CCAT tertiary mirror
CCAT-TM-76
Version
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1.
Date Author Revisions
7/31/11 S. Padin 1st draft
8/7/11
“
Added Fig. 5
8/8/11
“
Added Fig. 6
9/22/11
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Added Fig. 7
SUMMARY
This note describes a simple concept for the CCAT tertiary mirror. The mirror has machined aluminum tiles
mounted on a carbon-fiber-reinforced-plastic (CFRP) subframe, similar to the primary segments. The emphasis in
this design is on low, uniform coefficient of thermal expansion (CTE) in the subframe, with a relatively simple, low-cost
assembly approach that will not require multiple prototype cycles. The subframe is made of large, flat, single-piece
CFRP ribs, glued in a box configuration. The tile adjusters are simple differential screws made of Invar.
2.
SUBFRAME
√
Figure 1: Rectangular (left) and square (right) tertiary tiling patterns. In both diagrams, the bold ellipse (2.6 × 2.6 2 m) is
◦
the mirror rim for 1 field of view, small circles are the tile adjuster positions, the 3 bold circles are the mirror mounting points
on the back of the subframe, and diagonal thin lines are ribs in the subframe core.
√
The tertiary mirror must be a 2.6 × 2.6 2 m ellipse (for 1◦ field of view) and it must be ∼ 300 mm thick (for
< 1 µm rms gravitational deformation). The main issue with a mirror of this size is controlling the subframe CTE.
The glue that is used to join CFRP has high CTE, typically ∼ 10−4 K−1 [1], cf. a few ×10−7 K−1 for the CRFP, so
it is important to minimize the glue thickness. This favors designs that have long, continuous pieces of CFRP rather
than many small pieces glued together. CFRP sheet can easily be cut with ∼ 1/4 mm tolerance, so if the overall CTE
is to be set by the CFRP, the distance between glue joints must be & 250 mm. This sets the minimum cell size in the
CFRP structure.
It is fairly easy to control the CTE of CFRP laminate in 1d (e.g., along the axis of a tube), more difficult in 2d
(e.g., in the plane of a sheet), and very difficult to control a 3d structure. This leads to a subframe design made of
thin sheets of CFRP with low in-plane CTE. A box with a core made of flat ribs is an obvious option, but other core
designs, e.g., tubes or hexagons, are possible.
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Figure 2: Subframe details. The exploded view (left) shows the intersecting slotted ribs. In the assembled view (right) the top
facesheet has been made transparent to shown the core.
The structure of the subframe core is closely coupled to the pattern of reflecting tiles on the mirror because the
tiles must be attached at points where the core is stiff. For a circular or elliptical mirror, this coupling naturally leads
to a radial rib structure, but the density of ribs near the center of the mirror becomes high, and it is difficult to run
ribs across the full diameter to reduce the amount of glue in the structure. If we instead allow the subframe to drive
the design, which is reasonable because it is the largest and most difficult part of the mirror, then a rectangular array
is more efficient for the core. This leads to the tiling patterns shown in Fig. 1. In both of these designs, the tiles
have a radial support at the center, so this point is placed over the intersection of 2 ribs in the subframe core. The
tile corner supports carry mainly axial loads, so these
√ can be placed over a single rib, but close to an intersection of 2
ribs. The rectangular array in Fig. 1 has 0.3 × 0.3 2 m tiles to just cover the required ellipse. The minor axis of this
mirror is 2.7 m, which will just fit through the 2.8 m hole in the center of the primary. The square array in Fig. 1 has
0.29 × 0.29 m tiles for a minor axis of 2.61 m, which gives more clearance for installation of the tertiary. The smaller
tiles will give smaller gravitational and thermal deformation, and smaller machining errors, but larger blockage due
to gaps and adjuster mounting holes.
The subframe ribs are made of 1–2 mm thick CFRP strips, slotted and glued where the ribs intersect, as shown
in Fig. x. The ribs are glued to facesheets made of similar CFRP sheet. The subframes are assembled and glued at
room temperature to minimize stress.
The choice between a rectangular and square tiling pattern is not obvious. The square pattern gives uniform
mechanical and optical properties in the plane of the mirror, but the field pattern on the tertiary is elliptical, which
seems to favor the rectangular pattern. The rectangular tiling gives the same number of gaps and adjuster holes
across the beam along the mirror axes, but the projected width of those features is less along the major axis. The
square pattern gives about the same projected area for gaps and holes in both directions. The difference between the
two tiling patterns is subtle, and is probably not important as long as the mirror is stable. The square pattern will
achieve smaller surface errors because the tiles are smaller and the subframe is stiffer.
3.
TILES & ADJUSTERS
The mirror tiles are similar to those on √
the primary, but a little smaller (see Fig. 3). The tile size is driven mainly
by the need to efficiently cover a 2.6 × 2.6 2 m ellipse.
The tile adjusters are simple differential screws shown in Fig. 4. The center adjuster has a sleeve that provides
radial support. The corner adjusters have rod flexures to accommodate the difference in CTE between the tiles and
the subframe. The flexures should really be blades, tangent to a circle centered on the center adjuster, but that would
require a more complicated non-rotating design. The diameter of the rod flexure must be small enough to give small
tile thermal deformation, yet large enough to limit in-plane rotation of the tile. Each adjuster is attached to the
tile with 3 small mounting screws. With #10 adjuster screws and #4 mounting screws, the blockage due to all the
holes in the tile is ∼ 0.5%. The adjusters in Fig. 4 require a spring to take up any backlash in the differential screw.
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Figure 4: Tile center adjuster (left) and corner adjuster (right). The adjuster screw has slightly different threads on the 2 ends.
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Figure 5: Center adjuster with lateral alignment tool. The tool slips over the Invar puck on the subframe.
This could be a single spring between the tile and subframe halves of the adjuster, or a spring at each end pushing
between the nut and a fixed flange machined into the adjuster screw. If the adjuster screw has threads with n and
n + 1 threads per inch, one turn will move the tile 1”/ [n (n + 1)]. With n ∼ 30, the tile moves ∼ 0.001”/turn, so we
should be able to achieve ∼ 1 µm resolution.
The adjusters mount on ∼ 40 mm diameter Invar pucks glued to the top of the subframe using a fixture. The
center adjuster on each tile sets the position of the tile on the mirror. The tilt of the center adjuster is particularly
important because a small tilt causes a large lateral position error. The adjuster must be translated and tilted, based
on measurements with a laser tracker and a level, to position the tile within ∼ 100 µm. Fig. 5 shows a simple tool
for adjusting the lateral position of a center adjuster. In this design, the tilt is adjusted using shims. Fig. 6 shows a
more complicated design with a spherical washer and screw adjustments for lateral position and tilt. During assembly,
each adjuster will be set within a few ×0.001” of the required length, the adjusters will be bolted to the pucks on
the subframe, the tile will be placed on the adjusters, the tile nuts will be aligned with the mounting holes in the tile
(which will require up to 1/3rd of a turn rotation of the adjuster screws), the tile will be bolted to the tile nuts, and
then the surface of the tile will be aligned based on measurements in a coordinate measuring machine.
The mirror is supported at 3 points on the back of the subframe. Each mounting point has a ∼ 1/2” thick ×3”
diameter Invar pad glued to the back facesheet over the intersection of 2 subframe ribs. A hexapod truss supports
the mirror on a rotator, as shown in Fig. 7. The rotator could be a standard rotary table for a milling machine.
[1] Hysol EA 9394 data sheet, Henkel Corp., Bay Point CA 94565.
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Figure 6: Center adjuster with a spherical washer for tilt adjustment (left) and with alignment tools (right). The upper set
screws adjust the tilt and the lower set screws adjust the lateral position.
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Figure 7: Tertiary mirror support. An Invar truss connects the back of the mirror subframe to an Invar plate mounted on top
of the rotator. Differential expansion between the Invar plate and the steel rotator is taken up by a ring of blade flexures.
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