Europe's New Eye on the Sun

With a wide-open design and a clear aperture of 1.5 meters (4.9 feet), the just-completed Gregor telescope will give European astronomers an important new tool for studying the Sun.

For these next few weeks, untold millions of casual skywatchers will turn their eyes sunward (safely, I hope) to witness an annular eclipse and a must-see transit of Venus. Then they'll go back to their everyday routines, not giving the solar surface much additional thought.

But meanwhile, on the island of Tenerife in the Canary Islands, astronomers are putting the final touches on a new telescope designed to watch the Sun all the time — and maybe to gather a little starlight at night as well.

With a clear aperture of 4.9 feet (1.5 m), the Gregor telescope ranks as the largest solar telescope in Europe and the third largest worldwide. The completely open structure allows for free airflow around its optics, which are housed in a clamshell enclosure rather than a traditional dome.

If you look closely at the image at right you might get a sense that there's more hardware along the light path than normal for a reflector — and you'd be right! That's because it uses Gregorian optics — akin to a traditional Cassegrain, except that the secondary mirror is placed beyond the focus of the f/1.7 primary mirror. This arrangement requires that the secondary be concave, not convex, but more importantly there's a field stop at prime focus that keeps stray light (and heat) from reaching the secondary.

Managing heat will be a major challenge for the Gregor telescope. Its primary, made of low-expansion Zerodur glass, is cooled from the back side to keep its reflective surface from heating up and degrading the mirror's optical performance. The telescope's secondary and tertiary mirrors were fashioned from a special ceramic glass called Cesic, which is silicon carbide reinforced with microscopic carbon fibers. Cesic transfers heat about 100 times faster than other low-expansion glass ceramics.

After bouncing off the Gregor's primary and secondary, sunlight will carom off more than a dozen other mirrors — including an adaptive-optics system — before reaching the telescope's detectors.

This circuitous routing can resolve features on the solar disk as small as 45 miles (70 km), and a cluster of imagers, spectrometers, and polarimeters at Gregor's business end will be used to study how magnetic fields evolve in the photosphere and chromosphere, particularly around sunspots and in active regions. The work won't end at sunset, however, as the night shift will use a double spectrograph to study Sun-like stars.

An international consortium, led by the Kiepenheuer Institute for Solar Physics (KIS), Leibniz Institute for Astrophysics, and the Max Planck Institute for Solar-System Research, has been working on the Gregor telescope for a decade. It sits atop a "solar tower" originally used by the 45-cm Gregory Coudé Telescope (GCT), which was removed and relocated in 2002 to make way for its state-of-the-art replacement.

The telescope's formal inauguration is slated for May 21st — hey, maybe you can sneak a peak of the assembled dignitaries using the observatory's webcam. You can also read more about this project at the KIS website.