Mirror Mount Concepts
Location:
•3-point support to locate mirror tiles
•lift mirror into place
•Accurately adjust mirror tiles in the plane
Material:
•Minimise photon scattering – hence minimise thicknesses
•Same type and lengths of plastic and metal as central pivot
Adjustment to focus Cerenkov light:
•Use 500 μm pitch screws to position reflected light spot
Geometry of a single tile
A
Central (load-bearing) pivot
B, C
Location & adjustment supports
Angle BAC = 90o
AB = AC = 400 mm
A
B
C
Screw pitch at B, C = 500 µm
Angular adjustment of 10o is equivalent
to 15 µm perpendicular to plane,
corresponding to 35 µrad mirror tilt
which equates to 0.5 mm at 14 metres
A little Engineering
• Most of the engineering is in the design of the
central, load-bearing pivot !!! Nota Bene !!!
• Forces at B and C are small, so the aims are:
– Minimise material seen by photons
– Match the longitudinal material structure to the
central pivot so that effects from temperature and
humidity changes cancel to first order
– Decouple horizontal frictional forces from
movement perpendicular to the mirror plane
Concept for Mirror Supports
Fibre glass composite
Glue to mirror
Screw thread
Bolt to mirror plate
Hollow aluminium cylinder
Mirror supports at B and C
•Standard mirror support with individual inserts for
mirror tile
•Central region is strictly vertical to decouple frictional
forces from screw-head
Aluminium insert to ensure:
•Contact surface with screw-head
does not deform
•Contact surface is vertical
Plane of mirror (exaggerated)
Tolerances dictate design
• Why don’t we have a spherical region in the
centre of the mirror supports B and C to locate
the spherical head of the screw?
– Tolerances (gluing, assembly) will give contention at B
and C and there will be large frictional forces;
– Any relative movement of the structure in the mirror
plane will translate into displacement perpendicular
to the mirror and hence defocus the Cerenkov ring;
• We cannot position the mirror by dead reckoning
but must do so (carefully) on assembly
Assembly Procedure
•Set screws at B, C to give required
mirror plane for each tile
•Load mirror tile onto central pivot
•Locate mirror tile on supports at B, C
•Adjust edge vertical using plumb line
•Load all tiles and make vertical all edges
•Fine adjustment of mirror plane using
fine-pitched screws at B and C
Tolerance on central pivot: Δx
Balancing moments:
300 F = Δx Mg
Δx
[mm N]
Mg = 250 N
Suppose Δx = 0.1 mm
Then F ≈ 0.1 N
F
Mg
This must be provided by friction at
the two lower supports.
Probably OK ... If not, then
Spacers to supplement friction
Spacers are ...
•Flexible plastic insert
•Trim to size
•Glue to edge of one tile
•No force perpendicular to mirror
NB: Force between tiles ≈ 0.1 N