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Laser Launch Facility Mini-Review
Contributors: Jason Chin, Thomas Stalcup, Jim Bell, Drew Medeiros,
and Ed Wetherell
Agenda
• 9:00AM
• 9:05AM
•
•
•
•
10:50AM
11:00AM
12:00PM
12:45PM
Introductions
Presentations: Overview, 5 min.
Beam Generation System, 30 min.
Beam Transport System, 20 min
Switchyard, 20 min.
System performance 20 min.
Break
Open discussion
Committee closed session
Review committee feedback to team
2
LLF Reviewer Charter
• Reviewers: Olivier Martin (WMKO, Chair)
Renate Kupke (UCO-Lick)
Viswa Velur (Caltech)
• Does the LLF team understand the critical requirements?
• Does the opto-mechanical design satisfy the requirements?
• Is the opto-mechanical design technical feasible to fabricate?
• Are the technical risks clearly defined and are there plans to mitigate
the risks?
3
Requirements
•
•
Laser Launch Facility Requirements
– From three sources within Contour.
• 2.0 Overall Laser Guide Star Facility Requirements.
• 2.3 Overall Beam Transport System Requirements.
• 2.3.6 Diagnostics (within BTS).
Functional requirements
– Opto-mechanical system to transmit or relay the laser beams for central
projection onto the sky.
– Generation of the 7 LGS with proper orientation.
– Beam steering and centering.
– Diagnostics such as beam quality, laser power, and polarization not
included in Laser Units.
– Operational range: elevation range (0° to 70° zenith angle).
4
Requirements
• Functional requirements (continued)
– Ability to operate with the existing Keck II dye laser.
– Meet all ANSI and laser safety related requirements.
• Implementation requirements
– Does not add any additional vignetting.
– Does not impact daily and nightly operations (maintenance and service).
– Re-use of a launch telescope similar to the Keck 1 unit.
• Performance requirements
–
–
–
–
–
Peak power for 3 lasers (4.5KW/cm2)
0.9” spot size on sky; work in progress to define this in terms of WFE.
60% throughput, including launch telescope (was 75%).
On-sky laser positioning range of 30” with position tolerance of 0.3”.
Circular Polarization 98%.
5
Focus of the review
• Opto-mechanical designs between the Laser Units and the launch
telescope.
• Does not include software for the LLF or the motion controller.
• Management issues will be presented at PDR.
• Assumptions.
– Laser Units: 3 lasers providing 25 watts each. Lasers will be housed in
one or two laser enclosures situated on the elevation ring.
– Re-use of a launch telescope design similar to Keck I; LT design is not
included in this review
6
Definitions
• BGS – subsystem within the secondary socket to generate asterism
and provide PNS.
• BTO – on
telescope
structure
to relay
the
beams.
• SYD – on
elevation
ring to
steer
lasers into
BGS.
7
Telescope References
8
Beam Generation System
•
•
•
•
•
•
•
•
Central Asterism Generator (CAG, 4 beams).
Point aNd Shoot (PNS) generator and positioning (3 beams).
Steering of all beams on sky.
Tracking of lasers for non-sidereal objects.
Imaging of pupil on the launch telescope secondary mirror.
Rotator control.
Polarization control (possibly if needed).
Sensing of position to control mirrors in the SYD for telescope
flexure.
• Beams and asterism diagnostics.
• Final (fast) safety shutter.
10
BGS (Iso-view)
11
BGS (Iso-view)
12
CAG
14
PNS Module
15
BGS Rotator / Diagnostics
19
BGS interface at top end (horizon view)
20
BGS on LTA
21
BGS Interface with Counterweight
22
Design Advantages
• Design is relatively compact.
• Allows for PNS to move about within field; not limited to individual
sectors.
• Allows for non-sidereal tracking.
• BGS fits on top of the launch telescope; minimize independent
motion between BGS and launch telescope.
23
BGS Motion Control
• Size and weight constraints require the use
of piezo linear motors.
• Selection of smaller and lighter stages
– The current design uses three PI M-683
piezo ultrasonic stacked stages (1350g).
• Stages may have difficulty moving in the
vertical direction when stacked.
– New design calls for SmarAct stages (183 g).
• Option with tensioning system to balance load.
• Blocking force can be increased.
• Reduce the support required for lighter stages.
– New stages should reduce the overall mass
by 8.5 Kg.
– SmarAct can be controlled by USB, LabView,
or RS232.
– Risk reduction during DDR phase to test
stage and tracking performance.
24
Risk Assessment
• Risk Definitions
25
BGS Risks
•
1. Polarization changing as the K-mirror rotates (3,3) (likelihood,
consequence).
– Need to understand whether several degrees in angle can adversely affect the
polarization due to coatings.
– Understand coatings from manufacturers to understand its dependencies.
•
2. Ability to fit the components within its volume and meet its weight
restriction (2,2).
– Need to complete design of supports and determine the final weight.
– New smaller stages should minimize this risk.
– The counterweight mechanism on the f/15 may need to be modified; resulting in
additional cost. $2.5K of procurement and 1 man month of labor.
•
3. Air breakdown due to internal focus (2,2).
– Further examination is needed to determine allowable peak power at focus.
– If needed, the beam expansion optics must be change to reduce power density.
27
Polarization Control
• The launch system introduces an arbitrary polarization shift
• This will be compensated by applying a conjugate shift in the system
• Currently, this is done per laser in the laser enclosure
– Assumes that polarization shift is identical over the multiple paths after
the beamsplitter(s).
– There are a few surfaces where the incidence angle varies by +/- 2.7
degrees which may result in per-beam polarization variations
• If the per-beam polarization shift is too large, control will have to
move to the BGS after the beam splitters
– This is undesirable, as it would result in adding up to 14 motion devices
to the BGS…
28
BGS Risks
• Telescope vibration (2,1)
– Known vibration modes need to be verified on its impact.
– If needed, input high rate correction into SYD pzt stages.
• Diagnostics (2,1)
– Further layout is needed to ensure all diagnostics can fit onto
breadboard.
– Layout of shutter to allow real time diagnostics even if shutter is closed.
• PNS motion devices (1,1)
– How well they will work in a changing gravity vector.
– Test device in house.
• Additional risk reduction
– Investigate the feasibility of using the BGS design layout for the center
launch system project on K2.
– Partial implementation of the focus, beam expander and diagnostics.
29
Beam Transport Optics (BTO)
• Relay the 3 beams from the Laser Units to the BGS.
• Operate over the elevation ranges of 0° to 70° zenith angle.
• Additional requirement to assure this will operate with the current
Keck II dye laser.
– Install this subsystem in FY’11 as part of the Keck II Center Launch
System Project.
31
Flexure
• Important to understand the amount of flexure in the design of the
BTO
• KOR 90 estimates motion to be 1.7mm with an additional 19” of
motion; combined of 2.9mm of motion
• Current K2 flexure pointing model shows 19” of flexure for the entire
system, telescope, laser, sodium distance, secondary module,
telescope pointing model and acquisition system
• Recent K2 flexure testing shows a maximum of 2.9 mm motion from
the elevation ring to the current L4 position (top ring)
32
Flexure displacement (LRD)
33
Closed Loop Operations
• Use of PSD to determine
beam location at the BGS
to control flexure along the
BTO
• On-Trak PSM2-4 provides
4mm range and 0.1um
resolution.
• May require modification to
sensor packaging to fit into
the compact design of the
three beams; 25mm
separations
34
Long Relay Design
35
Long Relay Design (LRD)
36
Long Relay Design
37
LRD entry into the secondary
+X
+Y
38
LRD considerations
• Entry into the secondary socket is from the –Y direction; allows
greater freedom in attaching equipment; minimizes any
interferences with the telescope as it comes to horizon.
• Entry into the secondary socket is oriented for location of the BGS (Y of the module).
• Entry into the secondary socket minimizes interferences with
servicing within the f/15 module for operations.
• Mounting of light tubes on the telescope in the –Y direction
minimizes interferences with the telescope as it comes to horizon.
• If an additional laser is needed to be installed at the RBC, it can
easily tie into the LRD.
• Beam path travel to the top ring at an angle resulting is smaller
flexure error.
• The –Y direction of the telescope is more difficult to access for
installation, alignment and service.
39
Short Relay Design (SRD)
+X
+Y
40
SRD Considerations
• Most direct path to the top socket from the Laser Enclosure (LE);
• May reduce the number of reflective surfaces by 1.
• Re-use existing Keck II L4 launch telescope tube support structure
to support new tube.
• Easier access to locations on the telescope for installation,
alignment and servicing.
• Entry into the socket from +y; may impact servicing of f/15 module.
• If additional Laser Unit is needed at RBC, an identical BTO is
needed.
• Entry into the top socket is from the +y direction, opposite of where
the BGS is located
• Interferences with the telescope as it comes to horizon.
41
SRD and Telescope Interference
• Limitations of volume for opto-mechanical mounts
• At the L4 corner, ~20 cm of clearance as the telescope approaches
20 degrees near the Nasmyth deck
• At top end socket, ~ 20 cm of clearance as the telescope
approaches horizon near the Nasmyth deck
Nasmyth
Deck
Nasmyth
Deck
42
Tube Design Considerations
• Tubes exiting the laser enclosure will be larger than those crossing
the spiders.
• Tubes at the laser enclosure; 3” x 4”.
• Tubes at spiders and secondary socket; 1” x 3”.
• Exterior of tube will be low emissivity paint.
• Interior of tube will be an uncoated dispersive surface.
• The attachment points to the telescope will be compliant to not
impact the telescope structure performance.
43
Keck II Center Launch System Applicability
•
•
•
Take advantage of the existing optical
trombone on the laser table.
Install a M1 and beam expander prior to
the current M3
Allow for switching between current Keck
II laser operations and with new
BTO/BGS.
New exit
location
Current
M3
New M1
44
f/15 module modifications
• Using either the LRD or SRD
designs requires modification to
the top end socket.
• Infrastructure modifications to
support existing glycol,
pneumatics, and cabling.
• Counterweight design may need to
be modified to fit the required
volume of the BGS.
45
BTO Risks
• Flexure is larger than expected (2,1) (likelihood, consequence)
– Still a possibility; but is significantly reduced due to testing on Keck II.
– Design to allow an additional stage for the LRD to compensate for this.
• Vibration on the telescope (2,2)
– More data needs to be gathered. IF data is mainly concerned with
focus (OPD) versus tip-tilt.
– Test to move current accelerometers from top end mirror to the LTA in
Keck 1. Should provide an idea of expected vibration seen by BGS.
• SRD allowable volume (2,1)
– Need to finish design to assure volume is sufficient to install optomechanics at the pinch points at L4 and the top end locations.
46
SYD Requirements
• Format of the three laser beams to relay via the BTO to the BGS.
• Control of beam polarization at the output of the laser for circular
polarization on sky.
• Support installation of the Laser Units onto the elevation ring.
• Support laser alignments.
– Control laser beam power for alignment purposes.
– Provide a separate method to align BTO/BGS without the requirement
of the 589nm Laser Units; rough alignment.
• Diagnostics not provided by the Laser Units.
47
SYD Location
• Installation inside existing LE
48
SYD Layout with Laser Units
• 3 Laser Unit heads to be installed on an optical bench.
• SYD will be located in the middle of the same platform
• Sized similar to existing laser table (6’x5’); can be increased to used
existing space for dye lines.
49
SYD Layout
50
SYD Design
• Quarter and half waveplates to control polarization for each laser
• Polarization is not expected to change much in real time.
• Afocal beam expander telescope to convert 3mm laser beams to
1.14mm diameter beams for BGS.
• BTO flexure will require piezo control of tip/tilt mirrors to properly
center beams from SYD to BGS; 0.57” resolution requirement.
• Piezo tip/tilt mirrors will also support vibration control if necessary.
• Alignment laser for rough alignments.
• Dual optics stage to reduce laser power for alignment through
system.
• Beam dump for excess laser power as well as measurement.
51
SYD Mounting
•
•
•
Similar mounting to the existing laser table and auxiliary units, estimate
1,700 Kg.
Attach to existing elevation ring supports via pads to elevation ring
structure.
Elevation ring was stiffened with internal gussets during K2 laser install
53
SYD Risks
• Laser Units larger than expected (3,3) (likelihood, consequence)
– We believe a minimum of 2 lasers can be implemented in the LE; the
third is in question. Once Laser Units design is further along, this risk
will be retired.
– A backup plan is to install a third unit at the RBC location.
• Polarization (3,3)
– Do not expected to change once system is set in place. Current laser
technology has 700:1 linearity ratio.
– If the polarization varies due to pointing, up to 14 separate polarization
controls may be needed at the BGS.
– Further understanding of the coatings is necessary to determine the
significance of this risk to meet the 98% circular requirement.
54
LLF Motion Control
• 29 Degrees of Freedom (DOF) in Laser Launch Facility
• Precision
– millimeter to 10 um linear accuracy
– milli-radian to sub micro-radian angular accuracy
• Locations
– laser enclosure (13 DOF)
– secondary (16 DOF)
• Types
–
–
–
–
–
–
rotation (asterism rotator, polarization waveplates)
translation (in/out, focus, PnS steering)
tip/tilt (beam steering, asterism pointing, vibration compensation)
servo (most things)
piezo (for precision: beam steering, PnS steering)
tracking (flexure compensation, off loading, sidereal motion, nonsidereal motion)
55
LLF Motion Control
• Actuator choices
– traditional linear and rotation stages
• Brushed DC servo motors, drive or load encoding, end of travel switches
• lots of experience with design and integration
• good match with either centralized or distributed motion control architecture
– traditional piezo tip/tilt stages
•
•
•
•
smooth, high precision, high speed actuation, strain gauge readback
experience with design/integration
more suited to centralized MC architecture
care must be taken with high voltage signals
– piezo translation stages (PnS steering)
•
•
•
•
smooth, very precise actuation
not currently used at WMKO
turnkey and OEM type controller/driver solutions available
more suited to distributed MC architecture
– should work with centralized architecture, cable length is the concern
56
Cleanliness of the optics
• To achieve the level of throughput, it is imperative the optics remain
clean.
• 20% throughput gain for clean vs. dirty optics.
• The entire system will be sealed for laser safety and cleanliness.
• A positive pressure of 1 to 2 CFM of facility dried air is planned for
the LLF.
• Dried air is already available at the BGS and SYD.
• Cleanliness is also important for the optics due to high laser power
density.
57
Safety
• Compliance with ANSI Z136.1 standards.
• Hazardous radiation containment.
– Within cover for BGS
– Within beam tubes for BTO
– Within laser enclosure for Switchyard
• Indicators providing laser status at each service point
• Interlocking with safety system to contain radiation hazards.
58
System Performance
• Assumptions for throughput standard
– Laser line coating reflectivity = 0.994.
– Laser line AR coating transmissivity = 0.996.
– Transmission due to dust/dirt accumulation = 0.995.
• Expected throughput
– PNS: 75.71% clean; 62.53% dirty.
– Central AG: 76.78% clean; 64.37% dirty.
– Current requirements 60%.
• Wavefront error
– 123nm rms PNS; 116.3nm rms Central AG; need to determine the effect
of WFE on spot size.
– Requirement of 0.9” without atmospheric error term.
• Polarization?
59
System Performance
-PNS spot size versus altitude for seeing percentiles of 37.5%, 50%, and 87.5%
corresponding to r0=0.14, 0.16, and 0.22 m at a wavelength of 500 nm.
60
System Performance
-CAG spot size versus altitude for seeing percentiles of 37.5%, 50%, and 87.5%
corresponding to r0=0.14, 0.16, and 0.22 m at a wavelength of 500 nm.
61
Key Requirement Compliance Summary
Requirement
Complied
Note
1
Asterism Generation (4 lasers)

2
PNS (3 lasers)

3
Central Projection

4
Beam steering of 7 beams on sky (Tracking)

5
Polarization Control
6
Operational elevation range (0° to 70° zenith
angle)

Based on flexure
testing
7
Operate with existing Keck II dye laser

Requires modification
to existing Keck II
laser bench
8
No additional vignetting

9
Diagnostics: laser power; beam quality;
polarization
TBD
TBD
Polarization not
continuously
changing
62
Key Requirement Compliance Summary
Performance Requirement
Design
Compliance
62.53%
(dirty optics)

123.4
TBC
1
The LGS facility optical transmission
(PNS) >60%
2
Wavefront Error (PNS)
3
Spot size at sodium laser without
atmospheric considerations; 0.9”
0.8”

4
The blind pointing of the LGS
asterism shall be less than 10” rms
(TBC) with a goal of 1” rms (TBC). 1
arcsecond TBD
TBC
TBC
5
On-sky laser positioning range of 30”
with position tolerance of 0.3”
TBC
TBC
Notes
PNS is worst
case
Atmospheric
model must be
confirmed
63
Plans prior to PDR
• Retirement of the risks
– Measure K2 vibrations
– Determine polarization impact with design and coatings
– Determine laser power intensities within the BGS; for air breakdown and
coatings.
– Continue to monitor Laser Units progress to validate SYD design.
• Completion of designs
– Verifying BGS layout and volume
– Examine the clearance issues at L4 and at the secondary socket for the
BTO Short Relay Design
– Complete diagnostics for the LLF
– Complete SYD models
– Complete BTO attachment point models to telescope
64
Plans prior to PDR
• Performance
– Produce a focus error budget
– Produce a pointing error budget
• Management
– Schedule, cost and effort estimates
65
MAHALO for your participation
66