The TMT
Laser Guide Star Facility
(LGSF)
Kai Wei
Institute of Optics and Electronics (IOE),CAS
August 30,2010
1
Presentation Outline
•
•
•
•
LGSF Requirements and Updates
The changes of the LGSF designs
What we have done for the LGSF?
The working plan for LGSF
2
LGSF Requirements and Updates
-Overall Description
• The LGSF is composed of 3 main sub-systems:
– The Laser System (LAS), which includes the lasers, the Laser
Service Enclosure (LSE) and all associated electronics.
– The Beam Transfer Optics (BTO) and Laser Launch Telescope (LLT)
System
– The Laser Safety System (LSS), which is composed itself by
several sub-systems dedicated to:
• Protecting people and observatory systems from laser light,
• Protecting aircraft from laser illumination,
• Protecting neighboring telescopes from laser beams within their field of
view
3
LGSF Requirements and Updates
-Overall Description
• System Functions:
– Project the early light NFIRAOS asterism.
– Project other asterisms as required by the AO modes
– Switch rapidly between asterisms.
– Use conventional optics for the Beam Transfer Optics and launch
the AO asterisms from a Laser Launch Telescope located behind
the TMT secondary mirror.
4
LGSF Requirements and Updates
-Specific Requirements
•
•
•
•
•
•
Interface Requirements
General Constrains Requirements (Lifetime, Standards)
Environmental Requirements
Functional and Performance Requirements
System Attributes (Reliability, Maintainability, Security)
Access and Handling (installation, removal)
5
LGSF Requirements and Updates
-Specific Requirements
•
Asterism generation requirement
– NFIRAOS asterism: consists of 6 LGS,
5 equally spaced on a circle of radius of
35 arcsec and one additional on-axis
guide star. (black)
– MIRAO asterism: consists of 3 LGS
equally spaced on a circle of radius of
70 arcsec. (red)
– MOAO asterism: consists of 8 LGS, 3
equally spaced on a circle of radius of
70 arcsec and 5 equally spaced on a
circle of radius of 150 arcsec. (blue)
– GLAO asterism: consists of 5 LGS, 4
equally spaced on a circle of radius of
510 arcsec and one additional on-axis
guide star. (green)
switch between asterisms within 2 minutes
6
LGSF Requirements and Updates
-Specific Requirements
•
Asterism generation requirement
– as the telescope tracks a science field for exposure times of up to 60 minutes, with
1-axis, 1-sigma tip/tilt error for each laser beam of no more than 0.050 arcsec
•
Telescope flexure compensation
– The LGSF shall be capable of correcting for the effects of flexure of the telescope
top end structure by up to ±15 mm in translation，± 2.5 mrad in tilt and ± 4 mm in
axial motion over the operating zenith angles between 1 and 65 degrees and at any
observing temperature.
•
Beam quality and polarization
– 1.2 times the diffraction limited of 1/e2 beam diameter, for a near field 1/e2 diameter
of 0.3m at the LLT.
– A Beam Cleanup AO system may be included in the design, to correct the
aberrations on the out-going laser beams. The Beam Cleanup AO system will
include one slow WFS and possibly up to one DM per laser beam.
– The LGSF system shall generate Laser Guide Stars which are 98% circularly
polarized.
7
Four Kinds of the LGSF configuration
•
•
•
•
LGSF Initial configuration
LGSF Baseline configuration
LGSF Elevation Journal configuration
LGSF Side Launch configuration
8
LGSF Initial configuration
-Overall Description
3.The diagnostic
2.Beam
Transportsystem
Optics
transport
directs
a small
the nine
fraction
output
1.Laser
switchyard:
beams of
(0.5%)
from
each
theofLaser
the laser
an
optical
bench
Switchyard
beams
through
out of
a with
the LSE
motor-controlled
and up the telescope
beamsplitter
into two
beamsplitters
andone
truss to the
camera
systems.
BTOOB
mirrors
which
accept the
behind the
focused
at asecondary
relatively
50W
beams
mirror.input
close
distance,
thefrom
other at
the
lasersinand
Twooperating
infinity.
of the
Themirrors
near-field
each
direct
to the
beam them
camera
position
is used
within
to evaluate
the
proper
outputs
at the
BTOintensity
the
path
areprofile
controllable
and
desired
One
can
in tip/tiltofpower.
quality
tothe
maintain
laser
beam
both
generate
two
centering
within
theeither
and
LGSF.
pointing
The farof
25W
beamsator
three
the beams
field
camera
views
the
input
the to
17W
beams
the BTOOB,
projected
LGS
correcting
at
for
the inevitable flexure of
diffraction-limited
9 its
the telescope
resolution
to evaluate
structure
with altitude.
image
quality.
LGSF Initial configuration
-Diagnostic system
• Measure the locations of the
laser beams at the input to
the BTOOB to ensure that
the centering and pointing
mirrors in the BTO are
properly compensating for
telescope flexure.
• Measure the profile of the
laser beams for comparison
with the specified profile.
• Assure that the LGS
beacons are properly aligned
with the telescope pointing.
• Evaluate the image quality of
the LLT by imaging a star.
10
LGSF Initial configuration
-Asterism Generator
The
twomirror,
mirrors
theback
periphery
theAsterism
AsterismGenerator
Generatorplate,
will be
controllable
in
The
first
onatthe
side ofofthe
is a
high-bandwidth
tip/tilt
to steering
maintain mirror
centering
of the
beams on the
and
pointing
tip/tilt
fast
(FSM)
to compensate
forLLT
jitterpupil
in the
position
of of
thethe
LGS as
LGS
beacons
on
the
sky
in
compensation
for
any
flexure
within
the
BTOOB
and
measured by the associated WFS
pointing
of the
LLTcorrection
itself resulting
froma flexure
themas
telescope
structure
The
budgeterror
for fast
tip/tilt
assigns
value ofof50
to the 1-axis,
1σ laser
with
attitude.
pointing jitter
11
LGSF Initial configuration
-Centering and Pointing mirrors
12
LGSF Baseline configuration
The beams are transferred across to the –X elevation journal along the telescope
elevation axis via two active steering mirror arrays and a fold array. The active
arrays are used to follow the rotation of the telescope elevation structure as the
zenith angle changes and to correct for any misalignments of the telescope top 13
end due to thermal and flexure effects.
LGSF Baseline configuration
Main advantage:
Lasers located within the
telescope azimuth
structure to provide
fixed-g orientation, allow
a large laser footprint,
limit vibrations into
telescope and reduce
wind obstruction.
Main disadvantage:
The long optical path
and in particular the
deployable/retractable
section of the optical
path.
14
LGSF Elevation Journal configuration
•
•
•
•
•
Lasers attached to the -X elevation journal
Laser beams transported from the elevation journal up to the
launch
telescope located behind the secondary
Reduced optical path and in particular no deployable/retractable
section.
But lasers operating in variable gravity orientation, with tighter
limits on mass and volume.
15
LGSF Elevation Journal configuration
16
LGSF Elevation Journal configuration
• Smaller output aperture 0.4m instead of 0.5m
• Large asterism (17 arcmin) requirement deleted
– Telescope field of view: +/-2.5 arcmin
• No Up link AO upgrade path required
• Calibration requirement with visible stars deleted
• New Laser Guide Star acquisition system added
Revised designs for launch telescope developed for new requirements:
Modified off axis reflective design
Refractive design
17
LGSF Side Launch configuration
18
LGSF Side Launch configuration
19
LGSF Side Launch configuration
• Lasers and launch telescopes in several locations around M1:
– 2 LGS per launch telescope (up to 4 locations) - SL2 configuration
• AO performance are slightly improved compared with center launch
configuration
– Required laser power reduced for equal wavefront error due to noise
– Fratricide effect also minimized/eliminated
• Beam transfer optics and launch telescope simplified:
– Very simple and short beam transfer optics
– Launch telescope requirements somewhat simplified:
• Smaller output aperture needed: 0.4m instead of 0.5m
• Field of view: +/-1.6 arcmin
• On another hand:
– 4 launch telescopes required instead of 1
– Doubled LGS elongation requires larger LGS WFS
– De-rotation mechanisms needed in LGS WFS optical path to follow
elongation
20
What we have done for LGSF
• Beam transfer Optics
System:
– the Optical Path
– the LGSF Top End
• 3 main sub-systems:
– Laser System
– Beam transfer
optics system
– Laser safety system
• the Diagnostic
System
• Asterism Generator
• Launch Telescope
Assembly (LTA)
• Acquisition System
21
LGSF Elevation Journal Configuration
22
Comparison of the two optical designs
for the LTA
the confocal paraboloid design
the refractive design
Two off-axis paraboloid mirror,
Focus adjustment, K-miiror system
Objective lens nearly 16kg,
Focus adjustment, K-miiror system
23
Comparison of the two optical designs
for the LTA
Radius of field-angle
(arc sec)
Confocal paraboloid design
Refractive design
Strehl Ratio @ 589nm
Strehl Ratio @ 589nm
210 km
90 km
210 km
90 km
0
0.99
0.99
0.94
0.94
35
0.97
0.98
0.95
0.95
70
0.97
0.97
0.97
0.97
150
0.94
0.95
0.98
0.98
Image quality for the two optical designs
The differences between the image quality of the two optical design is inconspicuous
24
Comparison of the two optical designs
for the LTA
Issue
Confocal Paraboloid Design
Refractive Design
Elements
10 (2 input folds, 3 K mirrors, 3
refractive elements to
correct off-axis curvature,
secondary, primary,
window); 14 surfaces
8 (2 input folds,3 K mirrors, 2
refractive elements,
objective); 10 surfaces
Element fabrication
and test
Paraboloid straightforward to
test.
4th order aspheric on convex
surface may be harder to test.
Alignment
Off-axis mirrors will be more
challenging to align.
Approx 4 times less sensitive than
reflective.
Thermal stability
Should be relatively insensitive
if the mount material is the
same as the mirror
substrates.
Certain to be more sensitive to
temperature changes, might
need active focus adjustment.
25
Comparison of the two optical designs
for the LTA - Thermal analysis for the refractive design
Ambient air temperatures range: -5℃ to + 9℃
Observing Performance Conditions :
-Ambient air temperatures range: -5℃
Temperature
Focus Adjustments
(℃)
(microns)
to +9℃
Design value @ 0℃
0
-1.0
-28
1.0
27
-5.0
-139
-2.7
-74
-0.3
-1
2.0
54
4.3
117
6.7
180
9
241
Changes from -5℃ to 9℃
380
Changes quantity per 0.5℃
13
the WFE RMS of the LTA while the
temperature changes 1 degree
26
Comparison of the two optical designs
for the LTA - the 4th aspheric on convex surface
4th
order
parameter
tolerance
Beforecompensator
Sr (field angle )
0
150
-4.0E-13
0.5121
0.2754
+6.0E-13
0.1629
0.3053
Compensator
Shift (mm)
Aftercompensator
Sr (field angle )
0
150
0.137
0.9054
0.8602
-0.203
0.9065
0.86
At the maximum cutting
position the manufacture error
should be controlled less than
3.35 microns
The acceptable manufacture error
27
Comparison of the two optical designs
for the LTA - The objective lens deformation
Deformation of the objective
lens convex surface while the
zenith angle 1°:
The biggest deformation is the
top and the edge of the
surface, and the PV=106.7nm
Deformation of the objective
lens concave surface while the
zenith angle 1°:
The biggest deformation is the
top and the edge of the surface,
and the PV=97.8nm
28
Risk and Choice for the LTA
Maybe the most important advantage of the confocal paraboloid design is that it
has been implemented at the Gemini Observatory, and if the same design is
considered the TMT LTA will benefit from the experience with the fabricating and
mounting.
Refractive Design
Confocal Paraboloid
Design
Image quality
Strehl at 589nm slightly
inferior to reflective
design
Image quality appear to
be related to
fabrication
specifications and
mounting, not with
design
Fabrication and
Test
4th order aspheric surface is
difficult to test
Fabricating of the two
mirrors may also
need to be careful
Subject
Comments
29
Risk and Choice for the LTA
Subject
Refractive Design
Confocal Paraboloid
Design
Comments
Alignment and
mounting
Approx 4 times less
sensitive than
reflective
Approx 4 times more
sensitive than
refractive
Mechanical
flexure
Insensitive
Sensitive
Thermal stability
Sensitive but easy to
compensate
Insensitive
Cover window
Not need anymore
Require an additional
optical element
not complicated
Mass and
moment
Objective weight~16kg.
Fold mirrors ~3.2kg.
Weight mostly at
middle-top
Primary, secondary
weight ~19.6kg.
Weight mostly at
bottom.
Not include mechanical
element mass
Cost estimates
lower
May be expensive
Both of these are very
important during the
operation process of
TMT
30
LTA Throughput Estimate
Element
Number surface
Surface
Throughput
Throughput
Objective lens 1
2
0.99
0.980
Collimator
lenses
2
4
0.99
0.961
Fold Mirror
2
2
0.995
0.990
K Mirror
3
3
0.995
0.985
0.918
31
Error budget of the LTA
LTA total error
0.241
Objective lens manufacture WFE(p-v)
L1 manufacturing tolerance
L1 convex surface radius
L1 convex surface conic
L1 convex surface 4th parameter
L1 concave surface radius
L1 middle thickness
L1 index errors
0.139
Nominal data
440.822mm
-0.484869
1.88495E-11
3476.4188
70
1.5168
Tolerance
0.55mm~-0.15mm
0.00015~-0.00018
2.5E-13~-2.5E-13
2.5mm~-3.2mm
±0.03mm
-0.0001~+0.00008
Compensator
1.37527mm~-0.37499mm
-0.00813~0.00976
-0.00845~0.00845
-0.09214~0.11815
-0.02555~0.02555
0.08339~-0.22811
LTA alignment
LTA mirror parameters
L1 decenter
L1 tilt
0.010
Nominal data
Tolerance
±0.1mm
±180μrad
Compensator
±0.1mm
±3414.6μrad
p-v wavefront error
0
0.01
Nominal data
tolerance
0.14
0.04
0.08
Number of elements
1
5
2
p-v wavefront error
0.065
0.179
0.048
Surface irregularity(p-v)
LTA mirror parameters
L1(surface and support deformation)
Folding mirrors
Lenses
p-v wavefront error
0.008
0.1
0.09
0.03
0
0.015
0.196
The surface irregularities and wavefront errors are given in P-V at 589.3 nm. The
RSS calculations use a factor of 2 for reflective surfaces and 0.43 for transmitting
surfaces.
32
Schedule for the LTA
ID
WBS Code Beginning Time
Ending Time
Expected
Month
1
WBS 1.1
2012-10-1
2014-2-28
74w
2
WBS 1.2
2012-10-1
2013-3-29
26w
3
WBS 1.3
2012-10-1
2013-3-29
26w
4
WBS 1.4
2012-10-1
2013-5-31
35w
5
WBS 1.5
2012-10-1
2013-7-31
43.6w
6
WBS 1.6
2014-3-3
2014-8-29
26w
7
WBS 2
2013-3-1
2014-9-30
82.6w
8
WBS 3.2
2014-10-1
2014-12-31
13.2w
ID
WBS Code Beginning Time
Ending Time
2012年
10月
11月
Expected
Month
1
WBS 1.1.1
2012-10-1
2012-10-31
4.6w
2
WBS 1.1.2
2012-10-31
2014-1-31
65.6w
3
WBS 1.1.3
2013-5-1
2014-2-14
41.6w
4
WBS 1.1.4
2013-5-10
2014-2-28
42.2w
5
WBS 1.1.5
2012-10-1
2014-2-28
74w
Ending Time
Expected
Month
ID
WBS Code Beginning Time
1
WBS 2.1
2013-3-1
2013-4-30
8.6w
2
WBS 2.2
2013-4-1
2014-7-30
69.6w
3
WBS 2.3
2014-8-1
2014-9-30
8.6w
2013年
12月
01月 02月 03月
04月
05月
06月
2014年
07月
08月
2012年
10月
11月
09月
10月
11月
12月
01月 02月 03月
2013年
12月
01月 02月 03月
04月
05月
06月
07月
04月
05月
06月
07月
08月
05月
06月
09月
10月
11月
12月
01月 02月
2014年
09月
10月
11月
12月
01月 02月 03月
04月
05月
06月
07月
07月
08月
09月
10月
11月
12月
–
–
–
–
–
2014年
08月
2013年
03月
04月
08月
09月
1.1 Objective Lens
1.2 Two Fold Mirrors
1.3 Focus Adjustment
1.4 K-mirrors System
1.5 Mechanical Flexure
Compensator
– 1.6 Assembly Mounting
33
Samples of our fabrication
a folding mirror surface test result
d=23mm
PV = 24.2nm(0.041λ)
with a goal of 0.03λ is reliable
a folding mirror surface test result
d=70mm
PV = 29.2nm(0.05λ)
with a goal of 0.03λ is reliable
34
Samples of our fabrication
Type
Diameter
/mm
Stroke
Resonance
frequency
1
20mm
±4′
1000Hz
2
 30mm
±3.6′
200Hz
3
 50mm
±1.5′
930Hz
4
 78mm
±2.5′
300Hz
5
 60mm
±20′
260Hz
FSM surface test result
d=80mm
PV = 40.2nm(0.068λ)
with a goal of 0.04λat about 30mm
diameter mirror is reliable
35
The working plan for LGSF
• A cost/performance trade study to increase the LTA
field of view from 5 to 17 arc minutes
• An update to the existing cost estimate for the
current LGSF design
• Update the conceptual design (and its cost
estimate)
36
Thank you!
37