Transcript Adaptive Optics Update

MCAO
BTO, LLT and periscope
subsystems
Celine d’Orgeville
MCAO
Outline
• Introduction – Celine d’Orgeville
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•
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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 d’Orgeville & Jim
Catone
May 24-25, 2001
MCAO Preliminary Design Review
2
MCAO
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
A
May 24-25, 2001
MCAO Preliminary Design Review
B,C
3
BTO and
LLT Schematic
MCAO
Near- and far-field beam profiles
Beam position & tilt
PA, CA
Miscellaneous
Closed-loop
Boresight
Periscope
Pre
Alignment
Calibration
Features
X-Shaping
Control
Mirrors
BS
PA, CA
CC
Top-End
Ring Mirror
Top
End
Center
Section
PAC
PAC
Relay
Pointing
Array (PA)
PA, CA
MCAO
CS
BDS
Centering
Mirror (CM)
Beam Dump
Mirror
Shutter
LGS WFS
KM
Pointing
Mirror (PM)
Fast Steering Array
Offload to KM, PM, CM
l/4
Plate
LLT
cover
MCAO
CS
PAC
Centering Array (CA)
l/4 Plate
Safety Shutter(SALSA)
May 24-25, 2001
PS
MCAO Preliminary Design Review
LLT
PM, CM
4
LLT and BTO top-level requirements
MCAO
Beam launch
On axis
Do not obstruct M2 hole  PERISCOPE
LGS constellation
Rotating X-shaped constellation
1 beam on-axis, 4 beams 42.5 arcsec away
Optical throughput
> 0.7 for BTO and LLT combined (laser mounted
on telescope)
Beam quality
Do not degrade beam quality significantly
compared to atmospheric turbulence
Beam pointing
accuracy
< 1 arcsec peak (blind pointing)
< 0.05 arcsec rms (dynamic)
Heat dissipation
< 10W for beam dump on top-end
< 10W above M2
< 10W along telescope truss
Misc.
Controlled by MCAO CS
Low maintenance
Preserve circular polarization
May 24-25, 2001
MCAO Preliminary Design Review
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Beam quality error budget
MCAO
• 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)
May 24-25, 2001
MCAO Preliminary Design Review
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MCAO
Atmo. Comp. Strehl in J, H, K bands
MCAO performance vs LGS signal
21% signal decrease
K
H
J
Design point
Photodetection event per subaperture @ 800Hz
May 24-25, 2001
MCAO Preliminary Design Review
7
Optical throughput error budget
LGS subsystem
Laser power output
CoDR
PDR
P = 6-15W
To be reviewed
according to sodium
measurements
BTO transmission
(assumes laser mounted T
on telescope)
BTO > 0.8
LLT transmission
(incl. beam clipping)
T
Projected laser power
P L = 4-11W
May 24-25, 2001
MCAO
LLT
> 0.9
T BTO = 0.95 (per D.
Gavel’s calculations)
T LLT = 0.96 (per D.
Gavel’s calculations)
TBR
MCAO Preliminary Design Review
8
Heat dissipation error budget
LGS
subsystem
CoDR
PDR
Laser system
< 100W
< 100W
LLT
< 10W for
beam dump on
top-end
< 10W for
BTOOB + LLT
Total
< 120W
BTO
May 24-25, 2001
MCAO
< 8 W for BTOOB
< 10W for beam dump on top-end
< 10 W for all other BTO element
combined
< 2W for LLT alone
< 130W
MCAO Preliminary Design Review
9
MCAO
Laser Beam Transfer Optics
Performance and Budgets
Donald T. Gavel
University of California
Lawrence Livermore National Laboratory
BTO Alignment Control
MCAO
• Two pointing and centering systems:
• 1. Alignment up the telescope and
across the top-end behind a spider
vane
– P&C mirrors for each of the 5 beams
separately: on the laser table
– P&C sensors: on the BTOOB .
• 2. Into the LLT & pointing on the sky
– P&C mirrors on the BTOOB do “common
mode” steering on the sky, to
compensate top-end flexure
– pointing sensed by the AO WFS and offloaded.
May 24-25, 2001
MCAO Preliminary Design Review
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Requirements
MCAO
• 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)
May 24-25, 2001
MCAO Preliminary Design Review
12
4 Stages of Adjustment
MCAO
• 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 P&C loops
• Stage 3: P&C closed loop puts beam into capture
range of high-bandwidth tip/tilt sensors
• Stage 4: High bandwidth uplink tip/tilt loops closed
May 24-25, 2001
MCAO Preliminary Design Review
13
Motorized range and accuracy
MCAO
• Range = mounting + top-end flexure compensation
• Accuracy = capture range of uplink tip/tilt = ~1” on the sky
• Motorized Adjustments:
– Laser P&C mirror array
• Range to cover 2mm top-end sag, 30” top-end tilt: ~30mr
• Acquisition range of diagnostic P&C 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
May 24-25, 2001
MCAO Preliminary Design Review
14
Closed Loop Uplink Tip/Tilt
MCAO
• 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 P&C mirrors ahead of the LLT
May 24-25, 2001
MCAO Preliminary Design Review
15
MCAO
BTO Laser Transmission Budget
• 17 surfaces in BTO path
X-Shaping
Mirrors
Top
End
Top-End
Ring Mirror
Center
Section
10
9
8
7
BS
12
11
Centering
mirror (CM)
13
14 15 16 KM
Pointing
mirror (PM)
Fast Steering Array
56
34
2
Relay
Pointing
Array (PA)
1
Centering Array (CA)
• Requirement: T>80% (CoDR, Table 24, p74)
May 24-25, 2001
17
MCAO Preliminary Design Review
LLT
16
BTO Transmission Budget
MCAO
• Reflection
– "V" band (narrowband 589 nm) coatings:
– >999%/surface => 98% total
• Surface roughness
– Pscat/Pinc = (4 p s / l)2
– “commercial” polish s=10-30A => 91% total, “precision” s=2-10A =>
99.1% total
• Dust
– Optical Cleanliness Specifications and Cleanliness Verification,
SPIE 3782, 1999
– 0.1% area coverage/surface => 98.6% total
• Composite Budget: 95.7% > 80% required
• Transmission is specified high in order to meet the heat
dissipation budget, which is more demanding
May 24-25, 2001
MCAO Preliminary Design Review
17
BTO Heat Dissipation Budget
MCAO
• Requirement: < 10 Watts total (CoDR, Table 24, p74)
• Heat budget spreadsheet:
Component
Location
Number
Power per
component
Total power
Optics
Secondary and Top
Ring
11
127 mW
1.4 W
Optics
Primary Ring
4
127 mW
0.5 W
P&C diagnostic cameras
Secondary
2
3.6 W
7.2 W
P&C motors
Secondary
4
Fast tip/tilt stages
Secondary
5
40 mW
0.2 W
Chopper wheel
Secondary
1
0.5 W
0.5 W
PA and CA motors
Primary Ring
20
negligible
Total
May 24-25, 2001
negligible
9.8 W
MCAO Preliminary Design Review
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BTO Heat Dissipation Budget
MCAO
• 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
May 24-25, 2001
MCAO Preliminary Design Review
19
MCAO
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 (D=30cm, r0=20cm)
• Surface error would have to be l/100 on each of 17 surfaces to
meet a 10 nm total error budget!
• Here’s a budget based on “premium surface” optics:
Optic
Flat
Lens
Beamsplitter
Total
surface quality, waves
P-V
0.05
0.05
0.25
rms
surface,
nm
6.25
6.25
31.25
rms
wavefront,
nm
number of
surfaces
12.5
3.125
15.625
10
2
3
rss sum of rms
wavefront, nm
39.52847075
4.419417382
27.06329387
48.10876349
• Gemini used 95nm rms for the performance error budget, split
in 88 and 35 nm rms between low and high order aberr. resp.
May 24-25, 2001
MCAO Preliminary Design Review
20
Scattered Light
MCAO
• 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
May 24-25, 2001
MCAO Preliminary Design Review
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Scattered Light from Behind the
Secondary Spider
Laser
beam
MCAO
laser beam
...
spider vane
10 mm
spider vane
10 mm
subapertures
qa
qa
subaperture
50 cm
50 cm
• 25.2 photons per frame per subaperture without
baffles (calculations in Appendix Q)
• Baffles are recommended along the sides of beam
May 24-25, 2001
MCAO Preliminary Design Review
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Scattered Light from Atmospheric
Path to the Sodium Layer
1
Gemini LGS
geometry:
viewed from
space
2
0
3
4
May 24-25, 2001
MCAO
MCAO Preliminary Design Review
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Scattered Light from Atmospheric
Path to the Sodium Layer
MCAO
• Wave-optic simulation of laser propagation
• Backscatter simulated from layered (1km spacing)
atmosphere
• Backscatter coefficients taken from Gardner’s
(1990) measurements
• Scattered light imaged onto WFS focal plane
Subap [1,0], lasers 0 and 2
(WFS for lgs 0)
May 24-25, 2001
Subap [6,0], lasers 1 and 2,
(WFS for lgs 1)
MCAO Preliminary Design Review
24
Pupil maps of Rayleigh scatter
WFS 0 (center LGS)
laser 3
MCAO
WFS 1 (corner LGS)
laser 1
laser 1
laser 2
lasers 0 and 3
laser 3
May 24-25, 2001
laser 4
laser 4
MCAO Preliminary Design Review
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Scattered Light from Atmospheric
Path to the Sodium Layer
MCAO
• 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 LGS’s beam is small, but it
is important to field-stop correctly
May 24-25, 2001
MCAO Preliminary Design Review
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MCAO
BTO Optics
Brian Bauman
University of California
Lawrence Livermore National Laboratory
BTO Beam Path and Control
Diagnostic Split
Surfaces
Centering
MCAO
Mirror
X-shaping
mirrors
K Mirror
Pointing
Mirror
Top-end
Ring Fold
Relay Optics
Fast
Tip/Tilt
Array
Centering
Array
Pointing
Array
*
May 24-25, 2001
MCAO Preliminary Design Review
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Top-End Layout
May 24-25, 2001
MCAO Preliminary Design Review
MCAO
29
BTO alignment diagnostics
MCAO
• 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
May 24-25, 2001
MCAO Preliminary Design Review
30
Pointing diagnostic
MCAO
• 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
May 24-25, 2001
MCAO Preliminary Design Review
31
Pointing diagnostic:
0, 42.5, 85 arcsec off-axis
May 24-25, 2001
MCAO Preliminary Design Review
MCAO
32
Centering diagnostic
MCAO
• 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
May 24-25, 2001
MCAO Preliminary Design Review
33
Centering diagnostic
May 24-25, 2001
MCAO Preliminary Design Review
MCAO
34
X-shaping mirror (XSM)
May 24-25, 2001
MCAO Preliminary Design Review
MCAO
35
MCAO
K-mirror
May 24-25, 2001
MCAO Preliminary Design Review
36
Relay telescope
MCAO
• 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
May 24-25, 2001
MCAO Preliminary Design Review
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Relay telescope
May 24-25, 2001
MCAO Preliminary Design Review
MCAO
38
MCAO
BTO electronics
Mark Hunten
Beam Transfer Optics Electronics
Overview of
the BTO
diagram.
BDS
X-Shaping
Mirrors
Top-End
Ring Mirror
Top
End
Center
Section
Centering
mirror (CM)
BS
KM
Pointing
mirror (PM)
Fast Steering Array
Relay
Pointing
Array (PA)
May 24-25, 2001
MCAO
Centering Array (CA)
LLT
MCAO Preliminary Design Review
40
Beam Transfer Optics Electronics
MCAO
• 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.
May 24-25, 2001
MCAO Preliminary Design Review
43
MCAO
PDR Agenda
Thursday, 5/24
0800 Welcome
0805 Project overview
0830 Science case
0930 Break
0945 System overview
1015 System modeling
1100 AO Module optics
1145 Lunch
May 24-25, 2001
1245 AO Module mechanics
1340 AO Module electronics
1400 Break
1415 Beam Transfer Optics
1510 Laser Launch Telescope
1545 Closed committee session
1800 Adjourn
MCAO Preliminary Design Review
44