The Dutch Open Telescope (DOT) at the Observatorio del Roque
de los Muchachos on La Palma is an innovative optical telescope for
high-resolution imaging of the solar atmosphere. It produces marvelous
movies of the photo- and chromosphere thanks to the open construction
of the telescope, the excellent site, and post detection image
restoration which removes all remaining atmospheric disturbances.
Examples of images and movies can be found at the DOT-website, http://www.dot.iac.es.
An audiovisual about the telescope is available in several languages:
The idea of an fully open telescope was proposed by prof. C. Zwaan at the Astronomical Institute of Utrecht University after analyzing data of solar site test campaigns, showing measurements with reasonable better atmospheric conditions 10-30 m above the heated ground surface. Shortly after, R.H. Hammerschlag started with the design of a stiff tower and telescope which can withstand windload without starting to shake, which resulted in the DOT with help of a small team of coworkers. The DOT was installed on the Observatorio del Roque de los Muchachos during 1996-1997 and received first light in 1997.
The DOT is an open telescope, placed on an open steel tower and is without a vacuum system which is generally used to diminish the atmospheric turbulence in the telescope and which is caused by the intense solar radiation concentrated in the telescope. The DOT instead uses wind to flush the telescope and surroundings. It exploits the often excellent La Palma conditions through minimal obstruction to the strong trade winds that make La Palma an outstanding site for solar as well as night-time astronomy. These trade winds go together with a low inversion, often keeping the cloud layer below the volcano rim, and confine local turbulent convection from solar ground heating to a thin layer below the 15 m high open-tower top. They blow right through the telescope, also flushing the 45 cm diameter primary mirror, so that no internal turbulence develops.
The idea avoids the need of precise entrance windows used in
evacuated systems for closing the vacuum tank and opens the way to much
larger solar telescopes.
Tower - only parallel platform motion, no tilts, height 15 m
Canopy - fold-away clam-like shell, diameter 7 m
Telescope - equatorial fork mount, weight 16 tons
Primary mirror - diameter 45 cm
Effective focal length - 19.75 m
Field of view - 92 x 73 arcsec, 0.071 arcsec/px
Cameras - total 6, 1296 x 1030 px, 12 frames/s
Imaging in selected spectral lines in the solar spectrum permits sampling solar phenomena simultaneously in multiple altitude layers in the solar atmosphere because the depth one can look into the solar atmosphere strongly depends on the colour. The images below 4 shows a resulting DOT image mosaic taken on September 29, 2004 showing three such slices through a sunspot about as large as the Earth (inset).
The G band (first image) shows the photospheric solar surface covered by convective granules and tiny bright magnetic elements between these. (Note: this is the new inset in the attached picture). The sunspot has a dark umbra containing small bright umbral dots and surrounded by a filamentary penumbra. The strong Ca II H line (second image) samples the low chromosphere, a few hundred kilometers higher up. At that height the granulation appears reversedly and the magnetic elements appear considerably brighter. The Halpha line (third image) shows fibrils in the high chromosphere that lie at a few thousand kilometer height and are obviously controlled by magnetic fields. They show that many (but not all) field lines originate in the spot and in the intergranular magnetic elements, and span wide distances to where they connect back to to the surface. Sometimes flares occur which are visible as dark filaments on the disk.
DOT-data provides science input in themselves, but are also highly valuable as context tomography for spectrometry and polarimetry at other telescopes, and often combined with coronal EUV imaging from space.
The telescope is equipped with a multi-wavelength imaging
system, operating at Ca II H (396.8nm), G-band (430.5nm), blue
continuum (432nm), Ba II (455.4nm), red continuum (654nm) and H-alpha
Standard data consists of image sequences taken in these wavelengths simultaneous with exactly the same fields and with identical cameras during up to 8 hours at 20-30 second sampling cadence. The field covers up to 90x70 arcseconds. After the observation all data is reconstructed by use of the speckle method which removes all remaining atmospheric disturbances. The total amount of data collected each day sums up to 1.6 Terabyte which is processed overnight. The resolution is approaching the theoretical diffraction limit (0.2 arcseconds at 430 nm) already at fairly bad seeing and is uniform over the full field.
The DOT is able to support much larger primary mirrors than the 45-cm now, and a detailed design study is being made to equip the DOT which a primary 3 times larger in size, bringing down the resolution limit to 0.07 arcsec (50km on the solar surface). It will keep the DOT together with its multi-wavelength imager at the frontiers of solar physics in the next era too.