dneill@lsst.org
1 520 318 8578
Final Design of the LSST Primary / Tertiary Mirror Cell Assembly
Douglas R. Neill*, Gary Muller, Ed Hileman, Joe DeVries, Constanza Araujo,
William J. Gressler, Paul J. Lotz, Dave Mills, Jacques Sebag, Sandrine Thomas,
Mike Warner, Oliver Wiecha
Large Synoptic Survey Telescope, 950 N Cherry Ave, Tucson, Az 85719
ABSTRACT
The Large Synoptic Survey Telescope (LSST) primary/tertiary (M1M3) mirror cell assembly supports both on-telescope
operations and off-telescope mirror coating. This assembly consists of the cast borosilicate M1M3 monolith mirror, the
mirror support systems, the thermal control system, a stray light baffle ring, a laser tracker interface and the supporting
steel structure. During observing the M1M3 mirror is actively supported by pneumatic figure control actuators and
positioned by a hexapod. When the active system is not operating the mirror is supported by a separate passive wire rope
isolator system. The center of the mirror cell supports a laser tracker which measures the relative position of the camera
and secondary mirror for alignment by their hexapods. The mirror cell structure height of 2 meters provides ample
internal clearance for installation and maintenance of mirror support and thermal control systems. The mirror cell also
functions as the bottom of the vacuum chamber during coating. The M1M3 mirror has been completed and is in storage.
The mirror cell structure is presently under construction by CAID Industries. The figure control actuators, hexapod and
thermal control system are under developed and will be integrated into the mirror cell assembly by LSST personnel. The
entire integrated M1M3 mirror cell assembly will the tested at the Richard F Caris Mirror Lab in Tucson, AZ (formerly
Steward Observatory Mirror Lab).
Keywords:
Borosilicate, mirror, hardpoint, actuator, hexapod, active optics
1. INTRODUCTION
The M1M3 mirror cell assembly (M1M3 assembly) is one of the three major optical components
1
of the LSST
telescope
2
, which is presently under construction in Chile
3
. This paper presents the final design and construction status of
the LSST primary / tertiary (M1M3), mirror cell assembly. It emphasizes the changes that have occurred relative to the
baseline design
4
. The primary / tertiary mirror cell assembly, Fig. 1, consists of the primary / tertiary, borosilicate glass,
mirror (M1M3), its active support system, its passive support system, its thermal control system, a light baffling ring, a
laser tracker and the mirror cell structure. At a mass of 53,000 Kg (117,000 lbs) it is the principle payload of the
telescope mount.
During observing the M1M3 mirror is actively supported by a set of pneumatic figure control actuators and a hexapod.
These systems are based on the legacy of the previous large cast borosilicate mirrors. The active figure control actuators
both distribute the load to safely support the monolithic mirror and actively control its shape (active optics) to maintain
adequate image quality. The position of the mirror relative to the mirror cell is controlled by a set of six hardpoints
(position controlled actuators) that form a large hexapod. When it is not supported by the active support system, the
M1M3 rests on a set of wire rope passive supports.
The center of the mirror cell supports a laser tracking head which through the use of retro reflectors on the M1M3
mirror, camera
5
and secondary mirror (M2)
6
determines the relative position of these optics. The M1M3 is held in its
optimum position relative to the mirror cell by six hard points forming a hexapod. This hexapod is not actively operated
during observing. The position of the M1M3 determines the optical axis. The camera and secondary mirror are aligned
to it by their hexapods
7
which are actively operated during observing
8, 9
.
The mirror cell assembly with a height of 2 meters was designed to provide ample internal clearance for servicing the
complex support systems required to operate the M1M3 mirror. Besides the active and passive support systems the
mirror cell also supports an extensive thermal control system to manage the thermal condition of the mirror
10, 11
. This
system utilizes on-board blower assemblies to provide cooling air to the honeycomb cells of the M1M3 and is based on
the legacy of the Magellan telescope.
Figure. 1. Primary Tertiary Mirror Cell Assembly.
To limit the risk to the borosilicate mirror, all servicing must be accomplished without removing the glass from the cell.
Therefore, during coating, the mirror cell must function as the bottom of the coating chamber. Since the coating must be
applied in a vacuum, the mirror cell must also function as a vacuum chamber. The vacuum-induced mirror cell
deformations must be isolated from the mirror support system to prevent overstressing the mirror. This is accomplished
by tailoring the structural interaction between the top deck, which supports the mirror and the truss systems which
supports the vacuum boundary. The mirror cell assembly will be removed from the telescope by a cart. The same cart
will be used to transport the mirror cell assembly to the coating facility. During the entire transportation and coating
process, the M1M3 mirror monolith will remain in the mirror cell, and the mirror cell will remain on top of the cart.
2. PRIMARY / TERTIARY (M1M3) MIRROR
The Large Synoptic Survey Telescope utilizes a unique three-mirror design to provide a 3.5 degree field of view
1
, Fig. 2.
The annular 8.4 m primary mirror (M1) and 5.1 m tertiary mirror (M3) share a single monolithic substrate, Fig.3. This
substrate is a cast borosilicate hexagonal honeycomb sandwich, fabricated by Richard F Caris Mirror Lab
12, 13, 14, 15
in
Tucson. It has a continuous planar 25 mm thick backplate and a 28 mm thick face sheet. The face sheet and backplate
are connected by 12 mm thick ribs in a hexagonal pattern with a center to center spacing of 192 mm. Each honeycomb
cell has an 89 mm diameter hole in the backplate. The hole is used both to remove the cell mold after casting and to
allow cooling air injection into the mirror.
Figure 2. Optical Layout (left) and Isometric View of M1M3 Mirror Model (right).
Figure 3. Primary Tertiary Monolith Mirror Design - Section View.
2.1
Primary/Tertiary (M1M3) Mirror Support System
The M1M3 mirror is supported in its cell by three systems: 1) an array of pneumatic figure control actuators, 2) a
hexapod consisting of six stiff hard point actuators for rigid body position/motion control, and 3) an array of wire rope
isolators called static supports for safely supporting the mirror during periods when mirror is not being supported by the
figure actuators / hard point hexapod, Figs. 4 & 5.
Figure 4. View of bottom face of M1M3 mirror showing: a) pneumatic figure control actuators, b) hard point hexapod. The
static supports are not shown.
Figure 5. Load spreader and Axial Actuator layout, Right side is load spreaders only, Left side shows associated actuators.
Pneumatic Figure Control Actuators: During observing operations, the weight and inertial forces of the mirror are
reacted by the pneumatic figure control actuators, Figs. 6 & 7. The actuators are uniformly distributed over the back side
of the mirror and support the mirror via pneumatic pistons/cylinders with compressed air. Load cells mounted in-line
with the piston/cylinders provide force measurements to Inner Loop Controllers (ILC) mounted to each actuator. Each
inner loop controller then communicates with the telescope control system
17
to provide overall mirror figure control.
The figure control actuators do not provide any position sensing capabilities. Position sensing is achieved with the hard
point hexapod and an independent measuring system (IMS). These systems are described in subsequent sections.
There are two types of pneumatic figure control actuators: Single Axis Actuators (SAA) and Dual Axis Actuators
(DAA). Both types have an axis that is nominally parallel to the optical (Z) axis to provide axial forces. The DAAs
have an additional axis that is nominally tilted 45 degrees from the optical axis to provide lateral forces which are
required to counteract the transvers component of gravity, wind and dynamic loading. Each axis consists of a pneumatic
cylinder with an 80 mm diameter piston capable of producing 3500 N force in both directions. The piston stroke is +/-
30 mm, which is sufficient travel to accommodate the total motion budget of the M1M3 mirror. Universal joints
mounted to both ends of the actuator cylinders allow them to pivot as the M1M3 mirror displaces laterally (x and y) in
the cell up to +/- 20.8 mm and vertically (z) +17.2 mm/-24.3 mm. The nominal distance between the upper and lower
universal joints at mid travel of the piston is 450 mm.
Figure 6. Left: Dual Axis Actuator (DAA). Middle and right: Two configurations of the Single Axis Actuator (SAA). Each
axis consists of a pneumatic cylinder/piston (a), a load cell mounted to the upper piston rod (b), a conical base attached to
the bottom of the cylinder body (c), and a universal joint at the bottom of the conical base (d). On the DAA, forks above the
load cells (e) connect to a common universal joint on top (f). This universal joint is designed to interface with triple and
quad puck load spreaders on the back of the M1M3 mirror and is common to one configuration of the SAA. A second
configuration of the SAA uses a different universal joint (g) to interface with the single and dual puck load spreaders. The
two different upper universal joint configurations on the SAAs require two different bases (h, i) to correctly position the
cylinders in the vertical direction. Servo valve modules (j) are mounted to the aluminum actuator housings and each cylinder
uses two servo valves. The valve modules are designed for easy removal/installation using quick release ball lock pins. The
Inner Loop Controller (k) mounts on the housing also.
Figure 7. The M1M3 pneumatic figure actuator system uses five different configurations. They are: a) Quad puck
LS/DAA, b) triple puck LS/DAA, c) triple puck LS/SAA, d) dual puck LS/SAA, and e) single puck LS/SAA.
One hundred DAAs are oriented with the two actuator axes parallel to a plane normal to the elevation (X) axis of the
telescope so that when the telescope moves off zenith, lateral (gravity) forces in the y direction can be reacted by the
second axis of the DAAs, Fig.8. Twelve DAAs are oriented parallel to the x axis (cross-laterals) and keep the mirror
centered in x-direction. Forty-four SAAs are mounted around the outer perimeter and around the center hole of the
mirror.
Figure 8. View of back side of M1M3 mirror showing the mirror support system. The M1M3 mirror support system
coordinate system is defined as follows: The x-axis is parallel to the telescope elevation axis, the y-axis points up when the
telescope is horizon pointing and the z-axis points into the page and is parallel to the optical axis. Mirror support
components are identified as follows: (a) 44 Single Axis Actuators (SAA), (b) 100 Dual Axis Actuators (DAA) oriented
parallel to the Y-Z plane (laterals), (c) 12 DAA oriented parallel to the X-Z plane (cross-laterals), (d) 6 hard point actuators
arranged in a hexapod. The static supports are not shown.
Hardpoints: Since the M1M3 is supported by force control actuators during operation, a set of six axially stiff linear
displacement actuators (termed “hardpoints”) are used to define
the location of the M1M3 relative to the mirror cell
16
. In
general, the M1M3 mirror is passively held in its optimum location relative to its support system by these hard points.
The M2 and Camera are positioned relative to the M1M3 by their hexapods.
The upper ends of each strut attaches to the glass wedges are epoxy bonded to the mirror's backface, and at the lower end
they attach to stiff structural pads inside the cell. Although they are also displacement actuators, these devices are
typically referred
to as “hardpoints” to distinguish them from the force based figure control “actuators." Their hexapod
configuration is shown in Fig. 9 on the left, the hardpoint design is shown on the right. They incorporate displacement
controlled linear actuators (positioning mechanisms) which control the overall length of the hardpoint and hence the
position of the M1M3 mirror. Since they define the location of the mirror and provide vibration resistance, they must
have a relatively high stiffness of 110 N/um. They must also be able to repeat the position of the M1M3 within 5 um
RMS.
When the figure actuators are depressurized, the load must be transferred to the passive static supports. This requires that
the hardpoint accommodate the necessary motion of the mirror. This is accomplished in the axial direction by a
pneumatic “breakaway” mechanism as part of, and inline with, the hardpoints.
These breakaway mechanisms limit the
force transmitted in the axial direction by the hardpoints. Two sets of biaxial flexures provide compliance in the lateral
directions and limit the load in the other four degrees of freedom. An additional flexure component on each strut
provides rotational flexibility about the long axis of each strut to prevent torque from being transmitted into the
connections at the glass wedges on the M1M3. The breakaway mechanism along with the flexures limit the stress in the
relatively fragile borosilicate mirror. Each hardpoint also contains an axial load cell for determining the axially
transmitted force which is necessary for operating the M1M3 mirror supports in active mode.
Figure 9. The M1M3 hardpoints shown with the mirror (left) and the design elements (right).
Static Supports: When the figure actuators are depressurized, the M1M3 mirror sinks onto a set of passive static
supports, Fig. 10. Wire rope isolators are the principal component of the static supports which also include some minor
mounting components. These static supports provide support in all three directions. Since only the 3 and 4 puck
loadspreaders are intended to carry transverse (perpendicular to the optical axis) load and the static supports transmit
transverse load, the static supports are only located with the pucks of these loadspreaders.
The pucks located directly adjacent to the inner hole of the M1M3 are not attached to static supports. Because of the
reduced areal density (thinness) at this location, the removal of these static supports reduces the stress in the glass.
RTV Bond (Purple)
on Steel Pad (grey)
Triple Invar
Spreader
Static Support
Wire Rope Isolator (Gold)
Wire Rope Isolator
RTV Bond (Purple)
on Steel Pad (grey)
Triple Invar
Spreader
Static Support
Wire Rope Isolator (Gold)
Wire Rope Isolator
Figure 10. Static Supports
–
Wire Rope Isolators (right).
2.2
The M1M3 is attached to its mirror support system by bonded pucks, loadspreaders and glass wedges.
The primary / tertiary mirror (M1M3) is connected to the mirrors support system through steel pucks bonded with Q3-
6093 RTV and borosilicate glass wedges bonded with Norland 61 adhesive, Fig. 11. The 426 steel pucks connect to
invar loadspreaders, Fig. 12, which connect to the pneumatic figure control actuators and the static supports discussed
below. The 6 borosilicate glass wedges connect to the hardpoints that form the hexapod, also discussed below.
Figure 11. Mirror attachments: a) steel puck, b) 4 mm thick RTV bond pad, c) invar load spreader frame, d) mirror support
actuator interface cup, e) borosilicate glass wedge, f) lip around wedge, g) hard point hexapod actuator.
Figure 12. Section view of the back surface of the mirror showing the load spreaders bonded onto the back face.
The steel pucks are bonded to the backface (bottom surface of backplate) of the mirror utilizing 4mm thick RTV bonds.
These bonds distribute the load reducing stress concentrations. The RTV experiences nontrivial creep in the transverse
direction. The 30 inch square pucks are specifically sized to produce 100% creep (4mm) over the 30 year design life of
the telescope. It has been demonstrated that these RTV bonds can safely experience 100% creep without rupture.
The steel pucks are grouped together and attached to loadspreaders, Fig. 13. These include 8 single pucks, 30 double
puck, 114 triple puck and 4 quad puck loadspreaders. Each loadspreader group is attached to a figure control actuator.
Figure 13. Load spreader layout.
2.3
The LSST primary / tertiary mirror support system has two operational modes:
Active mode: During observing, the M1M3 mirror must be in the active operational mode
8, 9
. In this mode, the figure
control actuators are operated to control the shape of the M1M3 optical surface, which is required for adequate image
quality. These actuators support the entire mirror load from gravity, manufacturing/assembly errors and the quasi-static
component of wind in both the optical axis direction and the lateral directions. The forces for these actuators are
determined by a combination of a lookup table, the forces measured by the hardpoint load cells and wavefront error
analysis.
The great majority of the force from the figure control actuators is used to counteract gravitational effects. An actuator
force distribution is applied that minimizes the gravitational distortion of the mirrors. Since the actuator forces
principally counteract gravitational effects and the optimum actuator force distribution for counteracting gravitational
effects varies with elevation angle, the principal input for determining the forces for the figure control actuators is the
elevation angle. The initial force values for the figure control actuators are determined through finite element analysis,
and updated through wave front measurements.
Besides the forces required to counteract gravity, a static manufacturing correction force is applied to the mirror to
correct for the imperfections of the mirror. The optical surface of a mirror is manufactured to a level that is correctable
by the figure control actuators with forces that are within the force budget allocation. Normally this process is referred to
as “figuring” however, “manufacturing” has been substituted
in this document to reduce confusion with figure control
actuators. The initial manufacturing forces are determined by the combination of laboratory measurements and finite
element analysis. The actual forces used on the telescope will be updated during commissioning by the actual on-sky
telescope measurements.
The software used to determine the repeatable force values for the figure control actuators is commonly referred to as a
“Lookup
Table.” This software determines the forces needed to counteract gravity and manufacturing error based
principally on the elevation angle. Experience has shown that the optimum values are often affected secondarily by
azimuth angle, camera rotator angle, temperature, etc. Consequently the lookup table is set up to incorporate multiple
input parameters. These secondary parameters represent repeatable but unpredicted forces, consequently, they evolve
over time during commissioning and operation.
Although the hardpoints are extremely stiff, they are only intended for locating the M1M3. They are not intended to
provide force for supporting the mirror. Any force that is transmitted through them is measured by their load cells and an
equivalent dispersed load is applied by the figure control actuators to counteract this force. Otherwise, the forces through
the hardpoints would distort the M1M3 and degrade the image quality. The effects of wind, telescope motions,
manufacture/assembly tolerances and all the errors from the application of the figure control actuator forces combine to
produce a set of 6 hardpoint forces. The forces are measured by load cells within the hardpoints and decomposed into the
six degrees of freedom. A distribution of corrective forces is applied through the force control actuators to counteract
these 3 forces and 3 moments. These distributed actuator forces are applied as offsets to the forces determined by the
lookup table.
After counteracting gravity, manufacturing errors and offloading the hardpoints, a minimal force is used to correct the
remaining optical surface distortions of M1M3. Applying the proper forces for counteracting gravity, etc removes most
of the figure distortion. The effects of thermal gradients, hysteresis and other unknowns produce enough surface
distortion that the remaining optical surface error may be unacceptable. The wavefront error is determined then by
corner sensors in the camera. The telescope control system used this information to apply small corrective forces through
the figure control actuators to remove the remaining figure errors. These forces are applied as offsets to the lookup table
forces and the hardpoint offloading forces. Since these forces required for removing the optical surface distortions
produce no net forces or moments on the mirror they do not interfere with the lookup table forces or the hardpoint
offloading.
The breakaway mechanism of each hardpoint is pressurized to give the hardpoint determinate axial breakaway forces in
tension and compression. These breakaway forces are set by the allowable stress within the mirror. If the axial loads
carried by the hardpoints exceed these values, the breakaway mechanisms incorporated into each hardpoint rapidly
reduce the axial hardpoint stiffness to a minimal value. Consequently, once the mechanisms have broken away, a near
constant force is maintained regardless of the compression of the mechanism. The stroke of the hardpoint positioning
mechanism is controlled to determine the location of the M1M3 relative to its cell.
During the active mode, the static supports cannot be allowed to apply any force to the mirror. This is accomplished by
maintaining an air gap between each static support and its spreader attachment. Air gaps must be maintained in the axial
direction and both lateral directions.
Static Mode: Whenever the M1M3 is not supported by the active modes it rest on the static supports (wire rope isolator).
During this static mode, figure control actuators and hardpoint breakaway mechanisms are depressurized allowing the
M1M3 to rest on the static supports. These must allow enough motion of the M1M3, relative to the mirror cell, to both
overcome the air gaps between loadspreader and static support, and compress (or shear) the support in order to transfer
load to the static support springs. This motion must be allowed in both the axial direction (zenith pointing) and the
gravity lateral direction (horizon pointing).
The static support springs must be capable of supporting the mirror for the combined loading of gravity at any elevation
angle and seismic accelerations in any direction. The forces produced by the static support must not over stress the
mirror and sufficient extension / compression of the springs must be available to prevent the static supports from
bottoming out.
3. THERMAL CONTROL SYSTEM
As a result of the large thermal expansion coefficient of borosilicate glass (28e-7/K), large mirrors fabricated from this
material require extensive thermal control to prevent excessive thermal distortion
10, 11
. For cast borosilicate mirrors this
thermal control is typically provided by blowing ambient temperature air into each honeycomb cell of the mirror as
shown on Fig.14-left. The air enters each honeycomb mirror cell through a 25 mm diameter nozzle (A). This air
circulates within the honeycomb cell, cooling the bottom surface of the face plate, the ribs and the top surface of the
backplate (B). The air then leaves through the 89 mm diameter hole at the bottom of each cell (C). The returning air
flows over the bottom surface of the backplate providing further cooling (D). The air is reconditioned via heat
exchangers in 96 fan/coil units shown on the left side of Fig. 14 and injected into the plenum (E) via the fan unit. Lastly
it reenters the original 25 mm diameter nozzles and repeats the process (F).
Fig. 14. On-Board Fan/Coil Assembly (Right) and M1M3 Air Circulation Thermal Control (Left).
A Magellan telescope type primary mirror thermal control system will be utilized for providing thermal control for the
LSST M1M3 mirror
10
. This type of system supplies coolant to multiple on-board fan/coil units, Fig.14-right, in the
mirror cell. Each blower contains both a fan and a heat exchanger. The blower units supply conditioned and mildly
pressurized air to the mirror cell. The mirror cell then acts as an air plenum which distributes the air through individual
nozzles to each honeycomb cell of the mirror. This type of system can meet the stringent temperature and flow rate
specifications required to meet the LSST image quality budget.
Maintaining adequate shape of the mirror optical surface requires sophisticated operation of the thermal control system
based on the following principles
10, 11
. The ambient temperature at the start of observing must be adequately predicted
based on measured and historical data
12
. The M1M3 mirror must be precooled to a temperature ~ 1C below the expected
ambient temperature at the beginning of observing. The air flow rate should be regulated to balance the image
degradation from mirror seeing with the image degradation from mirror distortion. This balance will be achieved by
regulating the cooling air flow rate by varying the fan speed. Since a mirror surface temperature greater than ambient
produces more image degradation than an over cooled mirror, the cooling air temperature will be target ~ 1C below
ambient.
Since the maximum temperature change rates usually occur at the beginning of the night time, theoretically the image
quality requirements can be met by a combination of mirror precooling and a moderate flow rate (~2.5 liters/sec per
nozzle). The requirement of 8 liters per second per air nozzle utilized on previous large borosilicate mirror cooling
systems (LBT) was retained to provide operational flexibility and to meet the -0.7 C/hr design requirement without
precooling. The -0.7 C/hr is the typical maximum nighttime temperature change rate. Variations in ambient temperature
change rates, and limitations in the ability to predict the initial ambient temperature will limit the ability to accommodate
the large change rates by precooling. The occasional greater change rates can be accommodated for short periods by the
combination of precooling, targeting a 1C below ambient and the -0.7 C/hr cooling change rate.
The M1M3 thermal control system will incorporate 100 watts of resistive electric heating elements in the supply duct of
each fan/coil unit. These will normally serve to trim the air temperature discharged from each unit for optimizing mirror
temperature control and uniformity, but have been sized to also provide a positive heating rate of the mirror substrate of
2 degrees C per hour when needed to recover from a subcooled mirror condition or to accommodate the worst case
temperature rise rate.
4. LIGHT BAFFLING
The LSST is very susceptible to stray and scattered light rays impinging directly and indirectly on the 64 cm diameter
detector because of the wide field of view (3.5 degree) and camera position. Substantial stray light analysis has been
performed to optimize numerous structural features within the telescope mount, mirror cell, and dome to minimize this
effect. Consequently, the LSST has substantially more baffling than is normally encountered on an 8 meter class
telescope.
As shown in Fig. 1, a light baffling ring / cooling air plenum and aperture stop encompasses the M1 outer diameter. The
transition between M1 and M3 is also baffled. The clear aperture of the primary mirror defines the optical system
aperture stop. This mirror will be polished to within 22.5
mm of mirror’s edge. A flexible aperture stop is attached to the
light baffling ring, covers the unpolished edge, and defines the edge of the M1 clear aperture. The outer diameter light
baffling ring supports the flexible aperture stop and is also required for thermal control. The transition from M1 inner
clear aperture to M3 outer clear aperture occurs within a 50 mm wide radial band between the two polished surfaces. A
light absorbing material (film or paint) will be applied over this annular region to define the two mirror clear apertures.
The exact nature of this material has not yet been determined.
5. LASER TRACKER
The principal method for aligning the optical system is through information gathered by the wavefront sensors inside of
the camera. In addition to the wavefront sensors, the telescope incorporates a laser tracker system as shown in Figs. 15 &
16. The laser tracker head resides in the central hole of the M1M3 where it has line of sight to spherically mounted retro
reflectors (SMRs) attached to the M1M3, M2 and camera. It uses these SMRs to determine the relative position of the
optics. For the M1M3 and the M2 the SMRs are attached to the outer edge band of the mirror. For the camera they are
attached to the first lens (L1) mount outside of the periphery of the lens as shown in Fig. 15 left.
Tower
(gray)
Flexure Base
(red)
M1M3 mirror
Optical Surface
SMR
(gold)
SMRs
Tower
(gray)
Flexure Base
(red)
M1M3 mirror
Optical Surface
SMR
(gold)
SMRs
Figure 15. Laser Tracker System (left) and SMR mount to outer band of M1M3 (right).
Figure 16. Laser Tracker System Measuring the position of the SMR mounted to outer band of M1M3.
The laser tracker has a lower accuracy (~15
μm)
but a much larger range than the wavefront sensor and does not require
the telescope's camera to align the optics through on-sky images. Consequently, the laser tracker system is only
moderately redundant. The principle utilization of the laser tracker will be for positioning the optical assemblies (M1M3,
M2 and camera) during commissioning, pre-aligning the optics before the start of a night's observing and realigning the
optical system if they become misaligned outside of the practical capture range of the wavefront sensor system.
6. MIRROR CELL STRUCTURE
6.1
Mirror Cell Structural Requirement.
The LSST M1M3 mirror cell structure supports the M1M3 cast borosilicate monolith mirror, all of its support systems
and its thermal control system. It provides this support while it is attached to the telescope, during its transportation to
the coating area and while under
vacuum during “recoating.”
The mirror cell structure provides a rigid deckplate for mounting the figure control actuators, the static supports and the
on-board blower assemblies. For proper operation of the mirror supports and to prevent overstressing the mirror, the
maximum gravitation deflection requirement of the deck was initially set to 1 mm peak to valley. This limit was adopted
from the requirements of the Large Binocular Telescope (LBT) which utilizes a similar mirror. Since the mirror cell is
attached to the telescope it must function at any elevation angle between zenith pointing and horizon pointing.
The mirror cell structure also provides stiff locations, relative to the mirror-cell-to-telescope mounting flanges, for
mounting the bases of the hardpoints. These base mounting locations are only allowed +/-0.25 mm of displacement in
any direction from the zenith to horizon pointing elevation change. Since the hardpoints define the location of the mirror,
their mounted stiffness influences the mirror's rigid body natural frequencies. To prevent coupling with the telescope's
8.0 Hz lowest natural frequencies the natural frequency of the mirror when mounted through its 120 N/um stiffness
hardpoints should be 15 Hz or greater.
In addition, the mirror cell structure provides a vacuum boundary during the recoating of the mirror. To avoid excessive
stress in the mirror, the vacuum loads will not produce excessive deck deflections. The mirror cell structure must
withstand the combined vacuum loading / gravitational loading, while supporting both the mass of the mirror and the
mass of the coating chamber. Furthermore, the mirror cell structure must safely support the mirror during movement on
the transportation cart to and from the coating area.
Finally, the mirror cell will be easily removable from the telescope, it will isolate the mirror cell from the deformations
of the telescope mount, provide sufficient access for maintaining the mirror support systems, and provide an access port,
interface with the transportation cart and support the laser tracker head. Furthermore, the mirror cell structure meets all
its requirements without exceeding an aggressive mass limit of 25,424 kg (56,000 lbs).
6.2
Mirror Cell Structural Layout.
Fig. 17 shows a structural design. The active and static supports are mounted to the deckplate. The deckplate is locally
stiffened and supported by the deck support truss. The vacuum loads are reacted by a vacuum cylinder and the vacuum
support truss. The deck support truss and the vacuum truss are intertwined but not interconnected. This configuration
provides substantial isolation between the vacuum-induced structural deformations and the deckplate.
Figure 17. Mirror Cell Structural Layout.
The mirror cell is only attached to the telescope mount by four pylons. The interface to these pylons are shown in Fig 17.
This simplifies the removal of the mirror cell assembly from the telescope and minimizes the interaction between the
mount deformations and the mirror cell deformations. The four pylon mounting locations are connected by hardpoint
support trusses. The hardpoint bases attach to the mirror cell structure at the locations shown in Fig. 17. This
configuration minimizes the length of the structural load path from the hardpoint base mounts to the pylon mounting
locations. Minimizing this path length maximizes the stiffness and the rigid body natural frequency of the mounted
mirror. Since this truss structure only supports the hardpoints it experiences minimal influence from the gravity-induced
deformations of the rest of the mirror cell.
The structural layout also accommodates mirror cell maintenance. An access port is provided into the cell and 2 meters
of interior height is provided to access the mirror support systems and thermal control systems. The design also provides
for the support and access of the laser tracker head, and interfaces to the transportation cart.
6.3
Mirror Cell Structural Analysis Results
To demonstrate that the stress levels, buckling factors and deformations are acceptable, the mirror cell structure was
analyzed using finite element analysis (FEA) at the two extreme values of elevation angle, horizon pointing and zenith
pointing
4
. The stress, buckling factors and deflection at any intermediate angle is a combination of these two
orientations. Although the absolute maximum values may occur at an intermediate angle it cannot be significantly larger
than either of these orthogonal orientations. Since the safety factors were large for horizon pointing and zenith pointing,
determination of the angle of minimum safety factors and their minimum values is not warranted. The results are
presented in Table 1. The vacuum loading structural analysis is provided in reference 4.
The mirror cell structure was analyzed for stress, buckling and deflection, by FEA for combined loading of gravity and
earthquake accelerations. Seismic accelerations used for the analysis are 2.65g horizontally and 1.77g vertically. These
values correspond to event having a return period of 300 years, and include the magnifying effect of the structure
between the cell and the ground. If a seismic event occurs when the mirror is supported by the figure control actuators
the active optics system will detect a fault and transfer the mirror to the static supports. For seismic events, the mirror
cell was analyzed with the mirror supported on its static supports.
The analyses were conducted for every combination of both elevation angles (horizon and zenith pointing) and
earthquake accelerations in all three orthogonal directions (X, Y, Z). While mounted to the telescope, the cell structure
meets all stress and buckling requirements, with no case having a safety factor against yield stress of less than 1.4, nor a
buckling load factor less than 4.3. The maximum deflection of the deck plate under gravity loading alone while
supporting the mirror in the zenith position is 0.8mm peak to valley. This meets a stringent requirement originating from
the mirror supplier, Steward Observatory Mirror Lab in Tucson. It is needed to limit the long duration mechanical
bending stress in the mirror substrate to a low value which precludes propagation of fractures in the cast surfaces of the
mirror interior. There were no earthquake deck deflection requirements imposed. The first 6 natural frequencies of the
mirror cell assembly are rigid body displacements of the mirror relative to the mirror cell of 14.2 - 20.1 Hz which only
marginally meets the 15 Hz first frequency requirement.
Table 1. Mirror Cell on Telescope Analysis Summary.
7. SUMMARY
Other than stated below, the LSST Primary Tertiary Mirror (M1M3) cell assembly design fulfills all the requirements for
supporting the M1M3 mirror both on the telescope and on the mirror cell transportation cart during mirror coating and
transportation. When imaging, the requirements are met by supporting the M1M3 mirror with a combination of
pneumatic figure control actuators and hardpoints forming a hexapod. A cooling air system provides thermal control to
limit the thermal distortions of the mirror and the mirror seeing. When not imaging the M1M3 is supported by a set of
static supports (wire rope isolators).
8. ACKNOWLEDGEMENTS
This material is based upon work supported in part by the National Science Foundation through Cooperative Agreement
Award No. AST-1227061 under Governing Cooperative Agreement 1258333 managed by the Association of
Universities for Research in Astronomy (AURA), and the Department of Energy under Contract No. DEAC02-
76SF00515 with the SLAC National Accelerator Laboratory. Additional LSST funding comes from private donations,
grants to universities, and in-kind support from LSSTC Institutional Members.
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