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SOAR AOS Testing

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  1. SOAR AOS Testing Mike Warner & Steve Heathcote

  2. Calibration Wave Front Sensor • Shack Hartman Sensor • Based on Wavescope from AOA • Micro lens array divides pupil into ~150 sub-apertures • Performance • R = 9 average 3 x 15s measurements • Wave front error (atmosphere limited) • < +/- 40nm for low order (Z3 – Z12) • < +/- 10nm for higher order • Total overhead for measurement and tuning of AOS ~10m • Solid & Reliable in Practice • Caveat Lector • The CWFS uses the “Zygo” ordering and normalization (unit amplitude) of the Zernike terms • This convention is used throughout this report except where explicitly noted

  3. Zernike Convention • Conversion from Zygo ordering & normalization (unit amplitude) to “Noll” ordering & normalization (unit RMS) is given in table below

  4. Link Strain Gauges • An “after-thought” to help in safe installation of mirror • M. Warner greatly improved sensitivity to allow diagnostic use • But: • Only had sensors on 4 load bearing links • Sensor has strong intrinsic temperature sensitivity • Inappropriate location of sensor on turn buckle nut • Useless when adjusting link • Only uses small fraction of sensor dynamic range Strain gauge bonded to Turnbuckle

  5. Link Strain Gauges Strain gauges bonded to flexures • Current enhanced design • Strain gauges mounted on Link end flexures increases sensitivity • Full bridge design reduces temperature sensitivity of sensor • Although temperature drift is still primary limit on long term accuracy • Novel use of 2 crossed bridges to provide measurement of • Link Strain (2 independent measurements) Precision +/- 0.25kg • Bending Moments in 2 axes Precision: +/- 0.001 Kg m • Custom Electronics • COTS “ADAM” modules provide Ethernet interface to AOCS computer • Identical Sensors will be used with Actuated Links for telemetry and as part of safety interlock system Adam Modules

  6. Radial Axial Mitutoyo Electronic Dial Gauges • Measure position of glass relative to cell • Inexpensive COTS hardware, robust and reliable • Radial and Axial gauge (12 total) near each link attached to same piers • 1 μm resolution & precision 3 μm accuracy, negligible Hysteresis • Vendor supplied electronics (MIG-2A) provides RS 232 interface to AOCS computer • Will be used to close position loop in final AOS upgrade

  7. S-Beam Load Cell Mirror Actuators • The 120 Axial Figure actuators also provide valuable feedback • Axial force from each load cell • In combination provide net “weight” and moments on mirror • Cumulative step count tracks repeatability of vertical position of mirror

  8. Tangential Force Radial Gauge Axial Gauge Link1 T1 Link2 R1 R2 T2 A1 A2 T6 Link6 A6 Gravity Down View from behind M1 R6 R3 A3 Link3 T3 A5 T5 A4 R5 R4 Link5 T4 Link4 Sensor Location & Nomenclature • Continuous telemetry from all sensors logged by AOCS every 5 minutes (or as required up to 1 Hz) • Snapshot included in “header” of each wave front sensor measurement

  9. Z4 (microns) Z5 (microns) Zernike (microns) Elevation Effect – WFE Other aberrations ~ constant

  10. M1 M2 M6 M3 M5 M4 Elevation Effect – Dial Gauges • Radial dial gauges measure the distance from the ring to glass • Positive mirror closer to ring • Negative mirror further from ring • If both mirror and ring were rigid • Expect only a vertical displacement of the mirror relative to the ring as the telescope moves in Elevation • Equal & opposite change at top and bottom • R4 = - R1 • In reality the ring distorts under the weight of the mirror • Sag is greater at the top of ring than at the bottom • R4 >> - R1 • Out of round by ~100μm at El=20° Elevation @ Az=180 Amplitude ~100μm

  11. M1 M2 M6 M3 M5 Error in Link Force v Elevation M4 Link Force v Elevation Elevation Effect – Link Forces • In the ideal case • Component of mirror weight supported by links ~ W .Sin (El) • Shared equally between 4 load bearing links • M2 = M3 = - M5 = - M6 ~ 0.25 W Sin (El) • M1 = M4 = 0 • However because of the distortion of the ring the two lower links (M3 & M5) take up a disproportionate share of the load as the telescope moves off zenith • Graph at left shows departure from ideal case as a function of elevation • Amplitude of astigmatism is proportional to Imbalance in Link Forces • 30kg  ~ 1μm of Astigmatism

  12. M3 M2 M6 M5 Elevation Effect – Independent of Azimuth • On-sky tests suggest that the Elevation effect is independent of Azimuth • But comparison is difficult because of thermal drift during necessarily lengthy sequence of measurements • Measurements of Link Force Versus Elevation at different Azimuths confirm this result with much greater precision and certainty

  13. Goodrich FEA Model Assumed Rigid ElevationRing Reality Elevation Ring Deforms Elevation Effect – Actuator Forces Open Loop Actuator Force at Different Elevations

  14. 1.5 μm Z4 Hysteresis (microns) Hysteresis in Astigmatism Hysteresis in Link Forces Elevation Effect – Hysteresis • Hysteresis in WFE • Z4 ~1.5 m Hysteresis (for Elevation changes > 20°, less for smaller changes) • Hysteresis in other aberrations is likely present, but undetectably small • Hysteresis in Link Forces • The proportionality between Force error & Astigmatism also holds for the amplitude of the Hysteresis loop !!!

  15. Elevation Effect – Hysteresis • Amplitude of Hysteresis versus change in elevation • Very hard to do on-sky due to temperature changes during the course of the test so link strain gauges used as a surrogate. • For various ranges of elevation motion (down then up) • Left plot: M2 Link Strain • Right plot: Hysteresis in M2 Link Force • ~ 30 kg for Elevation loops > 20° • < 10 kg for Elevation loops < 10° ~ 6Kg ~ 30Kg

  16. Elevation Effect – Summary • Calibration: ~30kg force error  1μm of Astigmatism • Force Error: ((Link3-Link2)+(Link6-Link5))/2 Astigmatism Distortion Unbalanced Link Forces Hysteresis in Structure Hysteresis in Link Forces Hysteresis in Astigmatism

  17. Azimuth Effect – WFE Other aberrations ~ constant

  18. M1 M2 M6 M3 M5 M4 Azimuth @ El=70 Amplitude ~ 10μm Azimuth Effect – Distortion & Forces • The Azimuth effect was totally unexpected • Thus a key to understanding the problem • Radial dial Gauges show that the shape of the ring varies with Azimuth • R1 + R4 ~ 6 μm Cos 2 (Az - 22.5) • R2 + R5 ~ 6 μm Cos 2 (Az + 45°) • R3 + R6 ~ 6 μm Cos 2 (Az + 90) • Consequently the link forces are also modulated • M2 ~ 20kg Cos (2 Az + 22.5°) • M3 ~ 20kg Sin (2 Az + 22.5°) • M6 ~ 20kg Cos (2 Az + 22.5°) • M5 ~ 20kg Sin (2 Az + 22.5°)

  19. Azimuth Effect – Independent of Elevation El = 88.5° • On-sky tests suggest that the Azimuth effect is independent of elevation • But comparison is difficult because of thermal drift during necessarily lengthy sequence of measurements • Measurements of Link Force & Mirror Position versus Azimuth at various elevations indeed show only a very weak elevation dependence • Amplitude is slightly smaller at lower elevations • Detailed departures from simple Cos(2Az) dependence differ at different elevations • Fitted curves at right have same amplitude and phase at both elevations • A single Cos (2 Az) dependence is a good first approximation at all Elevations El = 30°

  20. Azimuth Effect – Mount Mechanics • Az axis misalignment data from mount commissioning • Tilt ~24 arcsec (cosine fit to data points) • Bearing wobble (residual from cosine fit) • 1 arcsec amplitude, roughly Cos (3 Az) dependence • Blue curves show variation of astigmatism for reference • FEA modeling suggests that the azimuth effect is caused by a slight non-flatness (~0.08 mm P-V) of the Azimuth bearing • Tilt alone will not produce an Azimuth effect

  21. Deformation ~ 11 µm Azimuth Effect – Summary • Mount motion in Azimuth produces a periodic deformation of the Elevation ring & cell and hence link forces • Probably due to tiny non-flatness of bearing • Calibration: ~ 33kg force difference  1μm of Astigmatism Variation in Astigmatism ~1.2 µm (Z4) Variation in Lateral link Force ~ 2 x 20 Kg Force difference

  22. Zernike (microns) Zernike (microns) ~0.5μm Nasmyth Effect – WFE • When the Nasmyth instrument packages were first installed we discovered a strong dependence of Astigmatism on the instrument rotator angle • Z4 ~ 1.2 Cos (N2 - 45), with ~ 0.5μm of Hysteresis • Other aberrations ~ constant • Similar amplitude and phase for both Nasmyth packages • Misalignment of the inner & outer axles was identified as the likely culprit Inner Axle Outer Axle

  23. 2 4 Tuning the Optical Nasmyth • Adjusting height & stiffness of Pillow block & centering outer axle reduces amplitude of Nasmyth effect • Dummy mass – Rigid Cylinder with no attempt to center • Optical ISB prior to adjustment of pillow block • After adjustment of pillow block height • After Centering outer axle on instrument package Used dial gauge to minimize run out Checked with Link strain gauges • No instrument package • Impact of Nasmyth Effect reduced to insignificant levels 1 4 2 3 5

  24. Z4 (microns) Temperature Effect – WFE • On a given night Z4 Depends strongly on Temperature • ~1.5 μm / °C drift • Form of El & Az dependence is unchanged curves just shift vertically • Other aberrations have much weaker Temperature dependence • There are also large night-to-night changes in the value of Z4 • These are only loosely related to instantaneous Temperature • Thermal history over the preceding day(s) is also important • Form of El & Az dependence is stable • Other aberrations are much better behaved Z4 (microns) Z4 (microns) Time Temp Elevation Azimuth –180

  25. Link Strain v Temperature Temperature Effect – Distortions & Forces • Cell dimensions, hence Link Forces show a gross linear dependence on Instantaneous temperature. BUT: • With strong Hysteresis & Curlicues superposed • We are seeing the response to temperature changes of a complex composite structure with several different characteristic time scales • Link Force, hence astigmatism, depends on thermal history in a way which is hard to unravel

  26. Time and Temperature Domain Data • To illustrate this, a period was chosen when the telescope was “parked” (Azimuth, Elevation, N1 & N2 all constant) for ~40 hours (11-03-05-9:05UT to 12-03-05-22:28UT) • This allows study of the time and Temperature dependence of the Link forces and dial gauge values in isolation from other effects • Some useful summary parameters are • Force Error = ((Link3-Link2)+(Link5-Link6))/2 Kg • “Equivalent Astigmatism” = Force Error/30 μm • The ~30Kg/m calibration factor is as found from on-sky El & Az tests • The error (ghost) forces for the 3 axes • Xfe = 3250*sin(zd)-(Link2+Link3-Link5-Link6)*cos(30) Kg • Yfe = Link1-Link4+(Link2-Link3-Link5+Link6)*cos(60) Kg • Zfe = Link1+Link2+Link3+Link4+Link5+Link6 Kg

  27. Time Data Sets Radial Gauges Axial Gauges Mirror Temperature

  28. M1 M2 M6 M3 M5 M4 Time Data Sets Lateral Link Strain Gauges Equivalent Astigmatism Ghost Forces Note : Links that are opposite to each other have similar trajectories

  29. M1 M2 M6 M3 M5 M4 Temperature Dependence Radial Gauges Axial Gauges Note : The Radial gauges R1 and R4 do not follow the same trajectory as R2, R3, R5 and R6, after temperature reversal, this indicates that the cell is being constrained by the yoke which is distorting it’s shape.

  30. Temperature Dependence Lateral Link Strain Gauges Equivalent Astigmatism Ghost Forces

  31. 3-link 6-link Three Link Tests • 3 of 6 links removed so that M1 is supported kinematically • But Elevation range restricted to ZD < 30 degrees • Demonstrates that true kinematic support fixes problems • No Hysterisis in Astigmatism versus Elevation • Much Reduced Temperature Dependance • No Azimuth Effect, No Nasmyth Effect • But also illustrate downside of only 3 Links • Larger aberrations • With stronger dependance on Elevation

  32. Three Link Tests- Elevation Elevation Axial Radial Strain 6-Links 3-Links

  33. Three Link Tests- Azimuth Azimuth Axial Radial Strain 6-Links 3-Links

  34. Three Link Tests - Distortions • Corollary • Mirror Cell & Ring are also less constrained • Much Larger (about double) distortions of cell • Distortions also show correspondingly more Hysteresis Elevation Azimuth 6-Links 3-Links

  35. Three Link Tests – WFE • Stronger Elevation Dependence of Many Aberrations

  36. Figure Control Tests • “+/- N Tests” performed to test figure control using axial actuators • A known amount of each aberration is imposed using the actuators • The CWFS is used to determine the response & check for cross-talk • Astigmatism Z4 (left) & Z5 • Response is ~ linear over ± 3 μm, but slope ~ 1.6 (a puzzle) • No significant cross talk • Most other aberrations show similar behavior • Spherical Z8 Spherical (center) • Response is ~ linear over ± 1 μm, but slope ~ 1.5 • Weak cross talk with Z3 (focus) is probably due to aliasing in the SH sensor • 4th Astigmatism Z11 (right) & Z12 • Response is ~ linear over ± 1 μm • Strong Cross-Talk with corresponding Astigmatism – no big surprise, but a nuisance