*RSneed@moog.com
; phone 1 303 216 9777 ext 219
Final Design of the LSST Hexapods and Rotator
Ryan Sneed*
a
, Douglas R Neill
b
, Scott Kidney
a
, Constanza Araujo
b
, William Gressler
b
Paul J. Lotz
b
, Dave Milles
b
, Jacques Sebag
b
, Thomas A.Sebring
c
, Mickael Warner
b
, Oliver Wiecha
b
.
a
Moog CSA, 2581 Leghorn St., Mountain View, CA, USA 94043
b
Large Synoptic Survey Telescope, 950 N Cherry Ave., Tucson, AZ, USA 85719
c
Xoptx, 270 Bellevue Ave Unit 260, Newport, RI 02840
ABSTRACT
The Large Synoptic Survey Telescope (LSST) is a large (8.4 meter) wide-field (3.5 degree) survey telescope, which will
be located on the Cerro Pachón summit in Chile. Both the Secondary Mirror (M2) Cell Assembly and Camera utilize
hexapods to facilitate optical positioning relative to the Primary/Tertiary (M1M3) Mirror. A rotator resides between the
Camera and its hexapod to facilitate tracking. The final design of the hexapods and rotator has been completed by Moog
CSA, who are also providing the fabrication and integration and testing. Geometric considerations preclude the use of a
conventional hexapod arrangement for the M2 Hexapod. To produce a more structurally efficient configuration the camera
hexapod and camera rotator will be produced as a single unit. The requirements of the M2 Hexapod and Camera Hexapod
are very similar; consequently to facilitate maintainability both hexapods will utilize identical actuators. The open loop
operation of the optical system imposes strict requirements on allowable hysteresis. This requires that the hexapod
actuators use flexures rather than more traditional end joints. Operation of the LSST requires high natural frequencies,
consequently, to reduce the mass relative to the stiffness, a unique THK rail and carriage system is utilized rather than the
more traditional slew bearing. This system utilizes two concentric tracks and 18 carriages.
Keywords:
LSST, hexapod, rotator, optical positioning
1. INTRODUCTION
This document presents the final design of the Secondary Mirror (M2) Cell Assembly hexapod and the Camera
hexapod/rotator assembly of the Large Synoptic Survey Telescope (LSST). This document emphasizes the changes that
have occurred relative to the baseline design
1
. Interfaces with the Telescope Mount Assembly (TMA), electronics
locations, electronics access, cable routing, and sensors are described in more detail in reference 1.
The LSST
2, 3
is a large, ground-based telescope currently under construction that can survey the entire visible sky every
three nights. This achievement is accomplished via a three-mirror telescope design consisting of an 8.4-meter Primary
Mirror (M1), 3.4-meter Secondary Mirror (M2) and a 5.0-meter Tertiary Mirror (M3)
4
, Fig. 1. This system design
accommodates a 3.5-degree field of view, feeding a large three-lens refractive Camera
5
.
Figure 1: LSST Optical Configuration
M2 Mirror
Camera Focal Plane
Camera Lens
M1M3 Mirror Monolith
Since the optical system, Fig. 1, does not include a fast steering mirror, the telescope has stringent vibration limitations
during observation. This requires structurally efficient hexapods which produce high natural frequencies. The compact
optical configuration also requires a compact camera hexapod/rotator design and limited hexapod motions.
Both the Secondary Mirror (M2) Cell Assembly
6
and Camera
5
utilize hexapods to facilitate optical positioning relative to
the Primary/Tertiary (M1M3) Mirror
3
, Fig. 2, which is required for active optics control
7, 8
. Geometric considerations
preclude the use of a conventional hexapod arrangement for the M2 Hexapod. A rotator resides between the Camera and
its hexapod to facilitate tracking. The requirements of the M2 Hexapod and Camera Hexapod are very similar;
consequently to facilitate maintainability both hexapods will utilize identical actuators.
Figure 2: LSST M2 Hexapod and Camera Hexapod/Rotator Assembly
2. HEXAPODS AND ROTATOR OPERATIONS
The purpose of the hexapods is to maintain proper orientation of the three optical assemblies: (1) camera, (2) secondary
mirror (M2), (3) primary/tertiary mirror (M1M3). In general, the M1M3 is held in its optimum orientation relative to its
mirror cell and the M2 and camera are oriented relative to it.
The disorientation of the optical systems consists of despaces, tilts and decenters. These disorientations are principally
produced by variation in gravitational orientation as a function of elevation angle. Other influences include thermal
variations, wind and creep. The hexapods will be principally operated by a lookup table. The major input to the look-up-
table (LUT) is the elevation angle. Bulk temperature changes will also be included.
The initial lookup table will be determined by finite element analysis of the telescope mount. This lookup table will be
refined by measurements from the camera wave front sensor. The wave front sensor produces a measurement of the wave
front with every exposure. The wave front sensor will be used to both refine the lookup table and to provide temporary
offsets to the lookup table. The temporary offsets will counteract transient effects such as thermal gradients in the structure
and mean wind effects.
The hexapods’ purpose is to counteract the overall disorientation of the optical systems resulting from variations in
elevation angle, thermal gradients, etc. The deformations of the hexapod and rotator assembly will be minimal relative to
the overall telescope deformations. Consequently the hexapods and rotator assemblies do not need to be self-correcting
for their own deformations. All of the requirements for accuracy, repeatability, etc. are for a constant elevation angle and
temperature. The stiffness requirement, however, must be met for changing elevation angles.
The rotator is principally used to de-rotate the image. The mount is an Alt-Az (elevation over azimuth) configuration.
Azimuth motions produce a rotation of the image plane relative to the sky. The rotator is required to counteract this rotation
during an image exposure which is typically 15 seconds .
3. CAMERA HEXAPOD/ROTATOR ASSEMBLY
Positioning and tracking of the LSST camera will be accomplished by the camera hexapod/rotator assembly, Fig. 6. The
hexapod aligns the camera along the optical axis, and the rotator tracks the sky motions by rotating the camera. The camera
hexapod/rotator assembly is entirely electromechanical.
3.1 Camera Hexapod Configuration
The camera hexapod utilizes a traditional 3 "V" actuator orientation, and is very similar in design operation and
configuration to standard hexapods used for secondary mirror positioning on most large astronomical telescopes. The only
major difference is the larger load capacity required for this application. Not only must it support the large and cantilevered
3060 kg camera, and approximately 400 kg rotator mass, but it must do it within tight geometric limitations imposed by
the optical design which requires that the camera, rotator, and hexapod be installed through the M2 cell mirror assembly.
All the hexapod actuators are electromechanical. Since the assembly is located over the camera and M1M3 mirror a
significant drip hazard exists. This requires that all systems be sealed to prevent lubricant escaping and precludes the use
of any hydraulics. The requirements of precision and stiffness preclude the use of pneumatics. Meeting the tight
requirements of accuracy, resolution, and repeatability requires a drive system with a very high overall gear ratio which is
produced by a harmonic drive. The limitations imposed by these tight requirements are partially mitigated by the minimal
speed requirements. The hexapod is only required to produce small motions of the order of microns, during the telescope
4 second slew. The maximum stroke of the actuators is only ±16mm which represents a compromise between achieving
sufficient range of motion of the hexapod and avoiding potential collisions between the camera and the surrounding
structure.
To prevent unnecessary heat production the hexapod must have power off braking. When the telescope is operating, the
hexapod has a duty cycle of only approximately 10%. Consequently, if powered braking was utilized it would significantly
increase the overall heat dissipation. The inherent friction in the high ratio gear system is adequate to provide the power
off braking in this case.
3.2 Hexapod Actuator Design
The final actuator design, Fig. 3, incorporates typical electromechanical elements of a rotary motor, a screw, and a gear
reducer. A 40mm diameter recirculating roller screw with a fine, 1 mm lead is used in the final actuator design as the
preferred means for converting the rotational motion of the motor into translational motion. Due to a larger number of
contact points, roller screws have higher stiffness, larger load capacity, and longer life than comparably-sized ball screws.
Acme screws rely on sliding contact which results in high wear making them inappropriate for long-life applications.
Recirculating roller screws are available with finer leads than standard planetary or inverted roller screws allowing for
improved resolution and increased stiffness. A preloaded split nut arrangement is used to eliminate backlash. Backlash is
considered unacceptable since the active optics system runs essentially open loop which requires a high degree of
repeatability.
A high gear reduction ratio of 50:1 was selected as a balance between achieving the challenging accuracy, repeatability,
and resolution performance while still maintaining sufficient actuator velocity. A harmonic drive (strain wave) gearing
system is used for its multiple advantages over conventional gearheads including zero backlash and excellent positioning
accuracy and repeatability. Based on the strict volume and mass allowances for the actuators, the capability of high gear
ratios in a compact and lightweight package is another key benefit of the harmonic drive.
Figure 3: Hexapod actuator design.
A DC brushless motor allows for higher efficiency, better reliability, longer life, larger power density, and finer motion
control compared to brushed DC motors. The fine pitch of the screw coupled with a high gear ratio limit the torque and
heat dissipation of the motor and allow the system to be self-locking or non-backdrivable. This provides power-off hold
capability and eliminates the need for a brake and the resulting heat dissipation.
An absolute linear sensor measuring the actuator length is required to ensure the drive system is functioning properly and
maintain hexapod positioning knowledge during a power interruption. A high resolution, high accuracy optical encoder
provides this positioning feedback while eliminating the impact of errors associated with the roller screw and harmonic
drive gearing. A rotary encoder on the motor is included for motor commutation. Temperature sensors are required on
each motor to monitor their thermal states.
In addition to software range limits, the actuators incorporate mechanical limit switches whose positions can be adjusted
without disassembling the actuator. Hard end-stops provide a final physical layer of safety to prevent over-travel. A flexible
bellows encloses the roller screw shaft and nut to control contamination while also preventing any stray light from the
optical
encoder from interfering with the telescope’s image quality.
O-ring seals are included at all bolted interfaces to
further protect against contamination and filtered breather vents prevent pressurization or vacuum conditions from forming
inside the actuator as it strokes. Removable access ports allow for periodic re-lubrication of the roller screw with no
disassembly.
The properties of the hexapod actuator end joints have significant effect on the overall hexapod performance. Typically
either rolling elements (bearings) or flexures are utilized for these applications. Ball joints generally have insufficient
stiffness and excess stiction. Flexures are preferred because unlike rolling element joints, flexures produce negligible
hysteresis and stiction. The telescope and its active optics operate though a look-up-table (LUT) which requires a high
degree of repeatability. Consequently, utilizing flexure end joints minimizes hysteresis and improves the overall telescope
performance. The smoother, stiction free operation resulting from flexure end joints facilitates the operation of the hexapod
during imaging.
A relatively simple two degree-of-freedom (DOF) blade flexure design accommodates the tip-tilt deflections of the
actuator end-joints as well as a small amount of twist rotation, Fig. 4. Extensive finite element analysis was performed to
ensure the flexure design was capable of simultaneously meeting the range of motion, seismic loading, and axial stiffness
requirements. Careful material selection was necessary to achieve a viable flexure design. Although initially preferred due
to its high strength to mass ratio, titanium (Ti-6Al-4V) was the determined to be infeasible, but an acceptable design was
found for a PH13-8Mo H1000 stainless steel material. The profile of the flexure gap was tailored to allow for gap closure
just beyond the maximum deflections levels.
Figure 4: Two DOF blade flexure for hexapod actuator end-joints
High axial stiffness of the flexures comes at the expense of significant bending stiffness in the tip and tilt degrees-of-
freedom of the joint. This bending stiffness generates side and moment loads in the actuator which would create premature
wear and decreased performance of the roller screw. A pair of bushings serve as a linear guide and react the side and
moment loads that would otherwise be applied to the screw. Each bushing consists of a PTFE wear strip which was tightly
toleranced and analyzed to ensure contact was maintained against the bore surface at end of life.
Each actuator has a nominal end-to-end length of 620mm and a mass of 58.8 kg. Three lifting points for hoist rings are
radially distributed around the body of the actuator at the center of gravity to allow the actuator to be handled with a crane.
The axial stiffness of the actuator is predicted to be 135 N/μm based on a combination of finite element analysis and
component-level testing. The positioning resolution is expected to be less than 100 nm based on analysis and experience
with similar actuator designs, but this, along with the axial stiffness, will be confirmed by test.
3.3 Rotator
The LSST camera rotator assembly facilitates coarse slewing motion and precision tracking motion. The slewing motion
is responsible for rapidly repositioning the camera assembly to positions within a 182° rotation range. Once the system
has been coarsely positioned, precision motion of the rotator assembly allows the camera to accurately track position while
imaging occurs for 15 second intervals. During precision tracking, the rotator motion is less than 1°. The camera rotator
assembly also serves as the connection between the camera and camera hexapod assembly. A profile view of the camera
assembly, rotator assembly and camera hexapod assembly is shown in Fig. 5. The rotator and camera hexapod assemblies
are shown separately in Fig. 6. An exploded view of the rotator assembly is shown in Fig. 7.
Figure 5: Camera, Rotator and Hexapod Assemblies
Figure 6: Camera Rotator and Hexapod Assemblies
Figure 7: Exploded View of Rotator Assembly
The rotator assembly consists of two aluminum annulus plates. The lower plate interfaces with the camera assembly. The
upper annulus plate interfaces with the camera hexapod assembly. Due to the large environmental temperature range the
rotator will be subjected to, the annulus rings will undergo significant diametric thermal expansion and contraction. The
rotator assembly must cope with this expansion/contraction and minimize the amount of strain that is induced into the
camera assembly. To achieve this goal, the rotator assembly implements a sectional rotation bearing design. The primary
component of this sectional bearing is a THK curved linear guide system. Fig. 8 shows an image of a sample curved linear
guide section.
Figure 8: Curved Linear Guide, Section View
Image courtesy of THK
The rotator utilizes two concentric, continuous ring curved linear guide assemblies with radii of 620 mm and 770 mm.
Each of the two rings feature six 60° profile rail segments. The rails are fastened to the aluminum camera interface annulus.
As the annulus expands and contracts due to low frequency thermal gradients, the segmented profile rail design reduces
the buildup of hoop stresses in the bearing assembly by allowing relative motion to occur between adjacent profile rail
sections. Load is transmitted between two annulus rings via discretely located guide blocks. A total of 18 guide blocks
(nine per ring) are strategically located to result in a mass efficient load path between the two annulus plates of the rotator
assembly and into the camera hexapod and to produce maximum stiffness. Fig. 9 shows a profile view of the rotator and
camera hexapod assembly with rotator guide blocks directly beneath the camera hexapod actuators.
Figure 9: Rotator Guide Block Alignment
The rotator motion system is driven by two DC brushless servo motors. A biased current command scheme is applied to
the two motors to wind up the drivetrain eliminating backlash and allowing for extremely high accuracy position tracking.
High resolution rotary encoders on the motors and a high resolution, high accuracy absolute optical encoder attached to
the ring gear provide feedback to the motion system. The motors include power-off hold brakes to prevent backdriving
and temperature sensors to monitor their thermal states.
The rotator drive motors interface with precision 110:1 ratio gearheads which drive helical pinion gears. The pinion gears
transmit torque to a large diameter helical ring gear mounted to the camera interface annulus plate. The ring and pinion
gear ratio is 14.583:1 creating an overall gear reduction of 1604:1. The high gear reduction ratio allows for excellent
positioning accuracy, resolution, and repeatability and is allowable due to a relatively slow velocity requirement of 3.5
deg/s during slewing. As with the rotator bearing assembly, the rotator motion system must accommodate the thermal
expansion and contraction characteristics of the aluminum annulus plates. The design utilizes a C86300 manganese bronze
ring gear which closely matches the CTE (Coefficient of Thermal Expansion) characteristics of the aluminum interface
plates. Bearing guide blocks positioned between the rotator drive motors are intended to react the axial loads generated
by the helical gear train and limit the relative motion between the hexapod assembly interface plate and the camera
assembly interface plate that would otherwise occur.
Similar to the camera hexapod actuators, the rotator includes software limits and mechanical limit switches to restrict the
range of motion. End stops provide the final layer of protection against over-travel and include a compliant bumper to
limit accelerations into the camera. A locking pin can be inserted using a toggle clamp at any 15 degree increment within
the rotator’s range to lock its position in place during maintenance operations.
Contact switches provide feedback to the
telescope’s safety interlock system to indicate
whether the pin is inserted or retracted.
Lubrication manifolds allow for periodic maintenance on the ring and pinion gears and curved linear guides, and access
ports permit inspection of these critical components. Debris shields around both the inner and outer diameters of the rotator
form a quasi-labyrinth seal and serve several functions, figure 7. They prevent external contaminants from interfering with
operation of the gears or linear guides, preclude greases from escaping from the rotator and potentially falling on the optics,
and hinder stray light from the optical encoder from escaping.
3.4 Camera Hexapod Rotator Design Envelope
The design envelope for the camera hexapod/rotator is principally a short, hollow cylinder, Fig. 10. The outer diameter is
limited by the optical design which requires that the camera hexapod and rotator be installed through the central hole of
the M2 mirror cell assembly. The inner diameter is limited by the protrusion of the camera utility trunk through the rotator.
Figure 10: Design Envelope of Camera Hexapod/Rotator Assembly.
The camera utility trunk which protrudes through the rotator contains most of the camera electronics, Fig. 11. The
maximum outer diameter of the utility trunk of 970 mm is smaller than the internal diameter of the cylindrical envelope,
1136 mm. Sufficient clearance between the two has been provided to allow for the motion of the hexapod and provide
access to the inner ring of fasteners between the rotator and camera.
Figure 11: LSST Camera
3.5 Mass Budget
The 1000 kg mass budget of the camera hexapod/rotator assembly (hexapod/rotator) is principally driven by the pseudo
rigid body mounted natural frequency requirements of the camera. The fundamental natural frequencies of the telescope
installed on its pier are approximately 8 Hz. To minimize the vibration coupling between the camera and hexapod, the
mounted natural frequencies of the camera should be approximately 12 Hz or greater. If the natural frequencies fall below
this limit, vibration coupling will increase the image degrading vibration and the maximum seismic accelerations.
The natural frequency is affected by both the stiffness and the mass. The relevant stiffness is not only the camera
hexapod/rotator assembly stiffness, but also the overall stiffness which included the effect of the top end structure. The
stiffness requirements of the hexapod/rotator were set to the level where they have a similar effect as the top end structural
stiffness. Further increasing the hexapod/rotator stiffness would leave the structure stiffness the dominant effect and would
produce minimal natural frequency increase. Consequently, an increase in hexapod/rotator stiffness cannot be utilized to
accommodate an increase in mass.
The total mass of the camera hexapod and rotator final design is 997 kg, less than the budgeted mass by 3 kg. The rotator
accounts for 415 kg and the camera hexapod is the remaining 582 kg. The lowest natural frequency of the combined
camera hexapod and rotator system was just over 16 Hz assuming the hexapod is mounted to an infinitely rigid base and
the camera is infinitely stiff. The results, based on a detailed finite element model, were determined to be sufficient.
3.6 Payload
Under normal operations the payload of the Camera hexapod/rotator is the 3060 kg camera. During maintenance operations
the overall mass increases moderately to accommodate a filter changer, shutter changer or lens cover. Since these added
mass configurations are for very short durations, normal seismic requirements are not applied. Consequently the 3060 kg
camera mass combined with the seismic accelerations
9, 10
produces the design limiting loads. Not only must the camera
hexapod/rotator support the camera mass but it must also support its own mass of 997 kg. The hexapod and rotator were
thoroughly analyzed for the design limit loads and all components have positive margins of safety.
3.7 Rotator to Camera Thermal Compatibility
The major structural components of the rotator, which is bolted to the aluminum camera body, will be fabricated from
aluminum. Consequently, the camera body and rotator will be thermally compatible. The top flange of the hexapod will
be fabricated from steel and bolted to the steel interface of the telescope mount assembly (TMA). Consequently, the
hexapod and TMA are also thermally compatible. The differential thermal expansion between the steel TMA (and top
hexapod flange) and the aluminum rotator/camera body will be accommodated by the hexapod legs and their flexures.
3.8 Hexapod to Telescope Mount Interface
The hexapod mounts to the
“Offset” of the TMA.
This is a steel structure consisting of two flat rings connected by a
straight cylinder. One ring bolts to the rest of the TMA, and the other bolts to the camera hexapod. Since the only purpose
of this interface is to attach the hexapod, the hexapod contractor determined the fastener pattern, but the flange must be
fabricated from steel and possess a substantial cross section. The utilization of steel eliminates any significant differential
thermal expansion problems. Not only does this interface provide a mounting surface for the hexapod, but it provides
substantial radial stiffness to the offset. The required overall stiffness is divided equally between the offset flange and the
mating hexapod flange.
3.9 Thermal Control
As a result of its location relative to the optical system, thermal control of the camera hexapod/rotator assembly
(hexapod/rotator) is of paramount importance. This thermal control is partially achieved by limiting the heat released by
the various components. These components include the six actuators of the hexapod, the rotator drive motors, and the
sensors and electronics. However containment and removal of the remaining heat is necessary to preserve the image
quality.
Since the heat sources are dispersed, direct cooling of each source is impractical and the air surrounding the camera
hexapod/rotator assembly must be contained. Consequently a flexible, removable shroud is required around the entire
camera hexapod/rotator assembly. This shroud must be readily removable to allow maintenance access to the hexapod,
rotator, and camera utility trunk. It contains the 200 W of heat escaping from the camera's utility trunk which is located
inside the camera hexapod/rotator assembly. A simple system of non-reflective EPDM rubber sheeting attached with
Velcro in six 60 degree sections was designed for this application. The heated air contained by the shroud is removed by
interactive air circulation with the TMA.
4. M2 HEXAPOD
The M2 Hexapod remains attached to the Telescope Mount Assembly when the M2 Cell Assembly is removed. The M2
hexapod is required to utilize identical actuators as the camera hexapod and any actuator that will fit within the camera
hexapod/rotator assembly design envelope will fit within the space available for the M2 hexapod. Consequently, the M2
hexapod does not have a specific design envelope.
4.1 Hexapod Configuration
The bisymmetric Top End Assembly (TEA) of the TMA has 16 spiders which connect its spider spindle with the rest of
the telescope mount. A conventional three "V" hexapod configuration is geometrically incompatible with this spider
configuration. Consequently, the M2 hexapod requires an unconventional actuator arrangement, Fig. 12.
Figure 12: M2 Hexapod Assembly Attached to Spider Spindle.
The Spider Spindle of the TEA functions as the base flange for mounting the M2 hexapod on the telescope. The spider
spindle is the principle structural member of the TEA and is permanently attached to the telescope. Consequently, a
separate test flange is required for testing the M2 hexapod off of the telescope.
The TEA was designed to accommodate any hexapod actuator design that can fit within the camera hexapod/rotator
envelope. Sufficient clearance around each actuator is provided to allow for both the hexapod motion and the maximum
practical actuator diameter. Sufficient length is also provided for any practical length actuator. Spacing blocks will be
required to fill the difference between the actual actuator length and the space available.
Although the mass of the M2 mirror cell assembly (with M2 baffle) is greater than the camera, as a result of the geometric
configuration, the loading on the M2 hexapod actuators is comparable to the loading on the camera hexapod actuators.
Since, the M2 mirror is actively supported in the M2 mirror cell assembly by a set of electromechanical actuators, the M2
hexapod is moderately redundant. Consequently, any hexapod actuator that meets the requirements of the camera hexapod
will likely meet the requirements of the M2 hexapod. Therefore, to facilitate maintainability the M2 hexapod utilizes
identical actuators as the camera hexapod. This includes the entire actuator including the end joints and interfaces.
4.2 Mass Budget
The M2 hexapod and the Camera hexapod are required to utilize identical hexapod actuators. Since the camera
hexapod/rotator mass is limited, the mass of its actuators is indirectly limited. Any actuator mass that fits within the camera
hexapod/rotator assembly mass budget will likely be tolerable for the M2 hexapod. The M2 hexapod to M2 cell flange
was designed in corporation between Moog CSA and LSST and a mass budget was unnecessary. Consequently, a mass
budget for the M2 hexapod was unnecessary.
4.3 Payload
Under normal operations, the payload of the M2 hexapod is the 4,920 kg M2 mirror cell assembly combined with the 151
kg M2 light baffle. Not only must the M2 hexapod support the M2 mirror cell assembly mass, but it must also support its
own mass of 639 kg. During some maintenance operations the M2 light baffle is removed and replaced by a mirror cover.
This produces a slight increase in the overall mass. Since this added mass configuration is only used for short durations,
the normal seismic requirements are not applied. Consequently the 5,710 kg mass total combined with the seismic
accelerations
9, 10
produces the design limiting loads. The M2 hexapod was thoroughly analyzed for the design limit loads
and all components had positive margins of safety.
4.4 Thermal Control
Heat escaping from the M2 actuators only crosses the optical path once, from the sky to the M1 mirror. Consequently, as
demonstrated through computational fluid dynamics, the LSST telescope is substantially (approximately 1/4) less
susceptible to heat escaping from the M2 hexapod than the camera hexapod/rotator assembly. Since it is not combined
with a rotator, the M2 hexapod produces less heat than the camera hexapod/rotator assembly. The dispersion of the six
actuators around the spider spindle makes containment and removal of the heat significantly more difficult. Consequently
no thermal control is required for the M2 hexapod actuators.
5. CONCLUSION
The final design of the M2 hexapod and camera hexapod/rotator assembly meets all the functional and geometric
constraints. The design meets all the interfacing requirements with the telescope, the M2 mirror cell assembly, and the
camera. Detailed design and analysis has been performed to verify the positioning performance, mass, stiffness and
strength requirements can be met and the hexapods and rotator designs have successfully completed a final design review
and integration and test plan review.
6. ACKNOWLEDGMENT
This material is based upon work supported in part by the National Science Foundation through Cooperative Support
Agreement (CSA) 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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