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An alt-alt (altitude-altitude) mounting for an 8-metre telescope has several operational advantages over an alt-az (altitude-azimuth) design. In this alt-alt arrangement the yoke (or major) axis, which is horizontal, lies East-West and the second (or minor) altitude axis is located across the yoke and lies in the North-South plane. By comparison, the alt-az has a vertical, azimuth axis pointed at the zenith and its minor axis is always horizontal. Consequently, the altaz telescope cannot track through the zenith because of the infinitely high rotation rate required about the vertical (azimuth) axis, thus it cannot be used to observe at the zenith where the images are sharpest and the atmospheric transmission highest, and its axle and field rotation speeds are disadvantageously high throughout a considerable area of sky in the vicinity of the zenith. The rapid rotation of spectrographs at folded Cassegrain and Nasmyth foci can cause the spectrum to drift on the detector during an exposure because of fiexure within the spectrographs. By comparison, the alt-alt telescope tracks readily through the zenith at slow rotation speeds, and the field rotation rate is zero when tracking along the Celestial Equator where the telescope becomes pseudo-equatorial because one axis (the minor axis) is then parallel to the axis of the Earth. It is at the Eastern and Western horizons where the rate of rotation about the horizontal axis becomes excessive, which is not critical because telescopes are not scheduled to observe there, where the seeing and transparency are very poor, and where in practice the light is usually obstructed by the building enclosing the telescope. When pointed at the zenith, the primary mirror in an alt-alt mounting is suspended over the cooled observing floor, thus minimizing seeing degradation caused by heat generated in the bearings and drives. (It is when the telescope is pointed closer to the horizon that the light path within the telescope tube is over a bearing.) Because of the rapid rotation and acceleration during tracking the alt-az mountings are specified to have exceptionally high resonant frequencies to enable quick response to the drive motors. The alt-alt telescope does not need such high resonance frequencies because its rotation speeds are low, usually even lower than for an equatorial mounting. In summary, the characteristics of the alt-alt mounting are in phase with the astronomical requirements, whereas the alt-az mounting is 90 degrees out of phase with these requirements in the sense that its worst performance is at the zenith instead of at the horizon.
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