS, GFPI, BIC)
GRIS (GREGOR Infrared Spectrograph)
GRIS provides spectropolarimetric measurements in the near infrared. The grating spectrograph is installed in the 1.5 m GREGOR telescope located at the Observatorio del Teide in Tenerife. The spectrograph has a standard Czerny-Turner configuration with parabolic collimator and camera mirrors that belong to the same conic surface. Although nothing prevents its use at visible wavelengths, the spectrograph is mostly used in combination with the infrared detector of TIP-II (Tenerife Infrared Polarimeter) in standard spectroscopic mode as well as for spectropolarimetric measurements. A slit scanner allows to scan some 60 arcsec. The slit length also corresponds to some 60 arcsec, with a sampling of 0.13 arcsec.
GRIS operates in the range 1.0–2.3 µm. The experience at the German VTT (Vacuum Tower Telescope), located at the same observatory, demonstrated that the most used spectral regions are those at 1.083 µm (including the photospheric Si I and the chromospheric triplet He I) and at 1.565 µm (with a number of iron lines including a line with g=3). As an example for the 1.565 µm region: Five accumulations of 30 ms exposure time each in the 4 polarimetric states yields a signal-to-noise ratio of 1000. In this case one scan step takes 3 seconds.
GRIS is operated by the IAC (Instituto de Astrofísica de Canarias). Manolo Collados is the PI (Principal Investigator) of GRIS.
GFPI (GREGOR Fabry-Pérot Interferometer)
GFPI provides imaging spectroscopy. Fast processes on the Sun require instruments capable of acquiring data in a time span comparable to the evolution time-scale of solar features, which is on the order of minutes and sometimes even on the order of seconds. Imaging spectropolarimetry with the GFPI is ideally suited for this type of application. 4)
The instrument comprises two tunable etalons in collimated mounting, which provide a spectral resolution R ~250.000. Scanning a spectral line takes a few tens of seconds to from a few minutes depending on the sampling, the number of images acquired per wavelength position, and the observing mode (spectroscopy vs. polarimetry). Two cameras with 1376 x 1024 pixel acquire images strictly simultaneously in the narrow- and broad-band channels to facilitate post-facto image restoration including simple destretching, speckle masking imaging and deconvolution, and MOMFBD (Multi Object Multi Frame Blind Deconvolution). The FOV (Field of View) is 50 arcsec x 38 arcsec in the spectroscopic observing mode and about half the size for polarimetric observations. Small sunspots and substantial portions of active regions can thus be observed. The coatings of the etalons are optimized for the wavelength range 530–860 nm, while the polarimeter limits the spectral observing window to 580–660 nm. Many interesting photospheric and chromospheric spectral lines are accessible, and two of them can be observed sequentially.
GFPI can observe simultaneously with GRIS. Then the pentaprism beamsplitter feeds GFPI with wavelengths < 650 nm. All GFPI data (i.e., raw data and high-level data products) are stored in the GREGOR archive at AIP (Leibniz Institute for Astrophysics Potsdam), where they can be accessed and queried on this website (at the moment access is limited to GREGOR partners).
BIC (Blue Imaging Channel)
High-cadence imaging provides important context information for the GFPI observations. Small-scale magnetic features are more easily detected in particular wavelength regions, e.g., the Fraunhofer G-band. Furthermore, the broad Calcium H and K lines offer chromospheric diagnostics. Two CCD cameras (pco.4000 and pco.sensicam) can be used in the BIC of the GFPI depending on the availability of the GREGOR/VTT facility cameras. Three interference filters (Ca II H λ=396.8 nm, G-band λ=430.7 nm, and blue continuum λ=450.8 nm) are available. Recently, MPS contributed a 0.1nanometer-wide Ca II H λ=396.8 nm interference filter of the Sunrise mission. The maximum FOV of the pco.4000 CCD cameras is limited to 75 arcsec x 93 arcsec (2160 x 2672 pixel) because of dichroic pentaprism splitting the light between GFPI and BIC. The FOV of the pco.sensicam CCD camera is much smaller 34 arcsec x 26 arcsec (1376 x 1040 pixel) but the data acquisition rate is much higher (eight vs. three frames per second). However, in both cases post-facto image restoration is feasible (Knox-Thompson, speckle masking imaging, and multi-frame blind deconvolution). In 2016, new (faster) cameras will be integrated.
All BIC data (i.e., raw data and high-level data products) are stored in the GREGOR archive at AIP, where they can be accessed and queried on this website (at the moment access is limited to GREGOR partners).
Project status and events
• September 1, 2020: GREGOR, the largest solar telescope in Europe, which is operated by a German consortium and located on Tide Observatory, Canary Islands, Spain, has obtained unprecedented images of the fine-structure of the Sun. Following a major redesign of GREGOR's optics, carried out by a team of scientists and engineers from the Leibniz Institute for Solar Physics (KIS), the Sun can be observed at a higher resolution than before from Europe. 5)
The Sun is our star and has a profound influence on our planet, life, and civilization. By studying the magnetism on the Sun, we can understand its influence on Earth and minimize damage of satellites and technological infrastructure. The GREGOR telescope allows scientists to resolve details as small as 50 km on the Sun, which is a tiny fraction of the solar diameter of 1.4 million km. This is as if one saw a needle on a soccer field perfectly sharp from a distance of one kilometer.
"This was a very exciting, but also extremely challenging project. In only one year we completely redesigned the optics, mechanics, and electronics to achieve the best possible image quality." said Dr. Lucia Kleint, who led the project and the German solar telescopes on Tenerife. A major technical breakthrough was achieved by the project team in March this year, during the lockdown, when they were stranded at the observatory and set up the optical laboratory from the ground up. Unfortunately, snow storms prevented solar observations. When Spain reopened in July, the team immediately flew back and obtained the highest resolution images of the Sun ever taken by a European telescope.
Svetlana Berdyugina, professor at the Albert-Ludwig University of Freiburg and Director of the Leibniz Institute for Solar Physics (KIS), is very happy about the outstanding results: "The project was rather risky because such telescope upgrades usually take years, but the great team work and meticulous planning have led to this success. Now we have a powerful instrument to solve puzzles on the Sun." The new optics of the telescope will allow scientists to study magnetic fields, convection, turbulence, solar eruptions, and sunspots in great detail. First light images obtained in July 2020 reveal astonishing details of sunspot evolution and intricate structures in solar plasma.
Figure 4: Europe's largest solar telescope, GREGOR, reveals intricate structures of solar magnetic fields in very high resolution. The image was taken at the wavelength of 516 nm (image credit: KIS)
Telescope optics are very complex systems of mirrors, lenses, glass cubes, filters and further optical elements. If only one element is not perfect, for example due to fabrication issues, the performance of the whole system suffers. This is similar to wearing glasses with the wrong prescription, resulting in a blurry vision. Unlike for glasses, it is however very challenging to detect which elements in a telescope may be causing issues. The GREGOR team found several of those issues and calculated optics models to solve them. For example, astigmatism is one of such optical problems, which affects 30-60% people's vision, but also complex telescopes. At GREGOR this was corrected by replacing two elements with so-called off-axis parabolic mirrors, which had to be polished to 6 nm precision, about 1/10000 of the diameter of a hair. Combined with several further enhancements the redesign led to the sharp vision of the telescope. A technical description of the redesign was recently published by the Astronomy & Astrophysics journal in a recent article led by L. Kleint. 6)
Figure 5: A sunspot observed in high resolution by the GREGOR telescope at the wavelength 430 nm (image credit: KIS)
Figure 6: Left: The GREGOR telescope on Tenerife, Spain. Right: The newly redesigned optical laboratory of GREGOR (image credit: L. Kleint, KIS)
European researchers have access to observations with the GREGOR telescope through national programs and a program funded by the European Commission. New scientific observations are starting in September 2020.
Albert-Ludwig University Freiburg ,founded in 1457, offers undergraduate and graduate studies in all important disciplines today. The Leibniz Institute for Solar Physics (KIS) located in Freiburg is a public foundation and a member of the Leibniz Association. It carries out fundamental research on the Sun and other stars.
• December 2, 2016: A special issue of Astronomy and Astrophysics contains a series of scientific articles, which are based on data obtained with the GREGOR solar telescope in 2014 and 2015. This period represents the initial phase of scientific use, and was carried out jointly by all partners of the GREGOR consortium. These articles demonstrate the potential of the telescope and its instruments at this early stage of development. 7) 8) 9) 10) 11) 12) 13)
The publications in this issue cover a range of topics, including magnetic reconnection in a flare, sunspot magnetic fields in the photosphere and chromosphere, material flows in active regions, and weak magnetism of the very quiet Sun. Imaging data provide details of the solar photosphere at a scale of 60 km on the Sun (0.08 arcsec angular resolution). For the first time, GREGOR has resolved details smaller than 100 km in sunspot light bridges, which has advanced our understanding of magneto-convection. The excellent polarimetric sensitivity enables the measurement of magnetic field strengths down to a few Gauss, unraveling for the first time that even in the most quiet areas on the Sun, 80% of the area is covered with magnetic fields. The combination of high spatial resolution and high magnetic field sensitivity makes GREGOR a unique telescope. 14)
Figure 7: Example of high-quality magnetic field data from the solar atmosphere acquired with GREGOR. The three sets of panels show the strength (left), inclination (middle), and azimuth (right) of the magnetic field vector at the edge of a sunspot. The sub-panels show different heights in the atmosphere from the photosphere to the chromosphere (image credit: J. Joshi, GREGOR consortium) 15) 16)
1) "New Solar Telescope GREGOR," URL: http://www.leibniz-kis.de/en/observatories/gregor/
2) "GREGOR telescope: Zooming in on the sun," May 10, 2012, URL: http://phys.org/news
3) C. Denker, O. von der Lühe, A. Feller, K. Arlt, H. Balthasar, S.-M. Bauer, N. Bello Gonzalez, T. Berkefeld, P. Caligari, M. Collados, A. Fischer, T. Granzer, T. Hahn, C. Halbgewachs, F. Heidecke, A. Hofmann, T. Kentischer, M. Klvana, F. Kneer, A. Lagg, H. Nicklas, E. Popow, K.G. Puschmann, J. Rendte, D. Schmidt, W. Schmidt, M. Sobotka, S.K. Solanki, D. Soltau, J. Staude, K.G. Strassmeier, R. Volkmer, T. Waldmann, E. Wiehr, A.D. Wittmann, M. Woche, "A retrospective of the GREGOR solar telescope in scientific literature," Astronomische Nachrichten , AN Vol. 333, No 10. 1 – 6 (2012), DOI: 10.1002/asna.2012xxxxx, URL: https://arxiv.org/pdf/1210.3167.pdf
4) Klaus Gerhard Puschmann, "The GREGOR Fabry‐Pérot Interferometer (GFPI) — Technical Innovations and Results achieved in 2013," Solar and Stellar Astrophysics; Instrumentation and Methods for Astrophysics, URL: https://arxiv.org/ftp/arxiv
5) "Europe's largest Solar Telescope GREGOR unveils magnetic details of the Sun," University of Freiburg, 1 September 2020, URL: http://www.leibniz-kis.de/en/institute/pictures-of-the-month/single-view/europes-largest-solar-telescope-gregor-unveils-magnetic-details-of-the-sun-1/
6) Lucia Kleint, Thomas Berkefeld, Miguel Esteves, Thomas Sonner, Reiner Volkmer, Karin Gerber, Felix Krämer, Olivier Grassin and Svetlana Berdyugina, "GREGOR: Optics redesign and updates from 2018–2020," Astronomy & Astrophysics, Volume 641, Published online: 1 September 2020, https://doi.org/10.1051/0004-6361/202038208
7) "GREGOR first results published in special issue of Astronomy and Astrophysics," Space Daily, Dec. 2, 2016, URL: http://www.spacedaily.com/reports
8) M. Sobotka, J. Dudík, C. Denker, H. Balthasar, J. Jurčák, W. Liu, T. Berkefeld, M. Collados Vera, A. Feller, A. Hofmann, F. Kneer, C. Kuckein, A. Lagg, R. E. Louis, O. von der Lühe, H. Nicklas, R. Schlichenmaier, D. Schmidt, W. Schmidt, M. Sigwarth, S. K. Solanki, D. Soltau, J. Staude, K. G. Strassmeier, R. Volkmer, T. Waldmann, "Slipping reconnection in a solar flare observed in high resolution with the GREGOR solar telescope," Astronomy & Astrophysics, Vol. 596, December 2016, GREGOR first results, DOI: http://dx.doi.org/10.1051/0004-6361/201527966, URL of abstract: http://www.aanda.org/articles/aa/abs/
9) J. M. Borrero, A. Asensio Ramos, M. Collados, R. Schlichenmaier, H. Balthasar, M. Franz, R. Rezaei, C. Kiess, D. Orozco Suárez, A. Pastor, T. Berkefeld, O. von der Lühe, D. Schmidt, W. Schmidt, M. Sigwarth, D. Soltau, R. Volkmer, T. Waldmann, C. Denker, A. Hofmann, J. Staude, K. G. Strassmeier, A. Feller, A. Lagg, S. K. Solanki, M. Sobotka, H. Nicklas, "Deep probing of the photospheric sunspot penumbra: no evidence of field-free gaps," Astronomy & Astrophysics, Vol. 596, December 2016, GREGOR first results, DOI: http://dx.doi.org/10.1051/0004-6361/201628313, URL of abstarct: http://www.aanda.org/articles/aa/abs/2
10) M. Verma, C. Denker, H. Balthasar, C. Kuckein, S. J. González Manrique, M. Sobotka, N. Bello González, S. Hoch, A. Diercke, P. Kummerow, T. Berkefeld, M. Collados, A. Feller, A. Hofmann, F. Kneer, A. Lagg, J. Löhner-Böttcher, H. Nicklas, A. Pastor Yabar, R. Schlichenmaier, D. Schmidt, W. Schmidt, M. Schubert, M. Sigwarth, S. K. Solanki, D. Soltau, J. Staude, K. G. Strassmeier, R. Volkmer, O. von der Lühe, T. Waldmann, "Horizontal flow fields in and around a small active region — The transition period between flux emergence and decay," Astronomy & Astrophysics, Vol. 596, December 2016, GREGOR first results, DOI: http://dx.doi.org/10.1051/0004-6361/201628380, URL of abstarct: http://www.aanda.org/articles/aa/abs
11) M. Franz, M. Collados, C. Bethge, R. Schlichenmaier, J. M. Borrero, W. Schmidt, A. Lagg, S. K. Solanki, T. Berkefeld, C. Kiess, R. Rezaei, D. Schmidt, M. Sigwarth, D. Soltau, R. Volkmer, O. von der Lühe, T. Waldmann, D. Orozco, A. Pastor Yabar, C. Denker, H. Balthasar, J. Staude, A. Hofmann, K. Strassmeier, A. Feller, H. Nicklas, F. Kneer, M. Sobotka," Magnetic fields of opposite polarity in sunspot penumbrae," Astronomy & Astrophysics, Vol. 596, December 2016, GREGOR first results, DOI: http://dx.doi.org/10.1051/0004-6361/201628407, URL of abstract: http://www.aanda.org/articles/aa/abs
12) M. J. Martínez González, A. Pastor Yabar, A. Lagg, A. Asensio Ramos, M. Collados, S. K. Solanki, H. Balthasar, T. Berkefeld, C. Denker, H. P. Doerr, A. Feller, M. Franz, S. J. González Manrique, A. Hofmann, F. Kneer, C. Kuckein, R. Louis, O. von der Lühe, H. Nicklas, D. Orozco, R. Rezaei, R. Schlichenmaier, D. Schmidt, W. Schmidt, M. Sigwarth, M. Sobotka, D. Soltau, J. Staude, K. G. Strassmeier, M. Verma, T. Waldman, R. Volkmer, "Inference of magnetic fields in the very quiet Sun," Astronomy & Astrophysics, Vol. 596, December 2016, GREGOR first results, DOI: http://dx.doi.org/10.1051/0004-6361/201628449, URL of abstract: http://www.aanda.org/articles/aa/abs/
13) R. Schlichenmaier, O. von der Lühe, S. Hoch, D. Soltau, T. Berkefeld, D. Schmidt, W. Schmidt, C. Denker, H. Balthasar, A. Hofmann, K. G. Strassmeier, J. Staude, A. Feller, A. Lagg, S. K. Solanki, M. Collados, M. Sigwarth, R. Volkmer, T. Waldmann, F. Kneer, H. Nicklas, M. Sobotka, "Active region fine structure observed at 0.08 arcsec resolution," Astronomy & Astrophysics, Vol. 596, December 2016, GREGOR first results, DOI: http://dx.doi.org/10.1051/0004-6361/201628561, URL of abstract: http://www.aanda.org/articles/aa/abs/2016/12/aa28561-16/aa28561-16.html
15) J. Joshi, A. Lagg, S. K. Solanki, A. Feller, M. Collados, D. Orozco Suárez, R. Schlichenmaier, M. Franz, H. Balthasar, C. Denker, T. Berkefeld, A. Hofmann, C. Kiess, H. Nicklas, A. Pastor Yabar, R. Rezaei, D. Schmidt, W. Schmidt, M. Sobotka, D. Soltau, J. Staude, K. G. Strassmeier, R. Volkmer, O. von der Lühe, T. Waldmann, "Upper chromospheric magnetic field of a sunspot penumbra: observations of fine structure," Astronomy § Astrophysics, Vol. 596, December 2016, GREGOR first results, DOI: http://dx.doi.org/10.1051/0004-6361/201629214, URL of abstract: http://www.aanda.org/articles/aa/abs
16) T. Felipe, M. Collados, E. Khomenko, C. Kuckein, A. Asensio Ramos, H. Balthasar, T. Berkefeld, C. Denker, A. Feller, M. Franz, A. Hofmann, J. Joshi, C. Kiess, A. Lagg, H. Nicklas, D. Orozco Suárez, A. Pastor Yabar, R. Rezaei, R. Schlichenmaier, D. Schmidt, W. Schmidt, M. Sigwarth, M. Sobotka, S. K. Solanki, D. Soltau, J. Staude, K. G. Strassmeier, R. Volkmer, O. von der Lühe, T. Waldmann, "Three-dimensional structure of a sunspot light bridge," Astronomy § Astrophysics, Vol. 596, December 2016, GREGOR first results, DOI: http://dx.doi.org/10.1051/0004-6361/201629586, URL of abstract: http://www.aanda.org/articles/aa/abs
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (email@example.com).