1 Kai Wei Institute of Optics and Electronics (IOE),CAS August 30,2010 The TMT Laser Guide Star Facility (LGSF)

2 Presentation Outline LGSF Requirements and Updates The changes of the LGSF designs What we have done for the LGSF? The working plan for LGSF

3 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

4 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.

5 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)

6 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

7 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 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/e 2 beam diameter, for a near field 1/e 2 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.

8 Four Kinds of the LGSF configuration LGSF Initial configuration LGSF Baseline configuration LGSF Elevation Journal configuration LGSF Side Launch configuration

9 1.Laser switchyard: an optical bench with motor-controlled beamsplitters and mirrors which accept the 50W input beams from the operating lasers and direct them to the proper outputs at the desired power. One can generate either two 25W beams or three 17W beams LGSF Initial configuration -Overall Description 2.Beam Transport Optics transport the nine output beams from the Laser Switchyard out of the LSE and up the telescope truss to the BTOOB behind the secondary mirror. Two of the mirrors in each beam position within the BTO path are controllable in tip/tilt to maintain both centering and pointing of the beams at the input to the BTOOB, correcting for the inevitable flexure of the telescope structure with altitude. 3.The diagnostic system directs a small fraction (0.5%) of each of the laser beams through a beamsplitter into two camera systems. one focused at a relatively close distance, the other at infinity. The near-field camera is used to evaluate the intensity profile and quality of the laser beam within the LGSF. The far- field camera views the projected LGS at diffraction-limited resolution to evaluate its image quality.

10 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.

11 LGSF Initial configuration -Asterism Generator The two mirrors at the periphery of the Asterism Generator will be controllable in tip/tilt to maintain centering of the beams on the LLT pupil and pointing of the LGS beacons on the sky in compensation for any flexure within the BTOOB and pointing error of the LLT itself resulting from flexure of the telescope structure with attitude. The first mirror, on the back side of the Asterism Generator plate, is a high-bandwidth tip/tilt fast steering mirror (FSM) to compensate for jitter in the position of the LGS as measured by the associated WFS The budget for fast tip/tilt correction assigns a value of 50 mas to the 1-axis, 1σ laser pointing jitter

12 LGSF Initial configuration -Centering and Pointing mirrors

13 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 end due to thermal and flexure effects.

14 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.

15 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.

16 LGSF Elevation Journal configuration

17 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

18 LGSF Side Launch configuration

19 LGSF Side Launch configuration

20 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

21 What we have done for LGSF Beam transfer Optics System: –the Optical Path –the LGSF Top End the Diagnostic System Asterism Generator Launch Telescope Assembly (LTA) Acquisition System 3 main sub-systems: –Laser System –Beam transfer optics system –Laser safety system

22 LGSF Elevation Journal Configuration

23 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

24 Comparison of the two optical designs for the LTA Radius of field-angle (arc sec) Confocal paraboloid designRefractive design Strehl 589nm 210 km90 km210 km90 km Image quality for the two optical designs The differences between the image quality of the two optical design is inconspicuous

25 Comparison of the two optical designs for the LTA IssueConfocal Paraboloid DesignRefractive 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. 4 th 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.

26 Comparison of the two optical designs for the LTA - Thermal analysis for the refractive design Temperature ( ℃ ) Focus Adjustments (microns) Design 0 ℃ Changes from -5 ℃ to 9 ℃ 380 Changes quantity per 0.5 ℃ 13 Observing Performance Conditions : -Ambient air temperatures range: -5 ℃ to +9 ℃ the WFE RMS of the LTA while the temperature changes 1 degree Ambient air temperatures range: -5 ℃ to + 9 ℃

27 Comparison of the two optical designs for the LTA - the 4 th aspheric on convex surface 4 th order parameter tolerance Before- compensator Sr (field angle ) Compensator Shift (mm) After- compensator Sr (field angle ) E E The acceptable manufacture error At the maximum cutting position the manufacture error should be controlled less than 3.35 microns

28 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

29 Risk and Choice for the LTA SubjectRefractive Design Confocal Paraboloid Design Comments 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 4 th order aspheric surface is difficult to test Fabricating of the two mirrors may also need to be careful 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.

30 Risk and Choice for the LTA SubjectRefractive Design Confocal Paraboloid Design Comments Alignment and mounting Approx 4 times less sensitive than reflective Approx 4 times more sensitive than refractive Both of these are very important during the operation process of TMT Mechanical flexure InsensitiveSensitive Thermal stability Sensitive but easy to compensate Insensitive Cover windowNot 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 estimateslowerMay be expensive

31 LTA Throughput Estimate ElementNumbersurfaceSurface Throughput Objective lens Collimator lenses Fold Mirror K Mirror

32 Error budget of the LTA The surface irregularities and wavefront errors are given in P-V at nm. The RSS calculations use a factor of 2 for reflective surfaces and 0.43 for transmitting surfaces.

33 Schedule for the LTA –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

34 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

35 Samples of our fabrication FSM surface test result d=80mm PV = 40.2nm(0.068λ) with a goal of 0.04λat about 30mm diameter mirror is reliable TypeDiameter /mm StrokeResonance frequency 1  20mm ±4′1000Hz 2  30mm ±3.6′200Hz 3  50mm ±1.5′930Hz 4  78mm ±2.5′300Hz 5  60mm ±20′260Hz

36 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)

37 Thank you!