Updated 3/20/14
Large Synoptic Survey Telescope mount final design
Shawn Callahan
a
,
William Gressler
a
, Sandrine J. Thomas
a
, Chuck Gessner
a
, Mike Warner
a
, Jeff
Barr
a
, Paul J. Lotz
a
, German Schumacher
a
, Oliver Wiecha
a
, George Angeli
a
, John Andrew
a
,
a
Chuck
Claver
a
, Bill Schoening
a
, Jacques Sebag
a
, Victor Krabbendam
a
, Doug Neill
a
, Ed Hileman
a
, Gary
Muller
a
, Constanza Araujo
a
,
Alfredo Orden Martinez
b
,
Manuel Perezagua Aguado
b
, Luis García
Marchena
b
, Ismael Ruiz de Argandoña
c
, Francisco M. Romero
d
, Ricardo Rodríguez
d
, José Carlos
González
d
, Marco Venturini
e
a
Large Synoptic Survey Telescope, Tucson, AZ, USA
b
GHESA Ingeniería y Tecnología, S.A.
c
IK4 Tekniker Research Alliance
d
Asturfeito S. A.
e
Phase Motion Control
ABSTRACT
This paper describes the status and details of the large synoptic survey telescope
1,2,3
mount assembly (TMA). On June
9
th
, 2014 the contract for the design and build of the large synoptic survey telescope mount assembly (TMA) was
awarded to GHESA Ingeniería y Tecnología, S.A. and Asturfeito, S.A. The design successfully passed the preliminary
design review on October 2, 2015 and the final design review January 29, 2016. This paper describes the detailed design
by subsystem, analytical model results, preparations being taken to complete the fabrication, and the transportation and
installation plans to install the mount on Cerro Pachón in Chile. This large project is the culmination of work by many
people and the authors would like to thank everyone that has contributed to the success of this project.
Keywords:
LSST, telescope, mount, elevation, azimuth, linear drives, tape encoders,
1. INTRODUCTION
The Large Synoptic Survey Telescope (LSST) is an 8.4-meter wide-field (3.5-degree) survey telescope, which will be
located on the summit of Cerro Pachón in Chile. The design of the telescope mount assembly is by GHESA Ingeniería y
Tecnología, S.A. Asturfeito, S.A, is fabricating the TMA in northern Spain is shall reinstall the mount on Cerro Pachón.
The design of the mount control, camera cable wrap, and mirror covers is by IK4 Tekniker. The design and fabrication
of the azimuth and elevation linear drives and capacitor bank is by Phase Motion Control.
This telescope mount assembly supports the optics, camera, baffles, and the integral subsystems required to operate the
telescope. This report provides an overview of the final design of the mount and all supporting subsystems.
Figure 1. Telescope mount assembly CAD model
2. TELESCOPE STRUCTURE
The LSST telescope mount assembly (TMA) supports the primary/tertiary mirror cell assembly (M1M3), the secondary
mirror cell assembly (M2), and the camera.
The telescope structure has been optimized to be:
Rigid. The structure has a locked rotor frequency of 7.4 Hz.
Low hysteresis. The structure and joints have been designed to minimize pointing hysteresis.
Constructible. Materials and structural solutions compatible with current manufacturing technologies
Accessible. All critical parts can be inspected during the life of the observatory
Maintainable. Permit the accessibility to the different TMA components for inspection and maintenance
2.1 Structural Modeling
The design of the TMA was verified with a detailed finite element model (FEM). The FEM is fully representative of the
telescope structure developed in the design phase and is divided into two different units: Azimuth structure and
Elevation structure.
Various parts of the LSST finite element model are labeled in Figure 3.
2.2 Azimuth track assembly
The 66-ton azimuth ring supports the weight of the telescope mount assembly. The track is bolted to the concrete pier
with 400 bolts and is supported on high-strength grout.
Figure 3 Azimuth track assembly fabrication at Asturfeito
Figure 2. TMA finite element analysis model
The 66 tone azimuth ring assembly includes the following:
Azimuth brake disk
Azimuth drive stators
Oil tray
Azimuth encoder tape
2.3 Azimuth Structure
The azimuth assembly supports the elevation assembly and azimuth platforms.
Hatches are provide access to the pier.
Figure 5. Azimuth structure general view
Figure 4. Azimuth track assembly details
2.4 Elevation Structure
The elevation structure supports the optical unit assemblies including: M1M3 mirror cell, M2, M2 hexapod, rotator,
baffle, and the camera assembly with its hexapod and rotator. The combined weight of all of these components is 62.5
ton.
The top end of the telescope is supported on four pedestals reinforced with simple tube bracing.
Figure 6: Elevation Structure General View
The elevation structure provides the interfaces for the optical unit assemblies and the camera assembly.
3. MECHANICAL ASSEMBLIES
3.1 Drive System Description
The main drives use distributed dual-gap linear motors. The dual-gap configuration cancels most of the attraction force
between the motor and magnets providing a light and compact layout.
The azimuth motors rotate with to the azimuth structure. The FeNdB magnets are mounted in a ring mounted on the
inside diameter of the azimuth track assembly. The magnets are coated to protect against shock and corrosion.
A similar dual-gap configuration is selected for the elevation axis. For this application a dual radial air gap motor is
required. The magnets are attached to two large drive arcs mounted on the bottom of the elevation structure.
7: Figure Elevation drive stator layout
The number of motor sections for each axis is defined on the basis of system maintainability and peak power request.
The table below shows the length and weight of each section.
Figure 8. Azimuth drive stator layout
Rotor & Stator Segments Main Characteristics
Axis
Element
Number
Length (mm)
Weight per element (kg)
Azimuth
Rotor Segment
70
665
66.2
Stator Segment
16
1033
372
Elevation
Rotor Segment
16+16
660
50
Stator Segment
3+3
1080
415
The electronic drive system of the LSST is based on the most recent technology with the drive and control units
embedded into each drive segment (2 drives per elevation motor section and 1 drive per azimuth motor section). This
dramatically simplifies the wiring to a few power buses and allows the same field bus wiring to connect all motor
segments.
DC power is distributed via four power buses, two for azimuth motors and two for elevation motors. Each bus feeds a
junction box located as close as possible to the motor group. The motors are then star connected. These junction boxes
house fuses to protect each motor and centralize control power and signals to the motors.
The encoder system is connected directly to the mount control system via an Ethernet network. This network position,
velocity and acceleration information with all distributed drives (nodes). This development drastically simplifies the
system, offers extreme redundancy and availability even with multiple failures, while emptying the control cabinet of all
drive hardware.
An additional important advantage is the elimination of high frequency components from the power lines (the power
lines carries DC power to all modules) so that the RFI interference to sensitive instruments is minimized.
The capacitor bank consists of 360 polypropylene self-healing film-foil capacitors, which provide an extensive lifetime
far in excess of 100,000 h. The system is completed with an AC/DC converter for all auxiliary power and a fast
discharge chopper to discharge the energy tank during an emergency or shutdown, located in a dedicated control cabinet
in the vicinity of the capacitor bank cabinets.
The capacitor banks are distributed in eight identical cabinets attached to the keel beneath the azimuth structure. control
cabinet is located directly above the capacitor bank on the azimuth floor.
3.2 Brake System Description
The floating VCS35-FL caliper disc brake (Twiflex) used in both azimuth and elevation braking systems is a spring-
applied, hydraulically-retracted unit consisting of two parts: the spring module and the floating module. The disc has a
minimum thickness of 20 mm.
The azimuth brake system comprises 8 calipers with their corresponding thrusters, brake support and a brake disc. Their
arrangement, guarantees suitable load distribution during emergency braking.
The calipers are joined to the azimuth structure by mean of a brake support. When the thruster pressure is released, the
calipers press against the disc, applying a braking force that depends on the pressure applied by the brake shoes and on
the friction between the brake shoes and the disc.
Figure 9: Azimuth Brakes
The altitude brake system comprises 2 calipers, 1 on each side, with their corresponding thrusters, brake supports and
two brake discs, one on each side of the altitude structure.
The brake calipers are joined to the mounting structure in the same way as the azimuth calipers, by mean of a brake
support. The brake support will be mounted on the azimuth structure, more specifically in the lateral beams that belong
to the yoke. The following figure shows the position of the elevation brakes:
Figure 10: Elevation Brakes
3.3 Limits Description
The mount control system has software limits, directional safety rated limit switches, and power-off hardware switches
for the elevation and azimuth axis. Hard stops restrict the travel to safe limits.
Caliper
Brake
disc
Brake
Support
The limit switches warn when the Telescope is going to reach the end of its rotation range and orders the cut-off of
power supply to the azimuth motors and the activation of the brakes. If this measure were to fail (and the limits provided
in the software), the shock absorbers and mechanical stops would stop the Telescope. The software limits warn when the
Telescope is near the end of its operational range.
3.4 Hard stops and shock absorbers
The azimuth and elevation structures have hard mechanical stops to prevent over travel. The Azimuth Hard Stop System
consist of two (2) shock absorbers (one for each direction). A topple block beam keeps the Telescope from exceeding its
range of rotation.
Figure 11. Limit switch layout
Figure 12: Azimuth Shock Absorber
The elevation hard stops consist of four (4) shock absorbers, (a 2 + 2 shock absorber configuration, two for each
direction, and two shock absorbers on either side of the cradle) and four end stops. The steel end stops are welded to the
elevation central section.
Figure 13: Elevation Shock Absorber
3.5 Balancing System
The TMA includes a system consisting of four motorized Balancing Units. Each unit moves a mass to balance the
elevation assembly in the direction along the optical axis (2 Axial Units) and perpendicular to both the optical axis and
the elevation axis (2 Transverse Units).
The technical solution for the motorized fine balancing of the TMA is based on linear tables. Each table consists of
profile rail guides with ball screw drives. These tables show high guiding accuracy and stiffness across their motion
range. Their compact design allows for space savings. Each rail is actuated by a servomotor controlled by the Mount
Control System. Even though the TMA will leave the workshop completely balanced (within 200 N.m) with the
Balancing Units installed in their mid range, this system allows balancing the elevation assembly for any elevation angle
in case that the original balance is modified.
Both the Axial and the Transverse Balancing Units are attached to mounts embedded in the center section for security
reasons. This location reduces the risk of falling pieces (bolts, nuts…) to the mirror.
Additionally, coarse balancing of
the telescope elevation assembly around the elevation axis is provided by repositioning of the motorized balancing units
and by repositioning/installing up to 200 counter-balance weights (20kg approximately each).
Figure 14. Balancing unit
3.6 Encoder System
The main axis encoder system is based on high accuracy incremental optical encoder technology. The encoder system
current configuration is provided by Heidenhain based on ERA scanning heads AK ERA 8980, MSB ERA
8900C/8902C tapes and EIB 8791 interpolation electronics.
The azimuth encoder configuration is based on a 360º circular outside-mounted scale tape of MSB ERA 8900C type
located in a slot that is machined in an external radial surface of the 15840 mm azimuth ring, four AK ERA 8980
scanning heads and an EIB 8791 external interface box with serial output interface for incremental and absolute encoders
(8 encoder input) supported on the azimuth structure. The elevation encoder configuration is based on one 140.3º MSB
ERA 8902C segment scale tape outside mounted per side, located in an external cylindrical surface of the elevation
cradles at a radius of approximately 3300 mm, two AK ERA 8980 scanning heads per side and an EIB 8791 external
interface box with serial output interface for incremental and absolute encoders (up to 8 encoder inputs). This box is the
same one that is used for the azimuth encoder system.
3.7 Hydrostatic bearings and oil supply system.
The LSST TMA hydrostatic bearing system is designed for the particular characteristics of the telescope, especially
regarding its accuracy, high stiffness and low friction requirements, as well as its size, mass and performances.
The proposed layout of TMA hydrostatic bearings system is based on the hydrostatic bearing shoes (HSB) from the
company SKF.
Applicable to the bearings of either of the two TMA axes, it should be pointed out that the HSBs from SKF are
parallelepiped in shape, made of carbon steel with a bronze layer and equipped with a central recess or pocket.
There are a total of 14 hydrostatic shoe bearings for Azimuth movement, divided into axial and radial.
The TMA has 6 azimuth axial bearings of model HSBM 410, all master ones. All the HSB supports have been arranged
over the stiffer nodes of the Azimuth Structure to properly transmit and distribute the loads coming from the TMA to the
telescope foundation
There are a total of 8 azimuth radial bearings, distributed into two types depending on their behavior in terms of
stiffness, 2 master and 6 semi master bearings.
The azimuth radial bearings are place on the outside of the azimuth ring and very close to the axial ones. As mentioned
above, in the central part of the yokes there is a set of two semi-master radial bearings for each axial bearing, since the
geometry and payloads in these areas indicate the need for this configuration. The Azimuth Radial semi master bearings
have been developed specifically for this project so as, to meet the high stiffness requirements of the TMA. This new
design will enable obtaining a behavior that is in between that of the Master bearing and Slave bearing, with dynamic
stiffness. This is achieved by means of an oil chamber located in the back and the action of the special restrictor it
includes. This “special restrictor” is additional to the “feed restrictor”, which the
HSB will also have, and the two have
Figure 15. Hydrostatic bearing system
nothing in common. In this case, it is possible to control the stiffness and therefore, to make changes to the extent
allowed by the system.
The criteria followed to define the HSB arrangement for the elevation axis are identical to those of the azimuth axis, but
paying special attention to the dynamic behavior (eigenfrequencies) requirements and trying to mitigate as much as
possible any issues leading to hysteresis problems.
In each of the two telescope trunnions there is a set of two radial master bearings and two axial master bearings. Figures
3.15 and 3.16 show the typical layout of each assembly.
The purpose of the hydraulic system is to provide adequate flow of oil at the appropriate temperature, viscosity and
pressure. The HSB and bearing tracks surfaces are always fully separated from each other by an oil film of
approximately 70
m
(80
m
for Azimuth radial bearings) throughout the operating temperature. Vibrations and
pulsations in the hydraulic system are minimized by design. The hydraulic unit in principle is divided up into four
subassemblies: Circulation Oil Tank, Pumping Unit, Cooling Unit and Electrical / Control System.
Figure 16. Elevation hydrostatic bearings
Figure 17: OSS equipment
4. MOUNT CONTROL SYSTEM
The TMA is controlled through either the connection of the TMA control system (MCS) with the telescope control
system (TCS) or through the MCS stand-alone engineering user interface (EUI). The MCS design is based on the
following:
4.1 MCS coordination unit
This unit (MMCS), installed in the telescope computer room, is housed in a Siemens industrial computer
running Linux OSS (SIMATIC IPC547). The MMCS is in charge of managing the interface with TCS through
the Distributed Data Service (DDS), with the operator (EUI) and the execution of the requested commands
ensuring the actions coordination between the necessary subsystems
4.2 MCS subsystem controller
The MCS subsystem controller is based on the LabVIEW RT environment and a NI PXI chassis with an
embedded controller (NI PXIe-1086 chassis with CPU PXIe-8880.). The MCS is in charge of controlling and
commanding the main TMA subsystems including: azimuth and elevation axis, cable wraps, balancing system,
elevation locking pins, M1M3 mirror cover, and deployable platforms. Other specific systems including the
hydrostatic bearing system and oil supply system (HBS & OSS) or TEA thermal control are commanded by
dedicated controllers that are subordinated to this one.
4.3 MCS control architecture
MCS synchronization is achieved by receiving a Precision Time Protocol (PTP) synchronization signal from
TCS. The data and synchronization interface with TCS is done by the MMCS PC that act as a gateway. The
control system equipment is located in different areas in a distributed architecture to optimize the design and
minimize the cable lengths and facilitate maintenance and repair. Figure 18 shows the general control
architecture and summarizes the concepts that have been previously described.
4.4 Control Networks
Two different Ethernet and EtherCAT networks are defined in the MCS. The Ethernet one is used for communication
data link with the other TMA dedicated controllers, for the TMA encoder system and its synchronization as well as for
the interface with TCS. On the other hand, the EtherCAT network is mainly used for the communication system with
the TMA drives and with the I/O modules that are distributed on the TMA. The EtherCAT system provides a dedicated
redundancy system based on a ring configuration. It is organized in two loops due to high data traffic and time criticality
of some data transferred.
The Azimuth and Elevation control algorithms are the most time critical tasks so they use a dedicated EtherCAT loop
that integrates all main drives for azimuth and elevation and the encoder synchronization system. The other EtherCAT
loop is provided to command the position of the rest of subsystems drives as balancing, mirror cover, camera cable wrap
or azimuth cable wrap drives. This loop also includes the distributed I/O modules.
Figure 18:MCS Architecture
4.5 MCS Operation Modes
Two operation modes are available for the MCS: Observing and Engineering.
In observing mode, the MCS is driven by the TCS and the relevant mount subsystems operate as a whole, in automatic.
Engineering mode is used for maintenance or engineering tests. This mode allows the operator to bypass some of the
mount protection limits needed to test the operation of the system and for some maintenance tasks. In this operating
mode it is possible for the operator to command the TMA from the computer room by using the MMCS EUI or locally
by using a hand held device (HHD) based on a tablet PC running a LabVIEW.
4.6 MCS main functions:
The MCS handles position, velocity and time commands sent from the TCS with the accuracy needed to meet the motion
requirements. When slewing not under TCS control, the MCS shall use a motion profile that keeps position, velocity,
acceleration and jerk within the required ranges and meets the slew and settling time requirements.
In all operation modes, the main drives control avoids excitation of the natural frequencies for both slewing and tracking.
MCS continuously reports if it is position and velocity tolerance for all pointing and tracking motions.
4.7 Active damping results,
After thorough analysis of the damping of the most important modes of the TMA, it has been concluded that including
an active damping control loop acting on the azimuth and elevation main drives provides the best damping values. This
result has been obtained after doing an analysis carried out with 100 modes and several altitude positions, with the pier
model and considering a structural relative damping of 2% for all modes. The Altitude and Azimuth control loops have
been tuned for getting a speed control loop bandwidth around 9Hz and a position control loop bandwidth around 2.3Hz.
As a result of the above it can be concluded that the Altitude and Azimuth control loops provide extra damping to the
spider flexion modes in X and Y directions which is important for the performance. They are also able to provide
noticeable damping to: the torsion of the spider about the Z axis, altitude rotation of the top end, and to the rotation
modes of the azimuth structure.
4.8 Power Distribution System
All systems of the TMA are powered at 400V, 50Hz by a large, dual on-line uninterruptable power supply (UPS) with
output voltage stability typically ±1%. This UPS system is part of the Summit Facility and is not considered within the
TMA system. LV cables from the facility UPS system shall be provided by others to both the Main Electrical Panel at
the pier mid-floor and the Oil Supply System Panel in the Machinery Room.
The Main Electrical panel (TMA-PI-PD-CBT-0001) is located at the Pier Mid-floor. The main panel connects all TMA
consumers through the structure by means of sub-distribution panels located throughout the elevation and azimuth
assemblies.
5. CABLE WRAPS
The TMA uses three cable wraps to support azimuth, elevation, and camera rotation.
5.1 AZIMUTH CABLE WRAP
The azimuth cable wrap is made up of seven coaxial rings connected by four steel cables. As the azimuth platform
rotates the rings are allowed to displace vertically. Each ring is machined with holes for each cable or hose. Additional
holes are provided to allow the installation of future lines.
The azimuth wrap has its own drive system to decoupled the system from perturbing the azimuth servo control system.
Figure 19: Azimuth Cable Wrap
Limit switches are provided to protect against over travel or motor failure.
An LVDT is used to synchronize the motion of the wrap with the motion of the azimuth platform.
5.2 ELEVATION CABLE WRAP
Two elevation wraps are located on each side of the elevation axis. Each wrap has a rotation range of 108º.
The wrap uses a horizontal double chain arrangement supported by the azimuth structure and a cylindrical structure
mounted to the elevation structure.
Figure 20: Elevation Cable Wraps
5.3 Camera cable wrap
The camera cable wrap (CCW) accommodates all camera utilities and services. The CCW is located in top end
assembly. The rotation axis is collinear to the optical axis. The CCW has been designed to rotate over a 180 degree
operational range (+/-90 degrees either side of centered position). An additional 8 degrees (+/-4) are available to
accommodate the software limit, limit switch, and hard stop
Figure 21: Camera cable wrap
5.4 M1M3 Cover
The M1M3 Mirror Cover must protect the M1M3 mirror from falling objects (nuts, bolts and tools) and water leaks
through the dome or from and condensation from the thermal control system.
Figure 22: M1M3Cover requirements compliance
6. SCHEDULE
The following schedule for the TMA show the main milestones for the project.
Milestone
Date
Notice to Proceed
01-04-2014
Phase A Kick-off Meeting
02-05-2014
Phase B Kick-off Meeting
15-07-2014
Preliminary Design Review
02-10-2015
Final Design Review
30-03-2016
Integration complete at Asturfeito Shop (Northern Spain)
16-06-2017
Shipment to Cerro Pachón (Chile)
14-12-2017
Final assembly & Test Campaign completion at Site
14-12-2018
7. CONCLUSION
The final design of the TMA is the culmination of the sustained effort of a large team. This paper presents a snapshot of
the many complex subsystems that are now in production. At this stage of development all known major engineering
risks have been addressed, the design has been approved, and fabrication has begun.
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.
REFERENCES
[1]
Kahn, S., “Final design of the Large Synoptic Survey Telescope,” in [Ground-Based
and Airborne Telescopes IV],
Hall, H. J., Gilmozzi, R., and Marshall, H. K., eds., Proc. SPIE 9906, in press (2016).
[2] W. Gressler, J. DeVries, E. Hileman, D. R. Neill, J. Sebag, O. Wiecha, J. Andrew, P. Lotz, W. Schoening, National
Optical Astronomy Observatory (United States)
“LSST
Telescope and site status” SPIE Proceedings Volume
Ground-based and Airborne Telescopes V
September (2014).
[3] Jacques Sebag, William Gressler, Doug Neill, Jeff Barr, Chuck Claver, John Andrew, National Optical Astronomy
Observatory (United States)
“LSST
telescope integration and tests” SPIE Proceedings Volume 9145:
and Airborne Telescopes V
September (2014).
