WHT - Mirror Support Systems
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William Herschel Telescope

The Primary Mirror Support Systems

The optical performance of a telescope depends on controlling the deformation of the mirror surface when the mirror is contained in a mirror cell. In fact, a large mirror would bend by hundreds or thousands of times the optical tolerance if not mounted properly. The images would be useless. The problems for large telescopes become severe very rapidly, since the deflections of a structure increase with its weight multiplied by the lever arm at which the weight is applied, and divided by the cross-sectional area of its material, which provides the stiffness. The deflections of a telescope thus increase as the square of the telescope mirror size, so that in this sense a 4.2-m telescope is about three times more difficult to make than a 2.5-m telescope.

The mechanical engineer is therefore faced with the task of calculating the deformations of the structures proposed in order to see whether they will be adequate for the task, and which is the most cost-effective.

The primary mirror of the WHT has sophisticated axial and transverse support systems. The axial support system consists of two subsystems: an axial flotation system made up of an array of pneumatic cylinders ('belloframes') employing roll-diaphragms as seals, together with a pumping system providing the gas pressure needed to support the full mirror weight; and an axial defining system which locates the mirror in its correct position at three points around the mirror edge ('load cells'). Load sensors in the axial definers provide signals which control the pressure in the pneumatic cylinders; the system also allows fine height and tilt adjustments to be made. A system of spring-loaded rest pads supports the mirror when the pneumatic system is not pressurized.

The cylinders of the axial flotation system, a total of 60, are arranged in concentric rings on the floor of the mirror cell. The optimum arrangement was determined by use of a finite-element computer analysis of mirror deflections. The cylinders are divided into three sectors each of 120 degrees, symmetrically disposed about a diameter perpendicular to the telescope's altitude axis. All the cylinders in each group are connected by a system of manifolds and pipes to an individual controller housed inside the mirror cell. Each of the three controllers have two sets of electrically-operated valves: one connects the cylinders to a pressurized nitrogen reservoir, the other opens  the cylinders to a vacuum tank. The valves are controlled by the output of the associated load cell in such a way that the force exerted by the mirror on the defining point is maintained at 0±5 kg during tracking at all angles of the telescope tube from the zenith down to the horizon. The total weight of the primary mirror is 16.5 tonnes. This axial system is a servo one.

The mirror is supported in a transverse direction by mechanical weighted levers coupled by link arms to brackets connected to the edge of the mirror ('axial definers') in much the same way as a conventional push-pull radial support system. However, as the mirror will not rotate with respect to the gravity direction, the weighted levers and linkages are arranged to act only in the vertical direction and are spaced unequally in such a way that each pair of weighted levers, one pushing and the other pulling, effectively supports a 'slice' of the mirror equal to 1/2 of its total weight. This efficient arrangement is only possible in an altazimuth mounting. In plane view the force applied by each pair of weighted lever acts through the centre of gravity of its slice, but in elevation all the forces are applied in the one plane containing the centre of gravity of the whole mirror.

Finally transverse (or radial) definers take the form of tangential links tying the mirror to its cell at three 90 degrees positions.

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Last modified: 13 December 2010