Secondary mirrors for large ground-based telescopes often require positioning systems with payload capacities around
1000 kg, relative accuracies within a few micrometers, and resonant frequencies above 15 Hz. A suitable six-legged
parallel manipulator, or hexapod, has been developed for sub-micron level positioning of large optical payloads in six
degrees of freedom. This 1000 kg class hexapod has tip/tilt rotational ranges of ±1800 arcsec, relative accuracies within
1%, and resolutions of better than ±0.2 arcsec, along with a piston translational range of ±30 mm, relative accuracy
within 1%, and resolution of better than ±1 μm. The center of rotation of the system may be placed at an arbitrary
location within the overall range limitations. The axial stiffness of each of the six actuators tested greater than 100
N/μm. The actuators use high precision roller screws and employ two degree of freedom universal end-joints. The
preload on the joints eliminates backlash due to transitions from tension to compression and maintains friction moment
of <10 Nm. An additional rotational degree of freedom is allowed in the body of the actuator to achieve the proper
kinematic constraints for the motion platform. The actuators have power-off hold capability to protect against power
loss and reduce heat dissipation. Overall heat dissipation has been measured and techniques have been studied to reduce
its impact. The paper describes the actuator design and hexapod performance in support of planned use in ground test
and validation of the James Webb Space Telescope.
KEYWORDS: Telescopes, Kinematics, Spherical lenses, Space telescopes, Large telescopes, Electroluminescence, Actuators, Data modeling, Interfaces, Safety
Secondary mirrors and lenses in several planned ground-based telescopes have masses on the order of 5000 kg and
require a positioning system that is repeatable to one-tenth the pixel size of the optical sensors, nominally 10
micrometers or less. Hexapods, or Stewart Platforms, are frequently integrated into the support structure as six degree of
freedom parallel positioning and alignment systems. These systems are limited in resolution by friction in the 36
kinematic degrees of freedom (DOF) necessary for properly constrained motion of the platform. The 30 passive DOF,
typically implemented with one 3-DOF and one 2-DOF joint on each hexapod leg, introduce unwanted friction and/or
backlash into the positioning system. Backlash is generally unacceptable and elimination requires significant preloading
of the joints, which in turn increases joint friction. This paper will: review various joint types including rolling element,
plain bearing (sliding), and flexure; examine the backlash and friction tradeoffs involved in selecting joint type including
unwanted deflections due to joint moments, static position resolution limitations, dynamic positioning settling time
effects, self-locking mechanisms, and power dissipation; compare with experimental data and previously published
results; present methods for modeling both static and dynamic effects of friction; and suggest recommendations for
general positioning system design. Considerations for both equatorial and altitude-azimuth telescopes will be discussed,
along with variation of effects due to telescope positioning. Analyses will be reinforced with friction and backlash
measurements made on several physical joints.
Hexapods can be an effective means of positioning optics of all sizes, including those within large ground-based
telescopes. A hexapod is often a convenient geometry when multiple axes of positioning are required. The paper
reviews several small and mid-sized hexapods built for different applications, and emphasizes experience with a three-meter-
diameter unit built to position a large optical component. The discussion highlights design tradeoffs in precision,
including repeatability, resolution and accuracy, range in multiple axes, and bandwidth of operation, and addresses test
and verification of performance. The paper concludes with a discussion and presentation of hexapod concepts for
secondary mirror positioning for Thirty Meter Telescope and Cornell Caltech Atacama Telescope.
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