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.