Vacuum telescope

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The Swedish Solar Telescope on La Palma contains the world's largest vacuum telescope with a diameter of 1 m in this 30 m high tower. The Coelostat is visible on the roof .

As vacuum telescopes astronomical telescopes are with evacuated tube called. They avoid air turbulence inside (see dome seeing ) and were specially developed for solar observatories .

Reducing air turbulence

In solar telescopes, solar radiation heats parts of the instrument due to its high intensity compared to starlight. When the distance between the optical elements of the telescope is filled with air, heated air rises inside the beam path. Since the refractive index of the air depends on its temperature, and the convection is uneven, this deteriorates the quality of the image. In the case of large solar telescopes, these disturbances are significantly greater than the resolving power resulting from the aperture alone, without taking account of turbulence in the air .

An evacuation of the telescope tube avoids this source of error. The deflection of the telescope objective or a separate window due to the air pressure is particularly critical here , which limits the aperture to about one meter. Large vacuum telescopes are usually designed as tower telescopes - i.e. with a vertical beam path. These have the advantage that their very long tube is not the movement of the sun tracked must be - wind load would be a problem. Finally, the coupling of light into the thermally stable underground observation area is simplified. The tracking takes place through the mirrors of a roof-mounted Zölostat , which reflects the sunlight into the evacuated tube.

Alternative technologies

Instead of evacuation, it is also possible to fill the telescope tube with helium gas . It would solve the deflection problem, but is only at the experimental stage. Helium creates fewer streaks than air because it has a lower density (less propulsion for convection) and higher heat conduction.

In order to reduce seeing (the air turbulence that occurs outside of the observatory), some solar telescopes are also equipped with adaptive optics . This technology, which was developed around 1985, analyzes the incoming wave fronts a few 100 times per second and thus controls numerous actuators on the lens or on auxiliary mirrors. What is now standard on large telescopes for night observations , however, still causes problems with solar observations. They were solved satisfactorily for the first time at the Sacramento Peak Solar Observatory in the USA and around 2002 also at the Swedish Solar Telescope (SST).

The vacuum telescope with the longest focal length (46 meters, aperture 70 cm) is the VTT (vacuum tower telescope), operated by the Kiepenheuer Institute for Solar Physics (KIS) at the Observatorio del Teide on the island of Tenerife (Spain). The height of the tower also reduces the influence of external air turbulence. Special features are the vertical echelle spectrograph with a focal length of 15 meters, a system for simultaneous filter recordings of the sun and a Fabry-Pérot interferometer .

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