nst primary mirror cell: analysis and solutions bbso, 02/11/2004 leonid didkovsky
TRANSCRIPT
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NST PRIMARY MIRROR CELL:ANALYSIS AND SOLUTIONS
BBSO, 02/11/2004
Leonid Didkovsky
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Do we build a Nondifractionlimited Solar
Telescope?
Are YOU the author? (Who is responsible?)
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Should we be concerned?• Why the PM surface RMS = 21 nm (/15 WF RMS for 632.8 nm)
was requested? (See Error budget in LD, JK and PG SPIE 5171-47)
• Polishing may be stopped at 41 nm RMS. And What?
• Figure on polishing support is worse than 33.5 nm RMS; Astigmatism for model. Higher order in life. Add technology errors.
• May it be corrected by PM active system? No.
- “Independent” sub-apertures; My estimation is 12/6 Zernike modes;
- Moments on PM and the problem with the beam;
• By an AO? No in the full 3’ FOV;
• What to do: to minimize the polishing errors by re-measuring the mirror and recalculating actuators/forces geometry.
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1. PM Cell Subsystems / Components
Active Primary Mirror PM Cell Assembly Axial and Radial Actuators Load Cells Thermal sensors Entrance Aperture PM Cell Cover Air Knives and Cooling System
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2. Active Primary Mirror
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3. PM Assembly
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4. Axial and Radial Actuators
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5. Load Cells
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Deflection 0.014”/100 lb
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6. Major Requirements PM Surface Quality on short time scale; Real-Time control system: WFS and Loads; Mechanical stability (high resonance freq); PM Surface Quality on long time scale; Thermal/Position stability for PM Cell; Mirror seeing (goal:< 1 deg difference); Dimensions (thickness < 620 mm); Weight = 1275 kg (including PM);
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7. Analysis of PM Cell
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7.1. Some static/thermal issues
Connection axial/radial rods to pucks:± 1 mm tolerance requires about 3 to 6 kg transversal force for radial rod or about 0.6 kg for tangent;
A thermal introduced SE for PM, actuators and Cell is about 3900 nm for 20C cooling.
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An Axial (Radial) Rod deformation model
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1mm/6kg (short axial)1mm/0.6 kg (tangent)
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Forces map on PM surface for 20C cooling
1 mm tolerance for axial forces will add additional 3-6 kg to shown.
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Deflection of the platform (0.4 mm under 560 Kg PM weight).
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Deflection of the platform (critical points)
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Thermal SE for 20C cooling
About 3900 nm PV
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7.2. Resonant Frequencies
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For 36 axial supports and ideal (without any deflection) PM Cell:
First tone is 44.8 Hz; PM cell has its first natural tone about 66 Hz; Combined (Real Cell and PM):
First tone is less than 35 Hz, what is lower than the whole telescope requirement: 40 Hz.
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7.2.1. Mirror & Cell 1st tone
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7.2.2. PM & Cell Tip/Tilt 43 Hz
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7.2.3. PM & Cell Shifts 56 Hz
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7.2.4. Mirror & Cell 2nd Axial 92Hz
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8. Proposed PM Cell Improvements
Static / Thermal Issues Special Ball Joints with small backlash; Resonant Frequencies Three Hardpoints; More rigid Platform; Dimensions (thickness)
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8.1. Ball JointsSpherical plain bearing designs (in USA 1-888-753-2000)
http://www.skf.com/portal/skf/home/products/
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An axial actuator
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8.2. Three Hard points
Three types of hard points were analyzed: Three standard axial actuators with 100 lb
Load Cell (proposed in “old” design); Using “harder” Load Cells (1,500 lb
instead of 100 lb) at three locations of axial supports;
Using three real hard points (no motion in axial direction).
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“Harder” Load Cells: Mode 1,2 (shifts) 56 Hz, 3,4(tip/tilt) 63 Hz, 5 (axial) 69 Hz
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Real Hard Points:Modes: 1,2 (shifts) 56 Hz, 3 (torsional) 113 Hz, 4,5
(tip/tilt) 180 Hz, 6 (first axial) 283 Hz
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8.3. More Rigid Platform
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Rigid platform eigen mode is 192 Hz compare to 66 Hz in the Old one
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8.4. PM Resonances vs hard points
PM on standard 36 axial supports shows its first axial tone (mode 1) at 44.8 Hz;
PM on three “harder” points shows its first axial tone (mode 5) at 69 Hz;
PM on three hard points shows its first axial tone (mode 6) at 283 Hz.
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8.5. PM & Cell Resonances • PM & Old Cell with 36 Load Cells (100 lb)
showed 1st axial tone is less than 35 Hz;• PM & Old Cell with Load Cells (1500 lb)
has axial resonant on 40 Hz;• PM & Old Cell with three real hard points
has its first axial mode on 47 Hz;• PM & Modified Cell with three hard points
showed first axial tone on 135 Hz.
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Proposed platform
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CONCLUSIONS• Static and dynamical analysis of the
Proposed preliminary PM Cell Design showed that some of its assemblies/parts may be improved.
• Improved design with three hard points and ribbed platform has axial resonance tone about 135 Hz compare to 35 Hz with “Old” platform and no hard points.
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ACKNOWLEDGEMENTS• This work has been supported in part by NASA
grant NAG5-12782, NSF grant NSF-ATM-0086999 and AFOSR grant F49620-03-1-0271.
• Designing of the PM cell was done by Doug Neill and Bill Duch III.
• Analysis, modeling and suggestions for PM Cell improvements were prepared by Mansur Iliyasov and Alexey Zverev (Lavochkin Science-Production Associations, Moscow, Russia).
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