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MCAO BTO, LLT and periscope subsystems Celine d’Orgeville MCAO Outline • Introduction – Celine d’Orgeville – – – – • • • • Laser path BTO and LLT schematic Top-level requirements Error budgets summary BTO performance and budgets – Don Gavel BTO optics – Brian Bauman BTO electronics – Mark Hunten LLT and periscope designs – Celine d’Orgeville & Jim Catone May 24-25, 2001 MCAO Preliminary Design Review 2 MCAO Laser path • Five 10-W beams • Preferred Laser System location: on center section (A) • Mirror relay (fiber technology is not there yet) • On-axis LLT A A May 24-25, 2001 MCAO Preliminary Design Review B,C 3 BTO and LLT Schematic MCAO Near- and far-field beam profiles Beam position & tilt PA, CA Miscellaneous Closed-loop Boresight Periscope Pre Alignment Calibration Features X-Shaping Control Mirrors BS PA, CA CC Top-End Ring Mirror Top End Center Section PAC PAC Relay Pointing Array (PA) PA, CA MCAO CS BDS Centering Mirror (CM) Beam Dump Mirror Shutter LGS WFS KM Pointing Mirror (PM) Fast Steering Array Offload to KM, PM, CM l/4 Plate LLT cover MCAO CS PAC Centering Array (CA) l/4 Plate Safety Shutter(SALSA) May 24-25, 2001 PS MCAO Preliminary Design Review LLT PM, CM 4 LLT and BTO top-level requirements MCAO Beam launch On axis Do not obstruct M2 hole PERISCOPE LGS constellation Rotating X-shaped constellation 1 beam on-axis, 4 beams 42.5 arcsec away Optical throughput > 0.7 for BTO and LLT combined (laser mounted on telescope) Beam quality Do not degrade beam quality significantly compared to atmospheric turbulence Beam pointing accuracy < 1 arcsec peak (blind pointing) < 0.05 arcsec rms (dynamic) Heat dissipation < 10W for beam dump on top-end < 10W above M2 < 10W along telescope truss Misc. Controlled by MCAO CS Low maintenance Preserve circular polarization May 24-25, 2001 MCAO Preliminary Design Review 5 Beam quality error budget MCAO • At CoDR MCAO performance calculations assumed a 1.5 x DL combined beam quality for Laser + BTO + LLT • PDR revised beam quality error budget gives 1.73 x DL (resp. 1.58 x DL) for: – Use of 1.2 x DL laser beams – The LLT meeting its image quality specification (resp. goal) – Use of premium quality BTO optics • Impact on the MCAO overall performance : – Requires a 21% (resp. 4%) increase in laser power to balance the LGS spot size increase and maintain performance – Or accept small penalty in Strehl • NB: this variability is within the margin presented at CoDR (36% 0.5 magnitude in LGS signal level) May 24-25, 2001 MCAO Preliminary Design Review 6 MCAO Atmo. Comp. Strehl in J, H, K bands MCAO performance vs LGS signal 21% signal decrease K H J Design point Photodetection event per subaperture @ 800Hz May 24-25, 2001 MCAO Preliminary Design Review 7 Optical throughput error budget LGS subsystem Laser power output CoDR PDR P = 6-15W To be reviewed according to sodium measurements BTO transmission (assumes laser mounted T on telescope) BTO > 0.8 LLT transmission (incl. beam clipping) T Projected laser power P L = 4-11W May 24-25, 2001 MCAO LLT > 0.9 T BTO = 0.95 (per D. Gavel’s calculations) T LLT = 0.96 (per D. Gavel’s calculations) TBR MCAO Preliminary Design Review 8 Heat dissipation error budget LGS subsystem CoDR PDR Laser system < 100W < 100W LLT < 10W for beam dump on top-end < 10W for BTOOB + LLT Total < 120W BTO May 24-25, 2001 MCAO < 8 W for BTOOB < 10W for beam dump on top-end < 10 W for all other BTO element combined < 2W for LLT alone < 130W MCAO Preliminary Design Review 9 MCAO Laser Beam Transfer Optics Performance and Budgets Donald T. Gavel University of California Lawrence Livermore National Laboratory BTO Alignment Control MCAO • Two pointing and centering systems: • 1. Alignment up the telescope and across the top-end behind a spider vane – P&C mirrors for each of the 5 beams separately: on the laser table – P&C sensors: on the BTOOB . • 2. Into the LLT & pointing on the sky – P&C mirrors on the BTOOB do “common mode” steering on the sky, to compensate top-end flexure – pointing sensed by the AO WFS and offloaded. May 24-25, 2001 MCAO Preliminary Design Review 11 Requirements MCAO • Compatible with single beam system • Pointing control to 0.05 arcsecond rms on the sky for each of 5 beams • Beams overlap at the LLT entrance pupil (we take this to mean within 10% of beam diameter) May 24-25, 2001 MCAO Preliminary Design Review 12 4 Stages of Adjustment MCAO • Stage 1: Mirror mounts placed so nominal beam line is within capture range of motorized adjustment • Stage 2: Mirrors remotely aligned into the capture range of slow P&C loops • Stage 3: P&C closed loop puts beam into capture range of high-bandwidth tip/tilt sensors • Stage 4: High bandwidth uplink tip/tilt loops closed May 24-25, 2001 MCAO Preliminary Design Review 13 Motorized range and accuracy MCAO • Range = mounting + top-end flexure compensation • Accuracy = capture range of uplink tip/tilt = ~1” on the sky • Motorized Adjustments: – Laser P&C mirror array • Range to cover 2mm top-end sag, 30” top-end tilt: ~30mr • Acquisition range of diagnostic P&C sensor: ~20mr • Accuracy needed to stay centered behind spider: 0.5 mm, 0.1 mr • Accuracy in closed loop: 1” on sky or 0.2 mr/mirror – Top ring fold mirror – 5-beam shaping mirror array – Pointing and Centering mirrors • Range 30” on sky (LLT primary tilt) = 9 mr • Stability 0.12 mr/mirror to capture fast t/t May 24-25, 2001 MCAO Preliminary Design Review 14 Closed Loop Uplink Tip/Tilt MCAO • Range of fast tip/tilt mirrors: 1” on sky = 300 mr • Accuracy: 0.05” on sky = 15 mr • Off-load the average deviation from mid-range to the P&C mirrors ahead of the LLT May 24-25, 2001 MCAO Preliminary Design Review 15 MCAO BTO Laser Transmission Budget • 17 surfaces in BTO path X-Shaping Mirrors Top End Top-End Ring Mirror Center Section 10 9 8 7 BS 12 11 Centering mirror (CM) 13 14 15 16 KM Pointing mirror (PM) Fast Steering Array 56 34 2 Relay Pointing Array (PA) 1 Centering Array (CA) • Requirement: T>80% (CoDR, Table 24, p74) May 24-25, 2001 17 MCAO Preliminary Design Review LLT 16 BTO Transmission Budget MCAO • Reflection – "V" band (narrowband 589 nm) coatings: – >999%/surface => 98% total • Surface roughness – Pscat/Pinc = (4 p s / l)2 – “commercial” polish s=10-30A => 91% total, “precision” s=2-10A => 99.1% total • Dust – Optical Cleanliness Specifications and Cleanliness Verification, SPIE 3782, 1999 – 0.1% area coverage/surface => 98.6% total • Composite Budget: 95.7% > 80% required • Transmission is specified high in order to meet the heat dissipation budget, which is more demanding May 24-25, 2001 MCAO Preliminary Design Review 17 BTO Heat Dissipation Budget MCAO • Requirement: < 10 Watts total (CoDR, Table 24, p74) • Heat budget spreadsheet: Component Location Number Power per component Total power Optics Secondary and Top Ring 11 127 mW 1.4 W Optics Primary Ring 4 127 mW 0.5 W P&C diagnostic cameras Secondary 2 3.6 W 7.2 W P&C motors Secondary 4 Fast tip/tilt stages Secondary 5 40 mW 0.2 W Chopper wheel Secondary 1 0.5 W 0.5 W PA and CA motors Primary Ring 20 negligible Total May 24-25, 2001 negligible 9.8 W MCAO Preliminary Design Review 18 BTO Heat Dissipation Budget MCAO • Total power in the light scattered by the optical surfaces : – Coating 0.75 Watts – Surface roughness 0.45 Watts – Dust 0.7 Watts – Total 1.9 Watts • Heat budget is dominated by – Power used by diagnostic cameras (7.2 W) – Power scattered by optics (1.9 W). This drives the cleanliness and “premium” surface requirement for optics May 24-25, 2001 MCAO Preliminary Design Review 19 MCAO Beam Quality • Requirement: optical aberrations negligible compared to atmospheric distortions (CoDR Table 24, p. 75) • Atmosphere: s2 = 0.134 (D/r0)5/3 = 0.5 radians rms = 1/12 wave = 48 nm rms (D=30cm, r0=20cm) • Surface error would have to be l/100 on each of 17 surfaces to meet a 10 nm total error budget! • Here’s a budget based on “premium surface” optics: Optic Flat Lens Beamsplitter Total surface quality, waves P-V 0.05 0.05 0.25 rms surface, nm 6.25 6.25 31.25 rms wavefront, nm number of surfaces 12.5 3.125 15.625 10 2 3 rss sum of rms wavefront, nm 39.52847075 4.419417382 27.06329387 48.10876349 • Gemini used 95nm rms for the performance error budget, split in 88 and 35 nm rms between low and high order aberr. resp. May 24-25, 2001 MCAO Preliminary Design Review 20 Scattered Light MCAO • Categories of stray laser light (Rayleigh and aerosol scatter) – 1) Scattered light along the beam path up the side of the telescope – 2) Scattered light along the beam path across the primary – 3) Scattered light along the atmospheric path to the sodium layer May 24-25, 2001 MCAO Preliminary Design Review 21 Scattered Light from Behind the Secondary Spider Laser beam MCAO laser beam ... spider vane 10 mm spider vane 10 mm subapertures qa qa subaperture 50 cm 50 cm • 25.2 photons per frame per subaperture without baffles (calculations in Appendix Q) • Baffles are recommended along the sides of beam May 24-25, 2001 MCAO Preliminary Design Review 22 Scattered Light from Atmospheric Path to the Sodium Layer 1 Gemini LGS geometry: viewed from space 2 0 3 4 May 24-25, 2001 MCAO MCAO Preliminary Design Review 23 Scattered Light from Atmospheric Path to the Sodium Layer MCAO • Wave-optic simulation of laser propagation • Backscatter simulated from layered (1km spacing) atmosphere • Backscatter coefficients taken from Gardner’s (1990) measurements • Scattered light imaged onto WFS focal plane Subap [1,0], lasers 0 and 2 (WFS for lgs 0) May 24-25, 2001 Subap [6,0], lasers 1 and 2, (WFS for lgs 1) MCAO Preliminary Design Review 24 Pupil maps of Rayleigh scatter WFS 0 (center LGS) laser 3 MCAO WFS 1 (corner LGS) laser 1 laser 1 laser 2 lasers 0 and 3 laser 3 May 24-25, 2001 laser 4 laser 4 MCAO Preliminary Design Review 25 Scattered Light from Atmospheric Path to the Sodium Layer MCAO • Rayleigh from “fratricide” is significant, on the order of the same number of photocounts per subaperture as the guidestar itself, for some subapertures • Pulsed lasers with time-gated return will eliminate the fratricide issue. • Rayleigh from the sensed LGS’s beam is small, but it is important to field-stop correctly May 24-25, 2001 MCAO Preliminary Design Review 26 MCAO BTO Optics Brian Bauman University of California Lawrence Livermore National Laboratory BTO Beam Path and Control Diagnostic Split Surfaces Centering MCAO Mirror X-shaping mirrors K Mirror Pointing Mirror Top-end Ring Fold Relay Optics Fast Tip/Tilt Array Centering Array Pointing Array * May 24-25, 2001 MCAO Preliminary Design Review 28 Top-End Layout May 24-25, 2001 MCAO Preliminary Design Review MCAO 29 BTO alignment diagnostics MCAO • Pointing sensor • Centering diagnostic • Chopper wheel to isolate each beam plus dark & open positions • Outputs drive the PA and CA mirrors in a slow closed-loop May 24-25, 2001 MCAO Preliminary Design Review 30 Pointing diagnostic MCAO • Pointing diagnostic has 170 arcsec field, cf. 85 arcsec diameter LGS constellation; some vignetting at edge of field • Plate scale is 0.165 arcsec/pixel • CCD is 1.3K x 1K with 6.8 micron pixels • Can be used as a beam quality diagnostic May 24-25, 2001 MCAO Preliminary Design Review 31 Pointing diagnostic: 0, 42.5, 85 arcsec off-axis May 24-25, 2001 MCAO Preliminary Design Review MCAO 32 Centering diagnostic MCAO • Centering diagnostic re-images desired plane to the CCD • Designed as afocal telescope to avoid magnification errors with defocus • Beam fills ½ of short dimension of CCD (allows for misalignment) • Can image either LLT entrance pupil plane or FSA plane to CCD; either works for control purposes • Design imaging LLT entrance pupil shown in next slide May 24-25, 2001 MCAO Preliminary Design Review 33 Centering diagnostic May 24-25, 2001 MCAO Preliminary Design Review MCAO 34 X-shaping mirror (XSM) May 24-25, 2001 MCAO Preliminary Design Review MCAO 35 MCAO K-mirror May 24-25, 2001 MCAO Preliminary Design Review 36 Relay telescope MCAO • 1 to 1 relay with 5m focal length lenses • Pupils at CA (centering array) and FSA (fast steering array) • Exact prescription depends on exit pupil position of laser system • Very slow beam avoids air breakdown at focus and aberrations • Large (150mm diameter) lenses avoid clipping of beams May 24-25, 2001 MCAO Preliminary Design Review 37 Relay telescope May 24-25, 2001 MCAO Preliminary Design Review MCAO 38 MCAO BTO electronics Mark Hunten Beam Transfer Optics Electronics Overview of the BTO diagram. BDS X-Shaping Mirrors Top-End Ring Mirror Top End Center Section Centering mirror (CM) BS KM Pointing mirror (PM) Fast Steering Array Relay Pointing Array (PA) May 24-25, 2001 MCAO Centering Array (CA) LLT MCAO Preliminary Design Review 40 Beam Transfer Optics Electronics MCAO • The Beam Transfer Optics control electronics will be located on the main part of the telescope, probably on the Center Section. • These will be located in a thermal enclosure to keep the radiated heat to a minimum. • Items in the enclosure will be the VME control computer, servo control electronics, mechanism control electronics, monitoring electronics and camera interfaces. • System is distributed on the telescope from the center section to the top end. • This will require substantial coordination with the telescope operations groups for the installation period. May 24-25, 2001 MCAO Preliminary Design Review 43 MCAO PDR Agenda Thursday, 5/24 0800 Welcome 0805 Project overview 0830 Science case 0930 Break 0945 System overview 1015 System modeling 1100 AO Module optics 1145 Lunch May 24-25, 2001 1245 AO Module mechanics 1340 AO Module electronics 1400 Break 1415 Beam Transfer Optics 1510 Laser Launch Telescope 1545 Closed committee session 1800 Adjourn MCAO Preliminary Design Review 44