Telescope on Stilts

A revolutionary solar telescope caught its first rays this week at the Roque de los Muchachos Observatory on La Palma, Canary Islands. The 45-centimeter Dutch Open Telescope (DOT) could open the door to bigger solar optical telescopes by eliminating the need for putting the telescope mirror in a vacuum to prevent heat from distorting the image.

The $2.5 million DOT, built by Utrecht University and Delft University of Technology--both in the Netherlands--should give astronomers a view of the sun rivaling that of the finest ground-based optical telescopes. The reason: a novel design to deal with the tremendous heat load from staring at the sun, which creates an image disturbance called "seeing"--like the shimmering mirage sometimes seen on a hot highway.

This problem is usually avoided by putting solar scopes in a vacuum tube where there's no air for temperature fluctuations. But, says Jacques Beckers, director of the U.S. National Solar Observatories (NSO) at Kitt Peak, Arizona, the window covering the opening of a vacuum scope can't be more than about 1 meter across, the size of the largest available optical-quality windows. Solar astronomers need bigger instruments to discern, for example, minute changes in the sun's magnetic field.

To get past that limitation, DOT chief engineer Rob Hammerschlag of Utrecht University, came up with a disarmingly simple design which his team believes will do away with problems posed by both heat turbulence and ground-level atmospheric turbulence. They mounted the DOT mirror, domeless and tubeless, on a rigid 15-meter tower and left it uncovered, to be cooled by wind.

The DOT "will break new ground" if it works, says Beckers, showing the viability of technology "that will allow us to go to larger facilities." Indeed, Hammerschlag hopes within a year or two to replace DOT's 45-cm mirror with a 70- or 80-cm mirror, which would boost the instrument's resolution by at least 50%. Regular observations are to start next spring.

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