Laser Launch Facility Mini-Review Contributors: Jason Chin, Thomas Stalcup, Jim Bell, Drew Medeiros, and Ed Wetherell.

Laser Launch Facility Mini-ReviewLaser Launch Facility Mini-Review

Contributors: Jason Chin, Thomas Stalcup, Jim Bell, Drew Medeiros, Contributors: Jason Chin, Thomas Stalcup, Jim Bell, Drew Medeiros, and Ed Wetherelland Ed Wetherell

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Agenda

• 9:00AM Introductions• 9:05AM Presentations: Overview, 5 min.

Beam Generation System, 30 min.Beam Transport System, 20 minSwitchyard, 20 min.System performance 20 min.

• 10:50AM Break• 11:00AM Open discussion• 12:00PM Committee closed session• 12:45PM Review committee feedback to team

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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?

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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).

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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%.

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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

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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.

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Telescope References

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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.

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BGS (Iso-view)

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BGS (Iso-view)

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CAG

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PNS Module

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BGS Rotator / Diagnostics

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BGS interface at top end (horizon view)

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BGS on LTA

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BGS Interface with Counterweight

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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.

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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.

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Risk Assessment• Risk Definitions

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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.

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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…

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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.

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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.

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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)

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Flexure displacement (LRD)

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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

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Long Relay Design

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Long Relay Design (LRD)

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Long Relay Design

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LRD entry into the secondary

+X

+Y

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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.

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Short Relay Design (SRD)

+X

+Y

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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.

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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

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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.

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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

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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.

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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 opto-

mechanics at the pinch points at L4 and the top end locations.

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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.

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SYD Location

• Installation inside existing LE

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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.

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SYD Layout

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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.

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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

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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.

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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, non-

sidereal motion)

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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

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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.

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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.

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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?

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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.

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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.

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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 TBD

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 Polarization not continuously changing

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Key Requirement Compliance Summary

Performance Requirement Design Compliance Notes

1 The LGS facility optical transmission (PNS) >60%

62.53%

(dirty optics) PNS is worst

case

2 Wavefront Error (PNS) 123.4 TBC

3 Spot size at sodium laser without atmospheric considerations; 0.9”

0.8” Atmospheric model must be confirmed

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

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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

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Plans prior to PDR

• Performance– Produce a focus error budget

– Produce a pointing error budget

• Management– Schedule, cost and effort estimates

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MAHALO for your participation