Keck NGAO Proposal:
Technical Overview
Presenters:
Adkins, Dekany, Gavel, Wizinowich
Other Contributors:
Bauman, Bell, Bouchez, Flicker,
Macintosh, Neyman, Velur
SSC Meeting
June 21, 2006
1
Presentation Sequence
1. Introduction (PW, 5 min)
•
•
2.
3.
4.
5.
6.
7.
System Design Process
Requirements Flow Down
Point Design Overview (PW, 10 min)
Performance Budgets (RD, 30 min)
Point Design Feasibility (DG, 15 min)
Point Design Product Structure (PW, 10 min)
Point Design Instruments (SA, 10 min)
Summary (PW, 1 min)
2
PW
1. Introduction: System Design Process
• Starting point needed to support development of
science cases & requirements
• One pass made through the system design process
loop to provide this starting point:
Science
Requirements
Technical
Implications
Initial
Concept
Performance
Assessment
Repeat as required while making improvements & balancing trade offs
• Resulted in an initial architecture or “point design”
3
PW
1. Introduction: Science Requirements Flow
Down
• Primary functions to meet science requirements:
•
•
•
•
•
•
•
Wavelength coverage from ~0.6 to 5.3 mm
Near diffraction-limited correction in near-IR
Modest Strehl in visible (0.6-1.0 mm)
High sky coverage
Multiplex observations over 2 arcmin field of regard
High throughput & low thermal near-IR background
Calibration & control of AO systematic effects
4
PW
2. Point Design Overview
•
•
•
•
Architecture
LGS Asterism
Opto-mechanical Layout
Real-time System
5
PW
2. Point Design: Architecture
• This is only a starting point to demonstrate feasibility of
meeting the requirements (& for initial cost estimation
purposes)
• Design will evolve during the system design phase
6
PW
2. Point Design: Elements
Components/features needed to fulfill requirements:
• Multiple LGS (Strehl & sky coverage)
• Measurement of 3-D turbulence structure (Sky coverage,
Strehl, calibration)
• Multiple AO corrector elements (Strehl, sky coverage)
• ~ 1000 actuator (Strehl)
• 2 arcmin field of regard (Sky coverage)
• Multiple near-IR low-order NGS wavefront sensors with AO
correction (Sky coverage)
• Minimize number of optics (SNR, emissivity)
• Cooling of AO system (emissivity)
7
PW
2. Point Design: LGS Asterism
• 5 LGS (cone effect)
• ~ 30W each
• variable radius (high on-axis
Strehl vs wider field
correction)
• 3 corrected tip-tilt stars for
good sky coverage
• Wavefront sensors
• LGS: Shack-Hartman
• NGS: MOAO correction with
MEMS, near-IR, pyramid
WFS for tip/tilt/focus/astig
• NGS: Slow S-H WFS for
LGS calibration
8
PW
2. Point Design: Optical Layout
Cooled
AO
Enclosure
(-15 C)
Basic AO Relay
(MOAO or MCAO)
1.5 m
Nasmyth
Platform
Outline
9
PW
2. Point Design: Opto-Mechanical Layout
10
PW
2. Point Design: Dichroic Switchyard
Demonstrates a
Feasible Switchyard
Point Design
11
PW
2. Point Design: Real-time Architecture
• Massively Parallel Processing pipeline architecture
Wavefront
Wavefront
Sensors
Wavefront
Sensors
Wavefront
Sensors
Sensors
Image
Image
Processors
Image
Processors
Image
Processors
Processors
Tomography
Unit
Centroid algorithm
Cn2 profile
r0, guidstar brightness,
Guidestar position
Wavefront
Wavefront
Wavefront
Sensors
DM
Sensors
Sensors
Projection
DM conjugate
altitude
Image
Image
Processors
Image
Processors
DM
Processors
Fit
Image
Image
Processors
Image
Processors
Deformable
Processors
Mirrors
Actuator
influence
function
• Power & space requirements are reasonable
• 10-20 boards with 50-100 FPGA chips, <2 kW total power estimated
12
PW
3. Performance Budgets
•
•
•
•
Wavefront Error (WFE) Budget
Emissivity
Companion Sensitivity (high contrast performance)
The following performance budgets will not be
presented:
•
•
•
•
•
•
•
Throughput
Point Source Sensitivity
Photometry
Astrometry
Polarization
Observing Efficiency
Up-time
13
RD
3. Wavefront Error Budget
Wavefront error budgets developed for several scenarios
using measured Mauna Kea site conditions
• LGS
• KBO Astrometry – Narrow field
• Galactic Center – Low elevation
• GOODS-N - Sky coverage
• “Best possible” Narrow-field
• Current Keck II LGS – Cross check
• NGS
• Io – Bright object
NGAO point design does all this science!
• This iterative approach will continue during the system
design phase
14
RD
NGAO Performance Summary by Observing
Scenario
Keck II LGS System delivers 360 nm
15
RD
NGAO Error
Budget
High-Order Errors
121 nm
Atmosphere
93 nm
Based on
Lick, Palomar
& Keck
experience
Systematics
0.71 mas (9 nm)
Atmosphere
3.81 mas (50 nm)
0 nm
20 nm
28 nm
Tilt Measurement
Tilt Bandwidth
Tilt Anisoplanatism
Res. Centroid Anisoplanatism
Res. Atmospheric Dispersion
Res. Telescope Pointing Jitter
45 nm
Atmospheric Fitting
59 nm
Bandwidth
33 nm
High-order Measurement 41 nm
LGS Tomography
29 nm
Asterism Deformation
22 nm
Multispectral
25 nm
Science Scintillation
15 nm
WFS Scintillation
10 nm
Angular Aniosplanatism
Tip-Tilt Errors
3.88 mas (51 nm)
Calibration
76 nm
50 nm
KBO imaging
scenario
Total Equivalent WFE
131 nm
Science Inst. Mechanical Drift
Long-Exposure Field Rotation
Go-to Control
0 nm
Res. Na Layer Focus
High-Order Aliasing
3 nm
20 nm
2.39 mas
1.68 mas
0.00 mas
1.99 mas
0.95 mas
1.05 mas
0.50 mas
0.50 mas
17 nm
DM Finite Stroke
Hysteresis
Drive Digitization
Static WFS Zero-point Calibration
Dynamic WFS Zero-point Calibration
Unc. AO Sys. Aberrations
Unc. Instrument Aberrations
DM-to-Lenslet Misregistration
Unc. Static Telescope Aberrations
Unc. Dynamic Telescope Aberrations
25 nm
13 nm
3 nm
25 nm
15 nm
20 nm
25 nm
13 nm
44 nm
23 nm
16
RD
NGAO Error Budget
Example:
KBO imaging scenario
Engineering Tool
Key: green = allocation
All others supported by
detailed calculations
17
Error Budget  PSFs (Narrow field)
V
R
I
J
H
K
135 nm
+ 8 mas
+ 15 mas
+ 25 mas
Used by Science Teams for Simulations
18
RD
NGAO Error Budget (Narrow-field)
NGAO point design
Narrow-field science target performance
1.00
H-band Strehl ratio
0.90
0.80
0.70
1% sky fraction
0.60
10% sky fraction
0.50
30% sky fraction
0.40
99% sky fraction
0.30
0.20
0.10
0.00
10
12
14
16
18
20
Science target brightness (mH)
Almost every 2MASS object can be observed
with high Strehl by NGAO!
19
RD
NGAO
KII LGS
NGAO H-band Strehl improved by 5x in median conditions
Strehl > 40% for seeing < 1”
rms Wavefront Error (nm)
NGAO Improves Performance over Wide Range of
Conditions
20
RD
Variable LGS Asterism Size supports Good
Sky Coverage
Use of sharpened tip-tilt stars
is key to sky coverage
Tip-tilt sensor patrol field is a
driver for good performance
over large sky fraction
Up to 4-5’ may be beneficial
Cannot be done with current
AO system (field = 2’)
21
RD
Cooled AO System (-15 C) Lowers
Background Below Sky in K-band
Sky + Tel
background
Component
Emissivity
Temp.
0.00075
273
0.02
258
Rotator mirrors (3)
0.03 x 3
“
OAPs (2)
0.03 x 2
“
0.03
“
0.0005
“
0.05
“
0.0015 x 2
“
0.02 x 2
“
IR dichroic (reflection)
0.05
“
Camera CaF2 window
0.0005
“
CaF2 window (bulk)
CaF2 window (surface)
AO background
K-band
DM
IR/vis dichroic (bulk)
IF/vis dichroic (surface)
IR ADC (bulk)
IR ADC (surface)
Total =
L-band
0.37
M-band
22
RD
3. Companion Sensitivity: NGAO can achieve
10 mag contrast at 0.5”
• High-contrast
sensitivity goes up
rapidly with high
Strehl
• Limited by AO +
atmosphere, plus
static internal &
external wavefront
errors
NGAO can exceed current AO contrast by an order of magnitude
23
RD
4. Point Design Feasibility: Technology
NGAO wavefront control presents a significant
increase in the level of technical challenge over the
present AO system:
• Multiple laser beacons
• Multiple wavefront sensors: LGS & NIR Tip-Tilt
• Tomographic reconstruction of a 3-D volume of
turbulence, rather than just one 2-D phase
• Multiple Deformable mirrors, in series (woofer-tweeter) &
parallel (MOAO)
• Higher bandwidth – need to keep up with the atmosphere
24
DG
4. Point Design Feasibility:
Unique architecture (mix of MCAO & MOAO)
Laser
Beacons
Key Enabling
Technologies
MEMS DMs
Dichroic
Deformable
Mirror
Science
Instruments
& NIR Tip-tilt
Sensors
Tomography
Computer
MEMS DMs
LGS Wavefront
Sensors
25
DG
4. Point Design Feasibility: Lasers
Laser situation much improved in last few years
Demonstrated options include:
• LM/CT 14W mode-locked CW laser delivered to Gemini
• 50 W Gemini-S laser is our notional point design choice
• SOR 50W mode-locked CW laser
Potential future options include:
• Fiber Raman laser (ESO)
• Fiber sum frequency laser (LLNL)
• Waveguide sum frequency laser with programmable pulse format
(LM/CT)
• Mode locked sum frequency crystal (Palomar)
• Coherent DFB semiconductor laser (Telaris)
Note: Required power depends on sodium coupling efficiency
26
DG
4. Feasibility: MEMS Deformable Mirrors
• MEMS deformable mirrors:
• For IR tip-tilt correction; in deployable
IFUs
• Working beautifully in UCSC lab (1000
actuators)
• Funded by a consortium (up through
4000 actuators)
• Sky demonstration at Lick within the year
LGS, astro & NGS light
• Open-loop AO:
• For deployable IFUs
• Already demonstrated in field tests along
several-km horizontal path
• Tests in progress at Lab for Adaptive
Optics; working well
Wavefront
Sensor
MEMS AO
(NGS+IFU)
27
DG
4. Necessary MEMS Technology Available
Point Design MEMS
Test
Beam
Beam
Splitter
Reference
Beam
Hartmann
Sensor
Camera
Interferometer
Camera
Far Field
Camera
Point Diffraction
Interferometer measures
to 200pm rms
32x32 actuator MEMS DM
(Boston Micromachines Corp)
Top mirror surface
Membrane
Electrode
MEMS electrostatic actuator
28
DG
4. Technical risk reduction effort will
use lab and sky test results
• Tomography:
• In lab: Lab for AO + 3 others (past, present)
• Gemini-S MCAO (next year)
• Solar MCAO (in progress now at two
different telescopes)
• VLT Advanced AO Demonstrator (next year)
• Palomar & MMT with multiple wavefront
sensors (in progress now)
• Visible-wavelength AO:
• Demonstrated: Air Force & Mt. Wilson
• Planned: Palomar, Lick
29
DG
5. Point Design Product Structure
•
•
•
•
AO system
Laser system
Operations tools
Science Instruments
30
PW
5. Point Design: AO System
31
PW
5. Point Design: AO System
Subsystem
Key Features
Motion
Control
Discussion
5 Shack-Hartmann WFSs
Motion to register lenslets.
62x62 offers a graceful fallback
from 48x48. 31x31 for low
sodium return case
48x48 subaperture baseline
62x62 & 31x31 subaperture option
LGS Wavefront
Sensors (WFS)
4x4 pixels/subaperture
256x256 pixel CCD with 1e read-noise
at 1 kHz readout rate
Center WFS located on-axis
Four WFS translate radially from 1050". Also need z-adjustment for field
curvature.
x,y control for 5 lenslets for DM
registration & changing lenslets
x,y control for assembly
Tracking to stay conjugate to Na layer
CCID-56 under development
4
10
2
1
For dispersion correction
32
PW
5. Point Design: Trade Studies
Sample Key Trade Studies:
• Alternate optical relay designs
• MOAO versus MCAO (e.g., crowded fields)
• Iteration of science field of regard & IFU multiplexing
• Performance impact/mitigation of Rayleigh scatter
• High contrast performance requirements & performance
• Upgrade of current system to NGAO
• Background requirement & how to achieve
• Best way to support interferometer operations
42 technical trade studies documented to date (24 high priority)
Science
Requirements
Technical
Implications
Initial
Concept
Performance
Assessment
Repeat as required while making improvements & balancing trade offs
33
PW
5. Point Design: Laser System
34
PW
5. Point Design: Operations Tools
35
PW
6. NGAO Notional Instrumentation
•
Large parameter space, specialized
instruments
•
•
•
•
•
Simpler instruments
Emphasize diffraction limited image quality
Imaging
Spectroscopy
Nasmyth platform operation
•
•
Single axis of rotation
Up looking or down looking
36
SA
6. NGAO Instrumentation
• Design notions:
• Simple imagers and IFU spectroscopy
• Optical and thermal considerations define coverage
breakpoints
• Available detectors or evolutionary developments
• Hawaii 2RG -> Hawaii 4RG
• LBNL 2k x 4k -> 4k x 4k
• Heritage software (OSIRIS/MOSFIRE) closely
integrated with AO system
• Emphasize observing efficiency
37
SA
6. NGAO Instrument Priorities
• Science case priorities:
• Convolved with first light needs, development timelines:
38
SA
6. NGAO Instrumentation
1.0
H
0.9
Ca II
IRT
0.8
H
K
}
Deployable
near-IR IFU
Near-IR imager
Single object near-IR IFU
0.7
Strehl ratio
J
NGAO
0.6
Visible IFU
Deployable
near-IR IFU
Near-IR imager
imager
Single object near-IR IFU
0.5
0.4
0.3
0.2
IFU
Visible imager
0.1
0.0
200
400
600
800
1000
1200
1400
1600
1800
2000
2200
Wavelength, nm
39
6. Imagers
Instrument
Wavelength
coverage (µm)
FOV
Sampling
Visible Imager
0.6 to 1.1
20” x 20”
Nyquist
(6 mas)
Near-IR Imager
1.0 to 2.45
20” x 20”
Nyquist
(10 mas)
Thermal near-IR Imager
(L & M-band Imager)
3 to 5.3
25” x 25”
Nyquist
(25 mas)
• Filter wheels
• Coronagraph (near-IR)
• Possibly grisms for R ~100 to 400 spectroscopy
• Polarimeters (visible and near-IR)
40
6. Spectrometers
Instrument
Near-IR deployable
IFU (6 units)
Wavelength
coverage (µm)
1.0 to 2.45
Sampling
(Spaxels)
Spectral
Resolution
R ~4,000
30 x 34
Near-IR single object
IFU
1.0 to 2.45
80 x 50
Visible single object
IFU
0.6 to 1.1
60 x 60
R ~4,000
R ~3,000
• Near-IR deployable IFU 100 mas spaxel scale
• Single object IFUs may have additional spaxel scales
• Single object IFUs may have broad band and narrow band modes
• We will study the possibility that OSIRIS might meet many of the
needs for the near-IR single object IFU
41
Technical Summary
• NGAO point design demonstrates
feasibility of meeting science requirements
• Next technical steps understood
• Ready to proceed to system design phase
42
PW
43
1. Requirements: Observatory
(Existing infrastructure influences design)
• Facility
• Location – Keck II left Nasmyth
• Pupil – entire Keck pupil
• Secondary mirror – Use existing f/15
• Instruments
• OSIRIS & NIRC2
• Interferometer
• OHANA
• Operational
•
•
•
•
•
•
Facility-class
Operations transition review
AO downtime - < 6 months?
Telescope downtime - < 5 days?
Mauna Kea laser projection requirements
Operate under normal environmental conditions
44
CN
2. Point Design: Architecture
Optimized for high Strehl on-axis and wide field (lower Strehl)
• 5 LGS atmospheric tomography
• Variable radius
• Basic AO relay
• Image derotator
• Pair of OAPs with pupil conjugate to DM (dact = 0.177 m)
• Option to use OAP1 as 2nd DM in a MCAO system (conjugate to 9 km)
• Window to cooled AO enclosure acts as a field lens
• Dichroic switchyard to divide light amongst:
• LGS mode:
• 5x LGS Shack-Hartmann (S-H) wavefront sensors (WFS) (ds = 0.25 m)
• 3x NGS IR tip/tilt/focus/astigmatism pyramid sensors with 32x32 MEMs
• 1x NGS visible slow S-H WFS for LGS calibration
• NGS mode: 1x visible S-H WFS
• Science instruments (near-IR & visible)
45
PW
2. Point Design: Subapertures & Actuators
Actuator spacing
chosen to minimize
segment errors.
Subaperture size
chosen to balance
measurement error
with laser power.
Risk: Lenslets not
registered to
actuators. Fallback
is a lenslet array
matching actuator
spacing.
46
PW
3. Mauna Kea Site Conditions
Median seeing conditions at  = 0.5 mm:
• r0 = 18 cm.  = 3 cm. L0 = 75 m.
• 0 = 2.5 arcsec
• fG = 39 Hz
• L0 = 75 m
• Cn2 profile:
Median sodium density = 4E9 atoms/cm2
Altitude
(km)
Fractional
Cn2
Wind Speed
(m/s)
0.0
0.471
6.7
2.1
0.184
13.9
4.1
0.107
20.8
6.5
0.085
29.0
9.0
0.038
29.0
12.0
0.093
29.0
14.8
0.023
29.0
47
RD
3. Input Assumption - Site Data
Mauna Kea median seeing conditions
at  = 0.5 mm:
• r0 = 18 cm.  = 3 cm. L0 = 75 m.
• 0 = 2.5 arcsec
• fG = 39 Hz
• L0 = 75 m
• Cn2 profile from data
Altitude
(km)
Fractional
Cn2
Wind Speed
(m/s)
0.0
0.471
6.7
2.1
0.184
13.9
4.1
0.107
20.8
6.5
0.085
29.0
9.0
0.038
29.0
12.0
0.093
29.0
14.8
0.023
29.0
Sodium column density
Median sodium density = 4E9
atoms/cm2
48
RD
NGAO Error
Budget
Io Scenario
49
Color key: blue/green = allocation
NGAO Error
Budget
Galactic
Center
Scenario
50
Color key: blue/green = allocation
NGAO Error
Budget
GOODS-N
Scenario
51
Color key: blue/green = allocation
NGAO Error
Budget
“Best
Possible”
Narrow-Field
Scenario
52
Color key: blue/green = allocation
Keck II LGS
Error Budget
Narrow-Field
Scenario
(1% Sky
Fraction)
53
Color key: blue/green = allocation
Keck II LGS
Error Budget
Narrow-Field
Scenario
(20% Sky
Fraction)
(Requires acquisition of tip/tilt
star at angular offset of 49”
from science target)
54
Color key: blue/green = allocation
Current Keck II LGS Error Budget (target-guiding scenario)
Current Keck LGS
(Guiding on science target, TT from STRAP)
0.25
K-Strehl
0.20
0.15
r0 = 18cm, w = 10 m/s
r0 = 25cm, w = 5 m/s
0.10
0.05
0.00
10
12
14
16
18
20
22
24
Strap TT guide star magnitude
Assumes: tip/tilt information from target itself, b=30, Na laser power = 20W LP spigot, Na = 4e9 atoms/cm2, exposure time = 10 sec,
20x20 SH EEV39 HOWFS, STRAP tip/tilt sensor, optimal HO and LO WFS integration times
55
NGAO Improves Stability over Wide Range of Conditions
Time Evolution of NGAO and Keck II LGS performance
for 20% sky fraction
0.90
Mean = 0.65
/Mean = 16%
0.80
H-band Strehl Ratio
0.70
0.60
0.50
NGAO
Keck II LGS
0.40
0.30
0.20
Mean = 0.12
/Mean = 67%
0.10
507
488
468
449
429
410
390
371
351
332
312
293
273
254
234
215
195
176
156
137
117
97.
78
58.
39
19.
0
0.00
Time (minutes)
Performance estimate of NGAO and Keck II LGS
based on actual MASS/DIMM measurements (from Palomar) scaled to median r0 = 18cm
56
NGAO Error Budget (GOODS-N spectroscopy scenario)
57
RD
D-IFU has Excellent Encircled Energy
& Sky Coverage
58
RD
Variable Asterism Radius Improves NGAO Performance
With fixed 30” radius quincunx
Mean = 0.43
Sigma = 0.19
With variable (optimized) quincunx
Mean = 0.48
Sigma = 0.20
Performance Histograms from Statistical Model
(r0 (log-normal, mean=18cm, sigma=4cm), tip/tilt brightness (Gauss, mean = 18, sigma = 4),
tip/tilt dist (Gauss, mean = 45, sigma = 15)
In all cases LGS and tip/tilt integration times are optimized for each realization.
59
RD
NGAO Point Source Sensitivity Estimates
Point source limiting magnitude
(5 in 1 hr of integration)
Zero-point
(magnitudes)
Sky (mag.
arcsec-2)
105 nm
140 nm
195nm
330nm
V
27.09
21.3
29.9
28.7
27.6
27.6
R
27.10
20.4
29.9
29.0
27.1
27.1
I
26.98
19.3
29.6
29.0
27.7
26.5
J
25.47
16.1
27.3
27.0
26.5
24.4
H
25.51
13.8
26.0
25.8
25.6
24.4
K’
24.84
13.5
25.3
25.2
25.0
24.4
L’
23.60
4.31
19.5
19.5
19.4
19.2
Ms
21.42
1.10
16.6
16.6
16.5
16.4
Filter
The measured performance of LRIS and NIRC2 are faithfully used as the basis of these calculations, with only the
pixel scale varied to sample the diffraction limit at each wavelength. The expected transmission and thermal
background of the NGAO system (cooled to 258K) are included, as is the effect of varying optimum photometric
aperture from diffraction-limited to seeing-limited in the low Strehl limit.
60
3. Throughput
• Throughput requirement to science instrument (telescope +
AO)
• ≥ 70% at 0.6-5.5 mm
• Point design -> 77% w/o IR ADC
• Point design -> 51% w/ IR ADC
• ≥ 60% at 5.5-14 mm
61
RD
3. Emissivity
Sky
background
AO background
NGAO cooled to 258 K (red)
compared to that due to the sky
plus telescope (black),
assuming a total telescope
emissivity of 0.10
63
RD
3. Companion Sensitivity: careful
consideration of telescope effects essential
Can be mitigated by advanced wavefront sensing & coronagraph or improved
segment figure (warping & dimples)
64
RD
Point Source Limiting Magnitude (5 sigma in 1
hr)
3. Point Source Sensitivities
Filter
Zeropoint
mag
Sky
(mag*arc
sec-2)
V
R
I
J
H
K’
L’
Ms
27.09
27.1
26.98
25.47
25.51
24.84
23.6
21.42
21.3
20.4
19.3
16.1
13.8
13.5
4.31
1.1
Point source limiting magnitude
(5 in 1 hr of integration)
105 nm 140 nm 195nm 330nm
28.7
27.6
29.9
27.6
29
27.1
29.9
27.1
29
27.7
29.6
26.5
27
26.5
27.3
24.4
25.8
25.6
26
24.4
25.2
25
25.3
24.4
19.5
19.4
19.5
19.2
16.6
16.5
16.6
16.4
30
Sky (mag*arcsec-2)
105 nm
28
140 nm
195 nm
26
330 nm
24
22
20
18
16
0.5
1.5
2.5
3.5
Wavelength (microns)
4.5
65
RD
3. Photometry
Photometric accuracy requirement:
•  0.01 mag 0.7-2.5 mm for <5” from H<16 NGS
•  0.02 mag 0.7-3.5 mm for <10” from H<16 NGS
•  0.05 mag at 0.9-2.5 mm for < 20” off-axis & 20% sky
coverage
•  0.1 mag at 0.7-2.5 mm for < 20” off-axis & 20% sky
coverage
• Not yet evaluated
• Appropriate PSFs with field dependence produced
• Next step: evaluate sample science images,
produced with PSFs, to determine accuracy
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3. Astrometry
Astrometric accuracy requirement:
•  0.1 mas for Galactic Center
•  10 mas at 0.7-3.5 mm for 30% sky coverage
•  50 mas at 0.7-3.5 mm for 50% sky coverage
• Not yet evaluated
• Appropriate PSFs with field dependence produced
• Next step: evaluate sample science images, produced with
PSFs, to determine accuracy
• GC: Keck II LGS AO approaching 0.25 mas
• Astrometric accuracy ~ FWHM/SNR
• Strehl improvement from 30 to 75% at K  2.5x improvement
• Many issues could prevent astrometry at these levels (e.g., field
dependent stability)
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4. Feasibility: Low Noise Wavefront Sensing CCD
• Project spearheaded by Sean Adkins, Jerry Nelson, Jim Beletic, in
collaboration with MIT Lincoln Labs and Lick Observatory CCD lab
• Recent lab measurements: < 1 e- read noise
• Radial pixel layout design accommodates elongated LGS spot
Wavefront sensor
subapertures shown with
respect to telescope
primary mirror
Sodium
Layer
LGS
spot
Serial
Register
LGS spots in each
subaperture
Video
output
Pixel array for
corresponding LGS spot
Clock
Lines
Centrally
Projected
Laser
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4. Today’s FPGA Technology Meets Realtime Processing Requirements
Wavefront
Wavefront
Sensors
Wavefront
Sensors
Wavefront
Sensors
Sensors
Image
Image
Processors
Image
Processors
Image
Processors
Processors
Tomography
Unit
Centroid algorithm
Cn2 profile
r0, guidstar brightness,
Guidestar position
Wavefront
Wavefront
Wavefront
Sensors
DM
Sensors
Sensors
Projection
DM conjugate
altitude
Image
Image
Processors
Image
Processors
DM
Processors
Fit
Image
Image
Processors
Image
Processors
Deformable
Processors
Mirrors
Actuator
influence
function
• Very fast – can keep up with real-time reconstructor
requirements (1kHz frame rate)
• Implemented with massive parallel processing
• All the algorithms are “parallelizable”
• Today’s FPGA technology can do it with margin
• Power and space requirements are reasonable
• 10-20 boards with 50-100 FPGA chips, <2 kW total power
estimated
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MEMS Flattening: < 1 nm rms
Before
•
•
After
RMS WFE
RMS WFE in control band
Lowpass
Before
148.1 nm
144.1 nm
After
12.8 nm
0.54 nm
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Correction of Kolmogorov Test Plate
With spatially filtered Hartmann WFS
Power Spectrum
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MEMS Open-Loop Control
Plate, membrane, and capacitor model predicts forces and displacements
of the top mirror surface
Open loop control to 30 nm wavefront has been demonstrated in the lab.
We expect to get better than this with calibration refinement.
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Difficulties upgrading Keck II LGS to NGAO
• General
• Large majority of NGAO requires new subsystems (lasers, wavefront
sensors, real-time software, control software, etc.)
• Sum of
• A major retrofit to c. 1995 components imposes many interface
constraints and limits component choices
• Component aging
• Optical bench
• Imposes limit on sky coverage
• Passes only 2 arc min; NGAO would prefer 4-5’
• Insufficient space for multiple wavefront sensors (LGS or NGS)
• Integration & Testing
• NGAO will require significant lab testing and characterization
• Operations impact
• Upgrade would significantly interrupt science operations
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6. System Design Technical Approach
• Coordination between science requirements & AO
development
• Key personnel: Project manager & Project scientist
• Builds on existing science & technical teams from NGAO proposal
• Facilitates rapid iteration of design
• Track overall performance metrics
•
•
•
•
•
•
Sky coverage
wavefront error
photometry
astrometry
emissivity
imaging & spectroscopic sensitivities
• Define & control interfaces between AO, instruments &
facility
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6. System Design Technical Approach
• Key Trade studies:
• Evaluate the performance impact of Rayleigh scatter
on NGAO performance & consider various mitigation
methods such as a pulsed laser.
• Determine the relative cost versus benefits of an
adaptive secondary mirror implementation
• Consider the cost/benefits of stand-alone tip/tilt mirror
as opposed to mounting another necessary optic on
the tip/tilt stage
• Consider cost/benefits of different options for achieving
the NGAO science goals in the K through M-Bands.
• Determine the best way to support Keck-Keck
interferometer operations when NGAO is operational.
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