ASTRONOMY

Aug. 1, 1998
A pair of comets plunged into the atmosphere of the Sun on June 2--a rare event captured by the Large-Angle Spectrometric Coronagraph (LASCO) aboard the Solar and Heliospheric Observatory (SOHO). In the lower right-hand corner of this image, the comets brighten rapidly as they arc toward the Sun; the lead comet is partly obscured by the Sun`s corona. The disk in the center shows where the telescope blocks out the Sun. The white circle represents the edge of the visible Sun.

ASTRONOMY

Telescope captures death of comets

A pair of comets plunged into the atmosphere of the Sun on June 2--a rare event captured by the Large-Angle Spectrometric Coronagraph (LASCO) aboard the Solar and Heliospheric Observatory (SOHO). In the lower right-hand corner of this image, the comets brighten rapidly as they arc toward the Sun; the lead comet is partly obscured by the Sun`s corona. The disk in the center shows where the telescope blocks out the Sun. The white circle represents the edge of the visible Sun.

The LASCO consists of three coronagraphs, each with an occulter in its focal plane to eclipse the Sun and make the much-dimmer corona visible. Occulting in the first telescope is accomplished with a central hole in its objective mirror. Light from an area just slightly larger than the Sun falls through this hole, leaving a donut-shaped image of an area that extends to a radius three times that of the Sun. This telescope is equipped with a Fabry-Perot interferometer that acts as a narrow-bandpass tunable filter, covering wavelengths from 530.3 to 656.2 nm. A charge-coupled-device camera of 1024 ¥ 1024 pixels records a monochromatic image. All previous space-borne coronagraphs had external occulting disks, which blocked the innermost part of the corona. Superpolished mirrors and extremely smooth coatings make LASCO`s internal occulting possible.

The other two coronagraphs have external occulters, which cut down the amount of stray light within the telescope and let them see the dimmer outer parts of the corona. One instrument, which took this picture, images the area from two times the Sun`s radius to six times. The other reaches from four to 32 times the radius of the Sun, more than three times the previous maximum.

Launched in 1995, SOHO lies about a million miles from Earth [SOHO is reportedly no longer operational.--Ed.]. The comets belong to a family called Kreut¥Sungrazers, which pass through the Sun`s corona as close as 50,000 km from the surface. They are believed to be the remnants of a larger comet that broke apart several thousand years ago.

Neil S

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