WHT - The Mounting
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William Herschel Telescope

The Mounting

The William Herschel Telescope has an altazimuth mount. Maximum rotations are 0-95° from the horizontal and ±270° about East. While observing, however, the accesible altitude is limited to 89.8°-12.0°. The reasons for these limitations are the existence of a zenith blind spot for zenital distances lower than 0.2° imposed by the speed limit in azimuth and that the telescope is partially obscured at elevations below 12° by the dome rail.

For each axis, the servo control system has three feedback loops: (a) current or torque, which is generated within the control electronics, (b) velocity, from the tachogenerator and (c) precision rate, which is developed either from a 17-bit incremental shaft encoder driven through a pinion off the main gear or a 20-bit roller driven encoder, each giving the same accuracy of 0.03 arc sec. Absolute position is obtained from a pair of gear driven shaft encoders on each axis of the telescope. For tracking, a local torque loop around the two motors in each axis maintains continous gear tooth contact during all tracking accelerations up to a maximum of 0.02°/sec². On the rare occasions when tracking an object which passes closer to the zenith than about 0.2°, observing has to cease temporarily while the telescope traverses a 'blind' region through which it will not be able to track continously. During this time, which is less than 3 minutes, the telescope slews through a large azimuth angle (180° if the object happens to pass through the zenith), to reacquire the object as it leaves the blind region.

In normal use, all the telescope drives are controlled from an operations desk by means of a computer system which communicates with each of the motors and encoders through a parallel CAMAC system. All the instrument turntables and cable wrap devices are controlled in sympathy with the telescope motion, as well as the positions of the dome observing slit and windscreen.

The altazimuth design of the telescope means that the field of view rotates as the telescope tracks. In order to compensate for this, it is possible either to mount instruments on a turntable or to place image derotation optics in the beam. The former option is used at the Cassegrain and prime foci, but at Nasmyth, only light instruments may be mounted directly on the turntables and heavy apparatus, which must be kept stationary, is used with image derotators.

The telescope mount weighs 80 tons, or 186 tons including the fork structure.

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Last modified: 08 April 2019