The Dutch Open Telescope (DOT) is an innovative solar telescope located at the Observatorio del Roque de los Muchachos of the Instituto de Astrofïsica de Canarias.
The DOT achieves high-resolution imaging of the sun simultaneously in multiple wavelengths which sample the solar atmosphere tomographically at different heights. All DOT data are public. More detail is given under DOT tomography, in our professional DOT publications and outreach descriptions, and in the reports under DOT documents.
The initiator of the DOT was Prof. C. Zwaan (obituary) while he led Europe-wide site testing for solar telescopes. At his suggestion of an open design the DOT was designed and built by R.H. Hammerschlag of the Sterrekundig Instituut Utrecht with a small team of coworkers at IGF Utrecht and DTO Delft.
The novel open design of the DOT is seen well on this photograph. 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. They 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 DOT's simple optical scheme permits precise optical alignment for optimum performance. The DOT's extraordinary mechanical stability gives high pointing precision even in strong wind buffeting. The fold-away clamshell canopy survives even the severe La Palma winter storms and heavy ice loads.
The first results after the DOT first light confirmed the viability of the open principle. P. Sütterlin's experiments with his version of the Göttingen speckle reconstruction code then gave such spectacular results that a multi-camera speckle acquisition system was realized. The combination of superb imaging and speckle restoration made the DOT the first solar telescope to regularly obtain 0.2 arcsec resolution throughout extended image sequences. Such movies are taken synchronously with identical cameras in blue and red continua, the G band, Ca II H including blue-wing tunability, and profile-sampling narrow-band Halpha and Ba II 4554. The laboriousness of the speckle processing limited the movie production severely until the installation of the advanced NWO-funded DOT Speckle Processor with 70 water-cooled processors which delivered fast on-site speckle processing and permitted an Open DOT program providing observing time to external colleagues. See Rutten et al., A&A, 413, 1183, 2004 for more detail.
We also ran a popular Students to the DOT education program for on-site tutoring in observational solar physics.
The open principle of the DOT may be upscaled to much larger aperture than is feasible for traditional vacuum telescopes requiring entrance windows. Its success together with the advent of adaptive optics inspired the 1.5 m German GREGOR telescope, the 1.6 m BBSO NST telescope, contributed to the ambitious US project to build a four-meter Advanced Technology Solar Telescope, and is upscaled far beyond that size in the GISOT concept. The mechanical stucture of the DOT itself permits relatively cheap upgrades from 45 cm to much larger aperture.
During its first decade the DOT operation was funded primarily by the Department of Physics and Astronomy of Utrecht University, NWO and NOVA. These sponsors terminated their contribution by the end of 2007. During 2008 and 2009 only part-time DOT observing could be performed, on EC funding. During much of 2010 Helio Research (USA) funded and ran full-time DOT operation on a grant from the US National Science Foundation. Such partnerships, in which external groups supply DOT operation costs, the on-site observers, and manpower for the data processing are the only way to un-mothball the DOT for new data gathering.
Since Utrecht University's support for the DOT, solar physics, and in fact all astronomy has gone (it terminated its whole astronomy department early in 2012), industrial sponsoring is obviously very welcome. The construction and engineering firm PelserHartman (English website) sets a good example as DOT Ambassador.
Rob Rutten 2018-08-13