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tracker and ccas tower

The system developed for tracking on the HET has no precedent in a large optical telescope. Objects are tracked by a moving instrument package located 13 meters above the mirror at prime focus. Essentially, on the HET, it is the eyepiece that does the tracking. As a star moves overhead, its light bounces off of the large stationary mirror and the tracker package moves to catch it, always precisely at the exact location of focus.

Accomplishing this is extremely complex. It combines focusing and tracking in a single system. If the primary mirror were flat, movement of the tracker within a single plane parallel to the mirror would be enough for it to accomplish its job. The mirror, however is spherical, so the focal surface is also spherical. The tracker must move within a six axis coordinate system (hexapod) to achieve focus as it tracks. The process requires the simultaneous motion of 10 motors to track a star precisely.

Other instruments also ride on the tracker, which weighs in total about 8 tons. These include cameras, correcting optics, fiber feeds to spectrographic instruments located in the basement below the HET, and the Low Resolution Spectrograph.


In 1814, Joseph von Fraunhofer first correctly identified the curious dark bands that are found in spectra when light is passed through a narrow opening and then a prism. The dark areas correspond to a reduction or absence of energy at particular wavelengths, and studying them, a wealth of information can be inferred. This is, in essence, Spectroscopy. It is no exaggeration to say that three-quarters or more of astronomical knowledge would be unknown if the optical spectroscope had never been invented. (Optical Astronomical Spectroscopy, C.R. Kitchin, 1995, p. xi).

spectra-vega vs. sun

The HET is designed for Spectroscopy. It is equipped with three spectrographs, of low, medium and high resolution. The Low Resolution Spectrograph (LRS) is located at prime focus on the tracker. The MRS and the HRS are located beneath the telescope in a climate controlled basement, and are fed by fiber optic cable. The configuration of the spectroscopes enables rapid switching between instruments which makes them well integrated with the observing program of the telescope.

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28 October 2008
Astronomy Program · The University of Texas at Austin
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