Adaptive Optics Overview (Astronomical)

Transcription

1 Adaptive Optics Overview (Astronomical) Richard Myers Durham University William Herschel Telescope with GLAS Rayleigh Laser Guide Star Photo: Tibor Agocs, Isaac Newton Group of Telescopes

2 Outline Generic Astronomical AO System Specifying AO for Atmospheric Turbulence Compensation and where this is not necessarily valid for more general applications Astronomical AO Components 2nd generation Astronomical AO modes Wide field AO Multi-Conjugate AO Ground Layer AO Laser Tomographic AO Multi-object AO Very high order correction extreme AO

3 Generic Astronomical Adaptive Optics Science target Laser * * Natural Guide Star Adaptive Mirror Basic Single Conjugate AO system control signals Control System wavefront information dichroic beamsplitter Visible light Wavefront Sensor Correcting the fluctuating aberrations caused by atmospheric turbulence above ground-based optical and near-infrared telescopes. atmospheric turbulence IR light Telescope Corrected Science focus Corrected Image Uncorrected image Uncorrected wavefront Corrected wavefront 3

4 The Atmosphere Kolmogorov model of turbulence Kinetic energy in large scale turbulence cascades to smaller scales Inertial interval Inner scale l 0-1cm. Outer scale L 0-10 to 100 m Refractive index Structure Function for separation r : D ( r) = n( + r) " n( ) n [!! ] 2 D ( r) n = C r 2 2 / 3 n J. Vernin, Universite de Nice. Cerro Pachon for Gemini IGPO

5 2D phase structure function at telescope for plane waves: D! & r ' = 6.88( r ) * 0 + 5/ 3 2 & 2" top atm 2 ' where r0 = & ' ( ( ) sec # Cn ( h) dh) 0 * $ +, * + and # is the zenith angle Fried parameter r 0 ( λ 6/5 ): Size of aperture where Diffraction width = Seeing width For infinite outer scale Kolmogorov turbulence in the near field, r 0 and the telescope diameter D are the only parameters required to: derive image profiles determine the number of Deformable mirror actuators required to produce a given residual wavefront phase variance (on average) ~(spacing/r 0 ) 5/3 Determine the required interactuator stroke But C n2 (h) will strongly affect off-axis performance and Scintillation (amplitude variation) is often important in nonastronomical AO - worst case: phase branch points Thermal blooming % 3/ 5

6 Types of Adaptive Mirror (J.C.Dainty) Laser Applications of AO

7 Continuous Facesheet Deformable Mirrors

8 Bimorph Mirror (J.C.Dainty, Imperial College) Laser Applications of AO

9 Bimorph Deformable Mirror Laser Applications of AO

10 The ELECTRA Segmented Adaptive Mirror (76 tip-tilt-piston segments) built by ThermoTrex, San Diego 228 degree of freedom adaptive mirror Laser Applications of AO

11 Fitting Error for Continuous Facesheet Deformable Mirror (and segmented) Flexible continuous phase sheet Minimum physical actuator separation ~ 7mm reflective surface Actuators: typically PZT or PMN throw: 2-20 microns Fitting error: σ 2 f =κ (r s /r 0 )5/3 rad 2 Lots of Astronomical assumptions! r s = projected actuator separation on sky κ = fitting coefficient for DM type. (continuous face sheet: ) Laser Applications of AO

12 Deformable Mirror Actuators 1st generation DMs all involved piezoelectric (PZT) / electrostrictive (PMN) actuators: Serious Hysteresis (typically 5-40% of full range) Curie Point (rapid change of level of hysteresis with temp) Often limited stroke (hence stacked actuators) Drive voltage (+/- 400V for low hysteresis hard PZT) OR magnetostrictive or voice call actuators for higher stroke applications Non-linearity, bulk, power Newer DMs are available with electrostatic, magnetic and electromagnetic actuators Electrostatic low hysteresis MEMS construction ( micron spacing) 4K actuator devices available But non-linear, stroke still limited (4-6 microns mechanical) Magnetic Essentially no hysteresis Low temperature operation High stroke 0.5V operation (COTS CMOS!)

13 Boston Micromachines MEMS deformable mirror Raw: 148 nm RMS WFE Flattened: 6-12 nm WFE In-band: 0.6 nm WFE 32x32 MEMS Evans et al 2006 Optics Exp Electrostatically actuated diaphragm Attachment post Membrane mirror GPI Courtesy: Bruce Macintosh, LLNL

14 4k MEMS prototype 4k MEMS prototype Courtesy: Bruce Macintosh, LLNL 64x64 MEMS prototypes now in testing 4 micron stroke Surface quality: nm RMS surface finish, 2-4 microns PV overall curvature

15 Parameter Value Comments Clear aperture disk diameter Number of actuators across the diameter of the clear aperture disk Yield 40 mm ±5 mm Range of acceptable D is 30mm to 100mm (TBC) N=64 N=84 N= % in mirror clear aperture for D as defined in DM19 If this specification cannot be met, please advice. It might be possible to accept a D between 30 mm and 100 mm (TBC). Difference in x and y: overall slightly elliptical shape might also be required. Regular Cartesian array assumed. Actuator Spacing D/(N-1) mm For D and N see DM19and DM20 respectively Actuator Geometry Square Might want to investigate the feasibility of having a different spacing in the x and y directions. Actuator Stroke (PV) Larger than or equal to 6 µm P to V mechanical stroke Inter-actuator Stroke 1.65 µm mechanical for N= µm mechanical for N= µm mechanical for N=112 High order WFE (wavefront error) Surface Roughness 30 nm rms TBC after flattening (refer to the definition of mirror flattening ) 3 nm (TBC) rms Not including provisions the manufacturer may take for flattening the DM. Errors of spatial frequencies greater than those corresponding to half the actuator spacing frequency (i.e. errors which can not be self-corrected by the DM). Scratch/Dig Ratio TBD Temporal Frequency Response < 5 phase lag at frequencies = or < 500 Hz (TBC) and 10% of max stroke (phase lag decreasing at lower frequencies) Hysteresis 1% of maximum stroke (TBC) For max stroke. In run actuator repeatability 6 nm RMS WFE over the entire clear aperture of the mirror when all the actuators are stimulated. In-run repeatability implies the variation in performance measured during a single power up using the same actuator commands (within the dynamic range).

16 Run-to-run actuator repeatability Reflectivity Thermal Radiation 6 nm RMS WFE over the entire clear aperture of the mirror when all the actuators are stimulated. > 80% from 0.5µm to 0.6µm > 95% from 0.8µm to 1µm > 98% from 1.0µm to 2.5µm When the DM actuators are operated, its optical surface temperature will not deviate from ambient temperature by more than 2%. Run-to-run actuator repeatability is the variation in measured performance across a number of device power-ups for the same actuator command set. These values should be treated as reflectance guidelines. The supplier should comment on feasibility. Durability specifications: any specified minimum reflectance should hold for a minimum TBC 10 years in the indicated operational and storage environments.

17 Shack-Hartmann Wavefront Sensor (WFS) Microlens Array Detector Each xy offset measures the local wavefront slope across the corresponding lenslet. Wavefront Laser Applications of AO

18 Curvature Wavefront Sensor Input Wavefront Focal Plane Sensing Planes Laser Applications of AO

19 Wavefront Sensors and Detectors The curvature sensor minimises the number of pixels required to remove a given wavefront variance (assumes Kolmogorov or similar) the use of noiseless fibre-coupled avalanche photo-diodes is therefore feasible Shack-Hartmann requires more pixels so a CCD is normally employed low read-noise multi-port frame-transfer specialised devices Including on-chip electron-multiplication to effectively eliminate read noise (multiplication noise effectively decreases quantum efficiency, however) Much of the above is driven by photon starvation in Natural Guide Star Astro AO Where there is PLENTY of guide light one can consider other detectors, especially CMOS.

20 Detector Quantum Efficiency for a CCD with a possible choice of dichroic filter

21 EEV CCD-39 Projected read noise (e - rms) versus pixel rate/port Laser Applications of AO

22 4-port frame transfer CCD Schematic of 4-port frametransfer CCD read-out port shift register shielded frametransfer area (1) frame transfer light sensitive area (4 quadrants, gaps exaggerated) (2) charge movement Total pixels LL CCID-11: 64x64 Loral: 64x64 EEV:80x80 Laser Applications of AO

23 Real-time Computer -RTC (please see poster) 1st generation astro AO systems used: Single PC or RISC device for Real-time (though inevitably accompanied by other housekeeping processors with typically shared memory) Multi-CPU Multi-DSP (most common) C40 DSP very popular and still running! TigerSHARC more recently 2nd/3rd generation RTCs incorporate FPGAs for some tasks (and high speed serial comms) Cell processor evaluated Not as good as might be thought for this application Future systems (for Extremely Large Telescopes) Evaluating large FPGAs And GPUs - very promissing!

24 Isoplanatic angle, temporal variation Angle over which wavefront distortions are essentially the same: % / 3 ' Cn h h dh & 2! " 0 = 2.91 & ' ( ( ) sec #, ( ) ) * * $ + + It is possible to perform a similar turbulence weighted integral of transverse wind speed in order to derive an effective wind speed and approximate timescale of seeing note the importance of C n2 (h) in both cases. LIMITS FIELD OF VIEW OF CONVENTIONAL AO

25 Correcting two turbulence layers Turbulence Layers Deformable mirror Does not work off axis: higher layer uncorrected, lower layer overcorrected Works on axis: both layers corrected Credit: Rigaut, MCAO for Dummies

26 Multiconjugate AO corrects both layers 2 Deformable mirrors Conjugated to each layer Turbulence Layers Credit: Rigaut, MCAO for Dummies

27 Ground Layer AO Very large field of view but only partial correction Use multiple LGS to isolate the groundlayer turbulence which applies to all lines of sight Apply correction with single deformable mirror Often implemented with an adaptive telescope secondary mirror [Courtesy ESO] Astronomical AO 27

28 Ground Layer AO with Adaptive Secondary Deformable Mirrors MMT; being built for LBT, VLT Compare with normal size DM 28

29 Laser Tomography AO Small field of view and high-order correction Use multiple LGS to perform tomography of the turbulent volume Apply correction with single deformable mirror Overcomes LGS cone effect Being built for VLT [Courtesy ESO] Astronomical AO 29

30 Large field of view and high-order correction Multi Object AO But individual fields are small Use multiple LGS to perform tomography of the turbulent volume Then the wavefront can be corrected for each individual target direction, by applying correction with multiple deformable mirrors one for each science target Correction is open-loop in that the wavefront is not nulled within a control loop Being studied for 42m E-ELT [Courtesy ESO] Astronomical AO 30

31 Extreme AO (XAO) Tiny field of view and very high-order correction Use single very bright NGS to analyse wavefront along single line of sight Block light from guide star and search for companions Apply correction with very high order DM Some interesting new technologies Very high order deformable mirrors (4K MEMS) Spatially filtered WFS Apodised pupil plane Lyot Coronographs Auxiliary focal plane and calibration WFSing [OSCA: built UCL, deployed: WHT ] Specialist Extreme AO planet finders being built for VLT (SPHERE) and Gemini (GPI) Astronomical AO 31

32 Calibration interferometer (JPL) measures slow aberrations to nanometer accuracy Average IR wavefronts Courtesy: Bruce Macintosh, LLNL