Transcript Document

Development and validation of vibration
source requirements for TMT to ensure
AO performance
Hugh Thompson and Doug MacMartin
AO4ELT3 Conference, Florence, Italy
26-31 May 2013
TMT.SEN.PRE.13.040.REL01
Presentation Outline
TMT AO error budget for vibration
Sensitivity of TMT structure to vibration
Examples of observatory vibration sources
Are
Sources × Sensitivities = Error budget ?
TMT.SEN.PRE.13.040.REL01
Rough scale of the problem
Many current AO systems are limited by vibration
–
ALTAIR on Gemini sees vibration of ~10 mas rms after
correction
–
Survey of similar problems at several telescopes:
Caroline Kulcsár ; Gaetano Sivo ; Henri-François Raynaud ; Benoît
Neichel ; Franҫois Rigaut, et al.
"Vibrations in AO control: a short analysis of on-sky data around the
world", Proc. SPIE 8447, Adaptive Optics Systems III, 84471C
(September 13, 2012)
–
For TMT the entire on-axis NFIRAOS budgeted wavefront error
of 187 nm corresponds to only ~ 5 mas of tip/tilt
TMT.SEN.PRE.13.040.REL01
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How do we flow AO
requirements down?
Delivered wavefront
First order turbulence compensation
LGS control loop
DM fitting error
DM projection error
LGS WFS aliasing error
Tomography error
Servo lag
LGS WFS non-linearity
LGS WFS noise
TMT pupil function
Opto-mechanical implementation
Telescope pupil misregistration
Telescope and observatory OPD
M1 static shape
M2 & M3 static shape
Segment dynamic mis-alignment
Dome seeing
Mirror seing
Field dependent astigmatism
NFIRAOS
Residual instrument
AO compomnents errors & higher order
effects
DM effects
LGS WFS & Na layer
Control algorithm
Simulation undersampling
NGS Mode WFE at 50% sky coverage
Residual tip/tilt jitter due to windshake
Residual telescope vibration
Residual telescope tracking jitter
Residual tip/tilt jitter due to turbulence
Residual plate scale mode due to
turbulence
Residual plate scale mode due to
windshake
Field dependent wavefront error
Contingency
On Axis WFE
187
117
117
Segment dynamic displacement
(due to vibration) 10nm
75
46
42
30
4
19
46
27
Telescope image jitter (due to vibration)
10nm equivalent to 0.275 mas
71
12
37
26
11
14
16
14
0
51
30
66
?
49
39
21
48
58
16
10
17
32
Pump impeller Balance Grade 6.3
35
5
20
80
TMT.SEN.PRE.13.040.REL01
The questions in more detail
What is the sensitivity of image quality to vibration?
–
How does this vary with amplitude, frequency and location?
What are the worst expected sources of vibration with
respect to these sensitivities?
What can be done to mitigate them?
Do we need to increase AO error budget allocation to
vibration?
What standards/requirements do we have/will we
develop to maintain acceptable vibration levels?
How will we assess and verify vibration performance
against predictions?
TMT.SEN.PRE.13.040.REL01
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Finite Element Model
FEM of telescope
structure includes
nodes for each M1
segment, M2, M3 and
each instrument
Optical sensitivity
combined with nodal
motions from FEM
determines
performance effects
due to:
–
–
image jitter
M1 segment motion
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Additional model details
AO rejection curves included (median conditions)
– 15 Hz Type II controller for tip/tilt
– 63 Hz DM bandwidth
– No additional narrowband rejection
Frequency-resolved calculations are smoothed
– Reasonable estimate of rms performance, not worst case
Using simple ground transmission estimates (no soil and pier model)
No direct transmission path measurements for comparison (either
soil or on telescopes)
Instruments modeled as lumped masses
– wrong above ~12 Hz
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Modelling Goals
Determine allowable vibration source amplitudes
Assess:
– Relative influence of location of sources
– Main contributors to image jitter (M1, M2, M3, focal plane)
– Sensitivity to source input frequency
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Modelled Sources
“Unit” forces are input at 6 locations
– Pier
Also covers sources in facility building with an additional factor to
account for attenuation through soil
– Instruments (NFIRAOS, MIRES) on Nasmyth platforms
– Laser Service Enclosure (LSE)
– Cable wraps (Az and El)
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Results combining M1
and image motion
After smoothing, after AO rejection
(a)
Pier Pier
forcing
Fz
0
(b)
-1
10
-2
10
-3
IJ
M1
10
AO-corrected rms wfe (nm)
AO-corrected rms wfe (nm)
10
NFIRAOS
forcing
NFIRAOS Fz
1
10
0
10
-1
10
-2
IJ
M1
10
Combined
Combined
-4
10
-3
0
10
1
10
Frequency (Hz)
10
0
10
1
10
Frequency (Hz)
In both cases image motion
is dominant above 10 Hz
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Check spatial correctability on M1
M1 response at 30 Hz
AO spatial
correctability is
good; correction is
dominated by
temporal bandwidth
TMT.SEN.PRE.13.025.DRF01
nm/N
11
Combined M1 and
image motion for all sources
1
AO-corrected rms wfe (nm)
10
Telescope
0
10
10x
-1
Pier
10
-2
10
-3
10
0
10
1
10
Frequency (Hz)
TMT.SEN.PRE.13.025.DRF01
Pier
NFIRAOS
MIRES
LSE
Elev
Az
12
Model Results Summary
All modeled telescope sources are roughly comparable in effect
– Pier forcing a factor of 10 less impact
– Locations in facility building likely reduce sources by an additional factor
of 5-10 relative to pier
Performance most sensitive to forces 5-20 Hz
M1 soft actuators reduce M1 response at 30 Hz by factor of 10
Motion of M2 largest contributor to image motion above 10 Hz
Residual dominated by image motion, not M1 above 10 Hz
– Means that feed-forward of M2 motion may be effective
– Narrowband rejection of tones may also help
Internal flexibility of instruments not accounted for
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(a)
Pier
1
(b)
Compare
actual
sensitivity with
fit to shaping
filter for each
source
Filter W(f):
-1
10
-2
10
5-20 Hz: 3.4 nm/N
-3
0
10
(c)
1
10
Frequency (Hz)
MIRES
1
10
(d)
0
10
-1
10
-2
10
LSE
1
0
10
-1
10
-2
10
5-20 Hz: 3.7 nm/N
5-20 Hz: 1.9 nm/N
-3
10
-3
0
10
0
10
(f)
0
10
-1
10
-2
10
Az
1
0
10
-1
10
-2
10
5-20 Hz: 1.3 nm/N
-3
10
1
10
Frequency (Hz)
10
Sensitivity (nm/N)
2
1
10
Frequency (Hz)
Elev
1
10
10
2
1
10
Frequency (Hz)
10
Sensitivity (nm/N)
Sensitivity (nm/N)
0
10
10
2
𝑖𝑓
𝑖𝑓
1+ +
𝑓2
𝑓2
-2
10
0
10
5-20 Hz: 0.43 nm/N
Sensitivity (nm/N)
2
-1
10
-3
(e)
𝑖𝑓
𝑖𝑓
1+ +
𝑓1
𝑓1
0
10
10
– f1=5 Hz
– f2=20 Hz
𝑖𝑓
𝑓1
NFIRAOS
1
10
Sensitivity (nm/N)
Sensitivity (nm/N)
10
5-20 Hz: 0.52 nm/N
-3
0
1
10
10
TMT.SEN.PRE.13.040.REL01
Frequency (Hz)
10
0
10
1
10
Frequency (Hz)
14
Vibration Budget
Specification on rms force after filtering by shaping filter (allows higher
vibration at low or high frequency)
Sensitivity
(nm per N)
Fraction of
budget
Allowable
force (N)
Pier
0.43
35%
20
Instruments
3.7
50%
3
LSE
1.9
5%
2
Cable wraps
1.3
5% each
2.5 each
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Source
example
ESO study of cryocoolers:
“Low-vibration high-cooling power 2-stage cryocoolers for ground-based
astronomical instrumentation”
Gerd Jakob, Jean-Louis Lizon
Proc. SPIE. 7733, Ground-based and Airborne Telescopes III 77333V (July 16,
2010)
Forces ~1N at 1- 2 Hz
Frequency is low but higher
harmonics can be problematic
Large numbers required for
TMT has led us to turbine
expander cooling with no lowfrequency reciprocating motion
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Source example
in the summit facilities
Large fluid cooler used to exhaust all TMT waste heat has 8 fans
of Balance Quality Grade 1
–
Results in 10 N of force per rotor
or worst-case in-phase imbalance
of all 8 rotors equal to 80 N
–
At 59 Hz even 1 kN should be
acceptable but careful tracking of
all equipment is required
4-pole induction motors on 60 Hz AC generates ~29 Hz but newer
VFD equipment moves frequencies with system demand
–
Do we want this?
Need tight imbalance requirements
and single or multi-stage isolation17
TMT.SEN.PRE.13.040.REL01
Pipe vibration
Konstantinos Vogiatzis has made some initial models of turbulent
flow in coolant pipes
Forces are low in straight runs, but elbows produce significant
broad-band forces
10
0
rms: 1.48 N
Power spectrum (N2/Hz)
TMT is considering
replacing water-glycol
with phase-change
refrigerant to reduce
coolant mass flow
(and forces) by a
factor of 10
10
10
10
10
-1
-2
-3
-4
0
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10
1
10
Frequency (Hz)
2
10 18
Impact of increasing the error
budget allocation to vibration
An increase from
14 nm to 30 nm
would not
dramatically
reduce observing
efficiency
– Roughly 3%
impact in J
band
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Things to do
On-going work needed to:
– Develop the allowable vibration source budget allocated to subsystems
– Improve estimate of propagation through soil (for enclosure and summit
facility sources)
– Improve all source estimates
– Hopefully through force measurements made at a telescope near you!
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Conclusions
Vibration sources on the telescope must be limited to a few Newtons
Vibration sources in the facility must be limited to a few hundred
Newtons
Possibly need to increase AO error budget allocation to vibration
Further mitigation may be possible via
– M2 feed-forward
– Narrow-band rejection algorithms
Conventional cryocoolers are not acceptable for TMT
Keep summit facility source frequencies at 60 Hz when possible
– Reduced sensitivities
– Allows effective use of ~ 5 Hz isolators
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Acknowledgements
The TMT Project gratefully acknowledges the support of the TMT partner institutions
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the Association of Canadian Universities for Research in Astronomy (ACURA),
the California Institute of Technology
the University of California
the National Astronomical Observatory of Japan
the National Astronomical Observatories and their consortium partners
And the Department of Science and Technology of India and their supported institutes.
This work was supported as well by
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–
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the Gordon and Betty Moore Foundation
the Canada Foundation for Innovation
the Ontario Ministry of Research and Innovation
the National Research Council of Canada
the Natural Sciences and Engineering Research Council of Canada
the British Columbia Knowledge Development Fund
the Association of Universities for Research in Astronomy (AURA)
and the U.S. National Science Foundation.
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You can build large structures without
vibration problems
Mass helps
– TMT dome = 2300 tons
– Brunellesci’s dome = 37000 tons
– The Duomo likely doesn’t have a vibration
problem!
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