The Swedish 1-m Solar Telescope (SST) on the island of La Palma,
Spain, had first light with a stopped down 60 cm aperture on March 2, 2002.
On May 21, the telescope was opened to full aperture and the adaptive
optics system was switched on for the first time.
Already on the second day of operation it delivered
diffraction-limited images, i.e. it reached the
theoretical resolution limit for a telescope of this size. This means the
SST has very small optical aberrations after compensation
by the adaptive optics system, designed to counteract blurring caused
by the atmosphere. This enables solar astronomers to see and
photograph solar details of smaller size than previously possible.
The SST will address current and important questions concerning solar magnetic fields and the dynamics of the upper solar atmosphere and will also be used to improve our understanding of the formation of stellar spectra
The front lens of the SST has a diameter of just under 1
meter, making it the largest optical solar telescope in Europe and the
second in the world, after the McMath-Pierce
telescope in Arizona, USA. Located on the best known site for
solar telescopes in the world, it can see details as small
as 70 km on the solar surface. This requires the use of a so-called
adaptive mirror that corrects for the blurring caused by the Earth's
atmosphere 1000 times per second. The SST is the first solar
telescope that is designed for use with such a mirror.
The SST is operated by the Institute for Solar Physics of the Royal Swedish Academy of Sciences but located within the Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias on the island of La Palma, Spain. The SST replaces a previous 50 cm telescope that has been a world leading solar research instrument for over ten years.
Here are some drawings of the SST.
In addition to the atmospheric effects, solar telescopes suffer
from heating by sunlight of the optics and the air within the
telescope tube. This causes the image to shiver and become blurred. Modern
solar telescopes are either vacuum telescopes, filled with helium or
use careful control of the optic's temperature to reduce heating of
the air in the telescope.
Vacuum telescopes cannot be built with very large apertures because such a design requires a vacuum window of extremely high optical quality, one that is as large as its aperture. It is then the first optical component in the telescope and has to be polished to high optical quality. This is difficult because of the thickness needed to withstand the enormous difference in pressure between the air side and the inside of the vacuum window. The practical limit for vacuum telescopes is probably a little over 1 meter in diameter or slightly larger than the SST.
By filling the telescope with helium instead of air, the harmful effects of heat inside the telescope are greatly reduced although not as much as by vacuum. This avoids the large forces on vacuum windows but still requires an optical window of high quality but much smaller thickness than for a vacuum telescope. Such telescopes up to diameter of 2.4 meters have been proposed (for example LEST, Large Earth-based Solar Telescope). A helium filled telescope that is presently under construction is SOLIS but its diameter is relatively small. For future very large solar telescopes, the only viable technique seems to be to build open air telescopes with temperature controlled optics. Examples of telescopes that use or plan to use such design are the Dutch Open Telescope, also in La Palma, GREGOR, planned to be built on Tenerife and the Advanced Technology Solar Telescope, for which a site has not yet been selected.
The SST is a vacuum telescope. Instead of a flat vacuum window, it uses a 1-meter diameter lens to seal off the vacuum. By using a lens of a single glass, excellent image quality is obtained through very narrow filters, that isolate a single wavelength or color. This is important to solar physicists because the solar spectrum is rich with narrow spectral lines, some of them covering less than 1/2000 of the visible colors. However, looking through such a telescope without filters would produce a poor image, because different colors are focused at different distances from the lens.
For other observations that require broad wavelengths to be observed simultaneously, the SST can redirect light from a small part of the Sun to a corrector that puts all colors together at a single focus. Telescopes that use such a corrector are called Schupmann telescopes after Ludwig Schupmann who proposed such optical designs 100 years ago. This may sound very old-fashioned but in fact all modern large night-time telescopes are based on the Cassegrain or Gregory designs from the 17th century.
Because the SST allows either direct use of the singlet lens to form an image or lets the light pass through a corrector of the Schupmann type, it cannot be described completely as a Schupmann telescope. Its design allows the astronomer to choose between two optical systems depending on the application.
Adaptive optics is the future of large telescopes. It is a new and exciting technique that is still developing quickly and has already demonstrated dramatic improvements of image quality on several large night-time telescopes around the world, e.g. the Canada-France Hawaii Telescope (CFHT), Gemini North, Keck and ESO VLT.
The principle of adaptive optics is simple but its implementation difficult. To build an adaptive optics system you need a deformable mirror and a wavefront sensor. The wavefront sensor can be of different types. A common type is called the Schack-Hartmann wavefront sensor which looks at a single star through many small parts of the telescope aperture and measures the position of the star as seen through each part. When the atmosphere disturbs the image, it causes these images to move differently depending on their position in the aperture. The positions are measured and translated to commands to the deformable mirror, so that it takes the shape that compensates for the distortions. The problem is that the atmosphere changes quickly so this has to be done very accurately and at a very high frequency. Adaptive optics systems have to correct the adaptive mirror at least several hundred and preferably more than 1000 times per second.
Measuring the position of a star can be done easily but requires the star to be bright enough that there is sufficient light to do this quickly enough. For night time astronomy a problem is that there may often not be a sufficiently bright star near the object that the astronomer wants to observe. Several projects are therefore in progress that use a laser to create an artificial star very high up in the Earth's atmosphere.
For solar telescopes, there are no stars or star-like objects that can serve as reference for a wavefront sensor. However, there is solar fine structure everywhere on the Sun's surface. The position of such structure can also be measured but requires much more complicated calculations than for a night-time adaptive optics system. There are therefore very few solar adaptive optics systems in operation. The first truly functioning solar adaptive optics system was built at the Sacramento Peak Solar Observatory for the R. B. Dunn telescope. This was followed a few months later by the system built for the previous Swedish 50 cm solar telescope in La Palma. Without such a system, it would not be meaningful to build the new 1-meter solar telescope.
Many people have been involved in this project. The
conceptual optical and mechanical design of the telescope and adaptive
optics system was proposed by Göran Scharmer. The mechanical
design was developed into blueprints mainly by Bertil Pettersson and
Klas Bjelksjö of Stockholms
Digitalmekanik AB, with important contributions from Robert
Hammerschlag, Henrik Sönnerlind, Hans Boesgaard, and Torben
Andersen.
Much of the large parts of the SST were produced by Svenska Bearing outside Gothenburg. Their quick and precise work is greatly appreciated, as is their enthusiasm for the project.
The final optical design and tolerance analysis was made by Bo Lindberg based on the work of Mette Owner-Petersen. Darrel Torgerson contributed to the tolerance analysis.
The control system is based on suggestions by Torben Andersen. Hardware for reading encoders was developed by Mark Shand, Martin Renard, Jérôme Martin, and Rolf Kever. Software for controlling the SST was developed by Göran Hosinsky, John Rehn, David Kennedal, and Pete Dettori.
The initial software of the adaptive optics system was developed by Wang Wei and Göran Scharmer. This software has since then been completely rewritten by Pete Dettori and its performance evaluated mostly by Mats Löfdahl.
The optics were meticulously polished by Tapio Korhonen and coworkers at Opteon Oy in Finland.
The electronics and fibre optical link for the CCD systems in the two finders were designed and built as in-kind contribution by the electronics department of the solar group at the University of Utrecht.
The Royal Swedish Academy of Sciences quickly approved 5 million SEK for the project and thereby made it possible to raise additional funds. The remaining 12 million SEK that were needed came from The Knut and Alice Wallenberg Foundation, Marcus and Amalia Wallenberg's Memorial Fund, The Marianne and Marcus Wallenberg Foundation, the LEST foundation and the Institute for Theoretical Astrophysics in Oslo.