The

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The Cobra Fiber Positioner, the WFMOS
Design,and Potential lessons for
DESpecMichael Seiffert, Jet Propulsion
LaboratoryRichard Ellis, Caltech
DESpec RAS and University College London March
2011
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Fiber Positioner Design Considerations
Density Should roughly match desired target
density. Practically, this means 1000
sources/deg2 At 4-8m telescope prime focus
with typical plate scale this translates to
positioner separation of 10mm. Number many
1000s. Maximize subject to budget
constraint. Throughput Efficiency Positioning
accuracy should be high, and losses due to tilt,
despace, and non-telecentricity should be small.
Reconfiguration Speed Fiber movement and
position verification should proceed rapidly
compared to exposure time. Robustness A
mechanically stiff system facilitates accuracy
and allows lenses at fiber tip or other treatment
of fiber ends.
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WFMOS Concepts Are Relevant to DESpec
Although the detailed designs are different,
WFMOS, PFS and DESpec may share system aspects
Fiber connector mounted on top end structure
Prime Focus Unit includes Wide Field Corrector
(WFC) and Fiber Positioner.
Spectrograph located above Naysmith platform
Fiber Cable routed around elevation axis and
brings light to the Spectrographs
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Prime Focus Instrument (PFI)
In the WFMOS and PFS designs, several Subaru
provided elements (field rotator, hexapod and
wide field corrector) are shared with the
HyperSuprimeCam
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Rotating Portion of PFI
Rotator Interface Ring
Alignment System
Cobra Optic Bench
Cobra Modules with Drive Electronics
2400 Cobra Fiber Positioners
Positioner Equipment Bench
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Positioner
Optical Bench with 2400 Positioner Units
1 Positioner Unit - Cobra
Room for gt4000 positioners 8mm apart in hexagonal
pattern to enable field tiling
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Positioner Element Cobra
Top View
Fiber arm
Patrol Region
Phi stage
Second axis of rotation
Theta stage
First axis of rotation
Fiber Tip

• Each motor rotates to provide complete coverage
  of the patrol region.
• Optical fibers mounted in fiber arm which
  attaches to upper postioner axis
• Fiber runs through the center of the positioner
  this couples the positioner and fiber system
  schedules and work efforts

Cobra
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Geometry
Cobra Positioner Patrol Area (9.5 mm dia.)
Phi Stage (2.4 mm radius)
Theta Stage (2.4 mm radius)
8mm
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Patrol Regions
Patrol Region Area of the focal plane
accessible to one fiber (9.5 mm diameter)
Adjacent patrol regions overlap with no gaps
Patrol Region may have zero or may have many
potential astronomical targets
Allocation efficiency describes the success rate
in assigning targets to fibers

• Low target densities degree of overlap between
  patrol regions is unimportant. Important not to
  have gaps.
• High target densities degree of overlap not
  important there are many targets in each patrol
  region to choose from
• Intermediate target densities (target density
  positioner density) there is some benefit to
  having larger overlap.

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Positioner Module

• A module is a subassembly of actuators and drive
  electronics boards
• Staggered production
• Parallel module integration
• Early mechanical and electrical functional
  testing
• Parallel fiber integration to reduce schedule
• Increases serviceability

Positioner Electronics Boards
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Motors
Commercially available rotating tube motor High
torque when stationary and unpowered 1 mN-m
powered torque 1 mrad resolution 1 10 rev/sec
speed

• Pairs of PZT plates oscillate in tandem bending
• Drive signals of the two PZT plates are phase
  shifted by 90 degrees
• This creates a traveling wave on the stator that
  excites the rotor like a harmonic gearbox to
  rotate the shaft by extremely small angles

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Cobra Prototype
2nd stage motor
1st stage motor
Fiber optic

• Ceramic friction drive
• Lubrication free, zero backlash
• Journal bearing limits motor side loads
• Hardstops to limit fiber twisting

• 5 um precision of fiber positioning
• Motor movement lt 1 sec/iteration

Cobra system tested at JPL in partnership with
New Scale Technologies Achieves 5µm positioning
accuracy in 6 iterations Prototype has also
successfully completed lifetime and environmental
testing.
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Prototype array of positioners is an essential
precursor to proposing for a 2400-4000 element
system
Cobra fiber positioners
fiducial fibers
Multiplexed motor electronics
Proposed laboratory and on-sky testing of
19-element system via NSF/Caltech
Proposed 7-element prototype to demonstrate
mechanical integration, tolerances, integrated
electronics
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Metrology Camera establishes science fiber
positions relative to fixed fiducial fibers on
positioner focal plane

• WFMOS concept
• Four camera systems each looking at a ¼ of the
  focal plane. Located on prime focus support
  struts looking back at positioner focal plane via
  primary. Each camera is 4k by 4k CCD with 15µm
  pixels. Cameras are defocused to allow
  centroiding.
• Future
• Larger format (10 k x 10k) single camera?
• Are back-illuminated CCDs (better centroiding)
  really required?

Metrology camera (1 of 4 shown)
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Science fibers are back-lit in a sequence to
allow discrimination between fibers in the
overlap regions between adjacent fibers, 1/3 at
at time. In one exposure only the fibers marked
(1) are illuminated, the next exposure only the
ones marked (2) are illuminated, etc.
Back-lit fiducial fibers used to establish
position of science fibers on positioner plane.
Encoder fibers used to establish rotation
orientation

• Positioner moves elements in 3 groups of 800
• Metrology camera views back-illuminated fibers.
  Fibers are illuminated in 3 groups of 800.
• Movement, illumination, camera readout,
  computation in parallel
• 6 iterations can be completed in lt 40 seconds

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Fiber connector options
APOGEE US Conec 30 fiber connectors ganging
with custom fixture allowing simultaneous mating
of 300 fibers. Wilson et al., 2010
WFMOS team B study Custom connector for 800
fibers with simultaneous mating. De Oliveira,
2008
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• Key challenge of fiber-fed spectrographs getting
  the fiber placed accurately on the astronomical
  target
• large field of view
• large number of fibers
• smaller diameter fiber
• Make sure the system design addresses these
  challenges
• Robust positioner design provides high precision
• attention to differential mechanical flexure in
  overall structure
• error budgets for mechanical tolerances
• Correction for non-telecentricity?
• Include imaging mode with fast readout for
  verification!

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Conclusions Future directions

• WFMOS design emphasizes instrument efficiency
  stiff, robust, precise positioner system with
  fast reconfiguration speed
• Lifetime, thermal, and simulated altitude testing
  of prototype complete. Dust or other
  contamination testing TBD.
• Now engaged in the PFS design effort
• Two proposals now pending for small array
  demonstration. Critical to demonstrate that
  technical risks are retired and costs are
  understood before scaling up to thousands of
  elements.
• Internal JPL proposal pending for 7 element lab
  module
• NSF proposal pending for 19 element, on-sky,
  Palomar demonstration
• Investigation now underway of improved fiber
  coupling. Concepts include elimination of fiber
  twist, tilt of fiber end for non-telecentricty
  correction, and inclusion of small lens at the
  fiber tip for f-ratio conversion.