Control of the Keck and CELT Telescopes. Douglas G. MacMartin Control & Dynamical Systems California Institute of Technology

Transcription

1 Control of the Keck and CELT Telescopes Douglas G. MacMartin Control & Dynamical Systems California Institute of Technology

2 Telescope Control Problems Light from star Primary mirror active control system (ACS) Correct mirror figure due to gravity, wind, Adaptive optics (AO) Compensate for atmospheric turbulence Adaptive optics Deformable mirror Primary mirror Instrument Wavefront sensor

3 Keck Primary Mirror: -36 segments actuators 0.9 m

4 Hale (Palomar) 5m (1948) Keck 10m (1992) CELT 30m (20xx)

5 Science object wavefront Distortion 1 (turbulence) Natural, Laser guide stars Atmosphere Segment Actuators System Block Diagram Gravity 2 Thermal Seismic Wind 3 Primary Mirror K PM 5 Other 4 Edge Sensors Gravity Wind Secondary Mirror K SM 6 AO Def. Mirror(s) Notes: 1 Distortion relevant at multiple layers 2 Gravity impacts segments directly, and through distortion of journals etc.? 3 Wind impacts segments directly, and through base motion of telescope on (non-rigid) ground 4 Other sources include machinery vibration; pumps, bearings, drives, FSM DM Actuators K TT 8 K AO 7 5 Wavefront sensor probably not required 6 May need to control secondary mirror; rigid body motion (hexapod or at supports) or flexibility (tertiary?) 7 Potential multiple AO mirrors, multiple wavefront sensors (Secondary as DM for AO?) 8 Tip/tilt control may require feedback from instruments ( final wavefront) Scientific Instruments Wavefront Sensors

6 Primary Mirror Control Problem Out-of-plane motion of segments is tightly controlled Performance goal (CELT): Primary mirror residual wavefront error < 45 nm rms * from ALL sources Segment out-of-plane alignment (controlled) wavefront error < 18 nm rms Sensor noise Actuator noise (precision) Uncompensated disturbances Disturbances: Gravity: slow, mostly predictable, 1.2 mm max (70 nm/s) Wind: approximate disturbance spectrum guessed at. 3 dof actuation on each segment Relative displacement sensors between each inter-segment edge * Specification with Adaptive Optics; only errors uncorrectable by AO (primarily segment edge discontinuities) contribute

7 Control Hardware CELT Actuator locations (three per segment) Sensor locations (two per intersegment edge) Keck Keck CELT Segments Actuators Sensors

8 Keck Actuator Roller screw, 24:1 hydraulic reduction Roller screw is reliable, but Keck has ~1 actuator failure per year due to hydraulic Some backlash Local feedback loop used to obtain desired output CELT Actuator Voice-coil with trim motor offload Local feedback loop used to obtain desired output and stiffness

9 Precision Sensor Design CELT design: Differential capacitance measurement Components mounted directly on segment edges Gain is sensitive to gap Keck design: Measure differential capacitance Sensor noise ~ 1 nm / Hz (Expensive) precision interlocking components b

10 Dynamics State vector: Displacement at actuator locations x k+1 = x k + g k + w k + u k NO dynamics! ( A = I ) Ignored for Keck, Need to include for CELT Disturbance: Predictable component, Random component Control input: treat as displacement actuator (assuming local loop) x k+1 = Ax k + Bu k y = Cx k + Du k Sensor y = C x + δ + η Sensor noise System influence matrix (from geometry) Desired sensor readings (from alignment camera)

11 Control: (i) estimate x from y, (ii) integral control (iii) feedforward of predicted disturbance component Control Problem

12 Gemini Mirror is 20 cm thick, deforms under gravity 120 actuators used to maintain optical quality

13 Adaptive Optics: Correct for Atmospheric Distortion Source is turbulence at multiple levels in atmosphere (Kolmogorov model) Detect wavefront (Hartmann- Shack sensor), m m array of subapertures Deformable mirror used to invert May need multiple mirrors, each to invert one layer of atmosphere, to increase isoplanatic angle Can use science object, natural guide star, or laser guide star May need multiple LGS to increase isoplanatic angle

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15 Keck Adaptive Optics on/off

16 Keck AO: Deformable Mirror Insert picture of deformable mirror here.

17 Shack-Hartmann Wavefront Sensing

18 Current Palomar AO Geometry 241 actuators ( o ) 256 element Shack-Hartmann wavefront sensor ( + ) (384 sensor measurements not obscured by secondary) Structural dynamics are outside of control bandwidth

19 Dynamics State vector: Displacement at actuator locations x k+1 = x k + w k + u k NO dynamics! ( A = I ) Disturbance: Random component Control input: treat as displacement actuator x k+1 = Ax k + Bu k y = Cx k + Du k Sensor y = C x + η Sensor noise System influence matrix (from geometry)

20 Control: (i) estimate x from y, (ii) Proportional-integral (PI) control Control Problem

21 Palomar AO FSM Transfer Function 0 Magnitude (db) Phase (deg) Plant: Nonlinear gain, differs for different channels Roughly constant Phase lag due to time delay High frequency modes Frequency (Hz)

22 Loop Transfer Function: PI controller Magnitude (db) Phase (deg) Loop transfer function, KI = 0.66 KP = 0.15 fsm A (max) fsm A (min) fsm B (min) Frequency (Hz) Controller: u = (K I /s + K P )x Integral to boost low frequency gain Proportional term to improve phase at crossover. Bandwidth ~ 10 Hz Gain margin ~ 10 db, phase margin ~ 60

23 Open-loop Closed-loop Performance Magnitude (db) Frequency (Hz) Computed sensitivity S=1/(1+L) Sensitivity matches prediction! Freq uency (Hz)

24 CELT AO Requirements ( diffraction-limited (180 nm rms) at 1 µm) Deformable Mirrors 3-4 mirrors, conjugate to different layers in the atmosphere Each mirror with ~7000 actuators Wavefront sensors Roughly double the number of actuator degrees of freedom Laser guide stars To provide information anywhere in the sky, create your own stars Excite sodium layer at 90km Need 7-11 lasers for reasonable field of view Control loop 20,000 actuators and 40,000 sensors Need to close loop at ~ 1kHz Wait 25 years for Moore s law to catch up, or Be more intelligent about local control

25 7368 Actuators

26 CELT views of Europa Visible Galileo image convolved to 0.5 arcsec resolution (AO off) (seeing-limited) Visible Galileo image convolved to arcsec resolution (AO on) (diffraction-limited at 1µm)

27 CELT: Galaxy Evolution Courtesy of M. Bolte

28 Mauna Kea Chajnantor