BTO, LLT and periscope subsystems

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BTO, LLT and periscope subsystems

• Celine dOrgeville

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Outline

• Introduction Celine dOrgeville
• Laser path
• BTO and LLT schematic
• Top-level requirements
• Error budgets summary
• BTO performance and budgets Don Gavel
• BTO optics Brian Bauman
• BTO electronics Mark Hunten
• LLT and periscope designs Celine dOrgeville
  Jim Catone

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Laser path

• Five 10-W beams
• Preferred Laser System location on center
  section (A)
• Mirror relay (fiber technology is not there yet)
• On-axis LLT

A
B,C
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BTO andLLT Schematic
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LLT and BTO top-level requirements
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Beam quality error budget

• At CoDR MCAO performance calculations assumed a
  1.5 x DL combined beam quality for Laser BTO
  LLT
• PDR revised beam quality error budget gives 1.73
  x DL (resp. 1.58 x DL) for
• Use of 1.2 x DL laser beams
• The LLT meeting its image quality specification
  (resp. goal)
• Use of premium quality BTO optics
• Impact on the MCAO overall performance
• Requires a 21 (resp. 4) increase in laser power
  to balance the LGS spot size increase and
  maintain performance
• Or accept small penalty in Strehl
• NB this variability is within the margin
  presented at CoDR (36 ?? 0.5 magnitude in LGS
  signal level)

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Optical throughput error budget
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Heat dissipation error budget
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Laser Beam Transfer OpticsPerformance and Budgets

• Donald T. Gavel
• University of California
• Lawrence Livermore National Laboratory

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BTO Alignment Control

• Two pointing and centering systems

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Requirements

• Compatible with single beam system
• Pointing control to 0.05 arcsecond rms on the sky
  for each of 5 beams
• Beams overlap at the LLT entrance pupil (we take
  this to mean within 10 of beam diameter)

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4 Stages of Adjustment

• Stage 1 Mirror mounts placed so nominal beam
  line is within capture range of motorized
  adjustment
• Stage 2 Mirrors remotely aligned into the
  capture range of slow PC loops
• Stage 3 PC closed loop puts beam into capture
  range of high-bandwidth tip/tilt sensors
• Stage 4 High bandwidth uplink tip/tilt loops
  closed

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Motorized range and accuracy

• Range mounting top-end flexure compensation
• Accuracy capture range of uplink tip/tilt 1
  on the sky
• Motorized Adjustments
• Laser PC mirror array
• Range to cover 2mm top-end sag, 30 top-end tilt
  30mr
• Acquisition range of diagnostic PC sensor 20mr
• Accuracy needed to stay centered behind spider
  0.5 mm, 0.1 mr
• Accuracy in closed loop 1 on sky or 0.2
  mr/mirror
• Top ring fold mirror
• 5-beam shaping mirror array
• Pointing and Centering mirrors
• Range 30 on sky (LLT primary tilt) 9 mr
• Stability 0.12 mr/mirror to capture fast t/t

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Closed Loop Uplink Tip/Tilt

• Range of fast tip/tilt mirrors 1 on sky 300
  mr
• Accuracy 0.05 on sky 15 mr
• Off-load the average deviation from mid-range to
  the PC mirrors ahead of the LLT

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BTO Laser Transmission Budget

• 17 surfaces in BTO path
• Requirement Tgt80 (CoDR, Table 24, p74)

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12
13
9
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14 15 16
8
17
7
5 6 3 4
2
1
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BTO Transmission Budget

• Reflection
• "V" band (narrowband 589 nm) coatings
• gt999/surface gt 98 total
• Surface roughness
• Pscat/Pinc (4 p s / l)2
• commercial polish s10-30A gt 91 total,
  precision s2-10A gt 99.1 total
• Dust
• Optical Cleanliness Specifications and
  Cleanliness Verification, SPIE 3782, 1999
• 0.1 area coverage/surface gt 98.6 total
• Composite Budget 95.7 gt 80 required
• Transmission is specified high in order to meet
  the heat dissipation budget, which is more
  demanding

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BTO Heat Dissipation Budget

• Requirement lt 10 Watts total (CoDR, Table 24,
  p74)
• Heat budget spreadsheet

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BTO Heat Dissipation Budget

• Total power in the light scattered by the optical
  surfaces
• Coating 0.75 Watts
• Surface roughness 0.45 Watts
• Dust 0.7 Watts
• Total 1.9 Watts
• Heat budget is dominated by
• Power used by diagnostic cameras (7.2 W)
• Power scattered by optics (1.9 W). This drives
  the cleanliness and premium surface requirement
  for optics

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Beam Quality

• Requirement optical aberrations negligible
  compared to atmospheric distortions (CoDR Table
  24, p. 75)
• Atmosphere s2 0.134 (D/r0)5/3 0.5 radians
  rms 1/12 wave 48 nm rms (D30cm, r020cm)
• Surface error would have to be l/100 on each of
  17 surfaces to meet a 10 nm total error budget!
• Heres a budget based on premium surface
  optics
• Gemini used 95nm rms for the performance error
  budget, split in 88 and 35 nm rms between low and
  high order aberr. resp.

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Scattered Light

• Categories of stray laser light (Rayleigh and
  aerosol scatter)
• 1) Scattered light along the beam path up the
  side of the telescope
• 2) Scattered light along the beam path across the
  primary
• 3) Scattered light along the atmospheric path to
  the sodium layer

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Scattered Light from Behind the Secondary Spider

• 25.2 photons per frame per subaperture without
  baffles (calculations in Appendix Q)

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Scattered Light from Atmospheric Path to the
Sodium Layer
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2
Gemini LGS geometry viewed from space
0
3
4
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Scattered Light from Atmospheric Path to the
Sodium Layer

• Wave-optic simulation of laser propagation
• Backscatter simulated from layered (1km spacing)
  atmosphere
• Backscatter coefficients taken from Gardners
  (1990) measurements
• Scattered light imaged onto WFS focal plane

Subap 6,0, lasers 1 and 2, (WFS for lgs 1)
Subap 1,0, lasers 0 and 2 (WFS for lgs 0)
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Pupil maps of Rayleigh scatter
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Scattered Light from Atmospheric Path to the
Sodium Layer

• Rayleigh from fratricide is significant, on the
  order of the same number of photocounts per
  subaperture as the guidestar itself, for some
  subapertures
• Pulsed lasers with time-gated return will
  eliminate the fratricide issue.
• Rayleigh from the sensed LGSs beam is small, but
  it is important to field-stop correctly

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BTO Optics

• Brian Bauman
• University of California
• Lawrence Livermore National Laboratory

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BTO Beam Path and Control Surfaces
Diagnostic Split
Centering Mirror
X-shaping mirrors
K Mirror
Pointing Mirror
Top-end Ring Fold
Fast Tip/Tilt Array
Relay Optics
Centering Array
Pointing Array

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Top-End Layout
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BTO alignment diagnostics

• Pointing sensor
• Centering diagnostic
• Chopper wheel to isolate each beam plus dark
  open positions
• Outputs drive the PA and CA mirrors in a slow
  closed-loop

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Pointing diagnostic

• Pointing diagnostic has 170 arcsec field, cf. 85
  arcsec diameter LGS constellation some
  vignetting at edge of field
• Plate scale is 0.165 arcsec/pixel
• CCD is 1.3K x 1K with 6.8 micron pixels
• Can be used as a beam quality diagnostic

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Pointing diagnostic0, 42.5, 85 arcsec off-axis
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Centering diagnostic

• Centering diagnostic re-images desired plane to
  the CCD
• Designed as afocal telescope to avoid
  magnification errors with defocus
• Beam fills ½ of short dimension of CCD (allows
  for misalignment)
• Can image either LLT entrance pupil plane or FSA
  plane to CCD either works for control purposes
• Design imaging LLT entrance pupil shown in next
  slide

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Centering diagnostic
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X-shaping mirror (XSM)
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K-mirror
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Relay telescope

• 1 to 1 relay with 5m focal length lenses
• Pupils at CA (centering array) and FSA (fast
  steering array)
• Exact prescription depends on exit pupil position
  of laser system
• Very slow beam avoids air breakdown at focus and
  aberrations
• Large (150mm diameter) lenses avoid clipping of
  beams

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Relay telescope
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BTO electronics

• Mark Hunten

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Beam Transfer Optics Electronics

• Overview of the BTO diagram.

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Beam Transfer Optics Electronics

• The Beam Transfer Optics control consists of
  items that are on the main part of the telescope.
  These would include

• Pointing Mirror
• Centering Mirror
• K Mirror
• Quarter Wave Plate
• Corner Cube Shutter
• Top End Ring Mirror

• Pointing Mirror Array
• Centering Mirror Array
• Fast Steering Mirror Array
• Near Field Chopper Wheel
• Near and Far Field Cameras
• Polarization Sensor

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Beam Transfer Optics Electronics

• The Beam Transfer Optics also has some items that
  are not in the main control loop. These are
  under user control and mostly for monitoring
  purposes

• Beam Dump Mirror
• Power Meter (part of Beam Dump Mirror)
• Alignment Camera selector (for 3 cameras)

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Beam Transfer Optics Electronics

• The Beam Transfer Optics control electronics will
  be located on the main part of the telescope,
  probably on the Center Section.
• These will be located in a thermal enclosure to
  keep the radiated heat to a minimum.
• Items in the enclosure will be the VME control
  computer, servo control electronics, mechanism
  control electronics, monitoring electronics and
  camera interfaces.
• System is distributed on the telescope from the
  center section to the top end.
• This will require substantial coordination with
  the telescope operations groups for the
  installation period.

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PDR Agenda

• Thursday, 5/24
• 0800 Welcome
• 0805 Project overview
• 0830 Science case
• 0930 Break
• 0945 System overview
• System modeling
• 1100 AO Module optics
• 1145 Lunch

• 1245 AO Module mechanics
• 1340 AO Module electronics
• 1400 Break
• 1415 Beam Transfer Optics
• 1510 Laser Launch Telescope
• Closed committee session
• 1800 Adjourn