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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's 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 properly track a star.
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.
Spectroscopy
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).
The HET is designed for Spectroscopy. It's 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 allows
for rapid switching between instruments which makes them well integrated with the observing program
of the telescope.
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