ASTRONOMY - Spectrograph's first light traveled 10 billion years

Sept. 1, 1999
This spectrum of light from a distant, luminous object allowed astronomers to confirm their belief that they had found a quasar-a powerful energy source believed to be caused by a gigantic black hole at the center of a galaxy. This confirmation was one of the earliest achievements of the Macario Low-Resolution Spectrograph (LRS), which achieved first light at the Hobby-Eberly Telescope (HET; Mount Fowlkes, TX) in April. The possible quasar had been identified by the Sloan Digital Sky Survey, a p

This spectrum of light from a distant, luminous object allowed astronomers to confirm their belief that they had found a quasar-a powerful energy source believed to be caused by a gigantic black hole at the center of a galaxy. This confirmation was one of the earliest achievements of the Macario Low-Resolution Spectrograph (LRS), which achieved first light at the Hobby-Eberly Telescope (HET; Mount Fowlkes, TX) in April. The possible quasar had been identified by the Sloan Digital Sky Survey, a project to make a detailed map of a quarter of the sky. The LRS data showed that light from the object, approximately 10 billion light-years from Earth, has been traveling since the universe was approximately an eighth of its current age.

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The first observation made by the LRS, run by the University of Texas (Austin, TX) and Pennsylvania State University (State College, PA), was a spectrum from Supernova 1999bv (see figure), made two days after the exploding star was spotted. The HET will follow the evolution of the supernova even after it fades too much to be seen by smaller telescopes.

The LRS rides inside a multiaxis tracker that allows the telescope to view approximately 70% of the sky. Because of space constraints and a weight limit of approximately 150 kg, it is designed with a refractive collimator and grism disperser-gratings that do the work of a prism. It can image at wavelengths from 365 to 1000 nm-up to 1400 nm for the collimator-and is attached to a charge-coupled-device camera with 15-µm pixels (see inset).

The device has several possible configurations for various observations. It has a single-slit unit with seven positions, capable of viewing areas 4.1 arcmin in one dimension and from 1 to 10 arcsec in the other, in addition to a hole for focusing and another for high-speed spectroscopy. The LRS can also perform multiobject spectroscopy by using a unit with 13 slits, each 1.5 x 15 arcsec. A 12-position wheel offers a choice of filters. There are two grisms-based on echelle gratings that break the spectrum into a square-which can be moved into the instrument's focus.

Two other spectrographs, one with medium resolution and one with high resolution, are being built and will be added to the HET later. Once everything is in place, the HET, with a primary mirror 11 m across made up of 91 spherical segments, will be conducting large spectroscopic surveys as well as observing phenomena that vary over time, such as supernovae.

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