NST PRIMARY MIRROR CELL:ANALYSIS AND SOLUTIONS

BBSO, 02/11/2004

Leonid Didkovsky

Do we build a Nondifractionlimited Solar

Telescope?

Are YOU the author? (Who is responsible?)

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.

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

BBSO, 02/05/2004

2. Active Primary Mirror

BBSO, 02/05/2004

3. PM Assembly

BBSO, 02/05/2004 Picture from Bill Duch’s report

4. Axial and Radial Actuators

BBSO, 02/05/2004

5. Load Cells

BBSO, 02/05/2004

Deflection 0.014”/100 lb

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);

BBSO, 02/05/2004

7. Analysis of PM Cell

BBSO, 02/05/2004

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.

BBSO, 02/05/2004

An Axial (Radial) Rod deformation model

BBSO, 02/05/2004

1mm/6kg (short axial)1mm/0.6 kg (tangent)

Forces map on PM surface for 20C cooling

1 mm tolerance for axial forces will add additional 3-6 kg to shown.

Deflection of the platform (0.4 mm under 560 Kg PM weight).

Deflection of the platform (critical points)

Thermal SE for 20C cooling

About 3900 nm PV

7.2. Resonant Frequencies

BBSO, 02/05/2004

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.

7.2.1. Mirror & Cell 1st tone

BBSO, 02/05/2004

7.2.2. PM & Cell Tip/Tilt 43 Hz

BBSO, 02/05/ 2004

7.2.3. PM & Cell Shifts 56 Hz

BBSO, 02/05/2004

7.2.4. Mirror & Cell 2nd Axial 92Hz

BBSO, 02/05/2004

8. Proposed PM Cell Improvements

Static / Thermal Issues Special Ball Joints with small backlash; Resonant Frequencies Three Hardpoints; More rigid Platform; Dimensions (thickness)

BBSO, 02/05/2004

8.1. Ball JointsSpherical plain bearing designs (in USA 1-888-753-2000)

http://www.skf.com/portal/skf/home/products/

BBSO, 02/05/2004

An axial actuator

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).

BBSO, 02/05/2004

“Harder” Load Cells: Mode 1,2 (shifts) 56 Hz, 3,4(tip/tilt) 63 Hz, 5 (axial) 69 Hz

BBSO, 02/05/2004

Real Hard Points:Modes: 1,2 (shifts) 56 Hz, 3 (torsional) 113 Hz, 4,5

(tip/tilt) 180 Hz, 6 (first axial) 283 Hz

BBSO, 02/05/2004

8.3. More Rigid Platform

BBSO, 02/05/2004

Rigid platform eigen mode is 192 Hz compare to 66 Hz in the Old one

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.

BBSO, 02/05/2004

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.

BBSO, 02/05/2004

Proposed platform

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.

BBSO, 02/05/2004

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).

BBSO, 02/05/2004