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Pointing stability of SOT against the microvibration

K.Ichimoto and Solar-B Team

SOT#17 2006.4.17-20

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Disturbance sources in the spacecraft

- Momentum wheel x 4- IRU-A (4 gyros) & B (2 gyros)- Moving mechanisms (many!)

in mission instruments (SOT/XRT/EIS)

Experimental evaluations of the OTA pointing error due to the micro-disturbances have been performed using the S/C-Mechanical Test Model (MTM) and the Flight Model (FM).

Vibration (shift/tilt) of M1 and M2 of OTA is the dominant cause for the pointing error.

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FPP

IRU-B

MWIRU-A

OTA

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Two complimentary configurations were adopted.

S/C hanged up by springs

PSD

theodolite

dolly

S/C installed on a dolly in the tower

Optical end-to-end measurement of SOT pointing error using PSD and FPP/CT.More environmental noise.

Transmissibility of microvib. is measured with accelerometers on M1/M2 of OTA.Less environmental noise.

630nm tunable laser

Test configurations

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Microvibration transmissibility measurement with accelerometers on OTA

Accelerometers on M1/M2 to detect shift and tilt. Sinusoidal force and torque injected at the locatio

ns of MW and IRU’s in MTM, and the response of OTA pointing was measured as a function of frequency at each location.

Net pointing error (f>20Hz) are calculated by using the component disturbance data.

MW and IRU are run in FM test to evaluate the SOT pointing error.

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FG-CCD

FPP

CT-CCD

Image planePSD

Data logger

Acc. sensors

Insertion pipe

180oBS

Optical layout

Pointing error measurement with optical sensors

Pointing error is measured by position sensitive detector (PSD, 3kHz) and FPP/CT camera (580Hz) (only in FM).

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Test historyTransmissibility test w/ MTM (2002) (dummy mass for IRU etc.+ shaker) (accelerometers on OTA)

Transmissibility test w/ MTM (2003)

Transmissibility test w/ OTA in vacuum chamber (2003) (accelerometers on OTA) confirm that damping by air is not significant

Pointing error measurement in 1st integration of FM (2004) (flight components & flight optics) (accelerometers on OTA) confirm the consistency of two test configurations

Pointing error measurements in 2nd integration of FM (2005-2006) (no accelerometers on OTA) monitor trend before/after Vib. test, after TV test

Components disturbance measurement (IRU/MW/PMU)

Evaluation of SOT pointing error and feedback to the flight hardware

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Example of microvibration transmissibility spectrum from IRU-A to M1 (red) and M2 (blue) tilt. FEM prediction (dots) fails to reproduce the transmissibility…

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Strehl degradation due to image jitter

Strehl degradation due to optical error

(requirement)

SOT requirement on image stability = 0.09” (3 = 0.042” (0-p)

psf with sinusoidal jitter, = 390nm

(sinusoidal jitter)

0.09” (3

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Momentum wheelsExample of data: power spectrum density of M2 tilt against MW-A spin rate

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Momentum wheelsSuitable windows of spin rate of MW were identified.

Hanged up config.Acc. data

Requirement = 0.03arcsec (rms)

X

Y

Operational spin range of MWs is decided to be 2800+100rpm so that disturbance of MW will not be a significant cause of SOT pointing error.

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IRU-A

PSD-X PSD-YOptical measurement in FM

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IRU-B

PSD-X PSD-YOptical measurement in FM

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IRU-A and B

X (arcsec rms) Y (arcsec rms)IRU-A 0.003 0.005IRU-B 0.005 0.006

For IRU-A, the observed disturbance level is much smaller (<1/30) than the prediction based on past experiments. The reason of this discrepancy is understood as the anti-resonance between IRU-A internal structure and the panel of bus module (but not conclusive). Trend of the disturbance level is being monitored during the final test period.

Total pointing error integrated for f >20Hz.

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FPP wheelsNFI filter wheel BFI filter wheel

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XRT wheelsXRT filter wheel-1 XRT filter wheel-2

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XRT VLS EIS coarse mirror

Since operation of EIS coarse mirror mechanism is very rare, we do not care..

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EIC/MIC2004.11

Before vib.2005.9.28

Post-vib.2005.10.26

Post-TV.2006.6

FPP-NFI-FW (110-120Hz)

PSD-X 0.0021 0.0019 0.0034

PSD-Y 0.0132 0.0076 0.0090

FPP-BFI-FW (110-120Hz)

PSD-X 0.0009 0.0019 0.0028

PSD-Y 0.0127 0.0076 0.0091

XRT-VLS (no cont.rotation)

PSD-X 0.18 (0-p) 0.10 (0-p) 0.09 (0-p)

PSD-Y 0.20 (0-p) 0.09 (0-p) 0.09 (0-p)

XRT-FW1 (63-72Hz)

PSD-X 0.0052 0.0015 0.0075

PSD-Y 0.0220 0.0042 0.0096

XRT-FW2 (63-72Hz)

PSD-X 0.0106 0.0054 0.0032

PSD-Y 0.0217 0.0194 0.0103

EIS-SHT (no cont.rotation)

PSD-X Neg. Neg. Neg.

PSD-Y Neg. Neg. Neg.

EIS-F-Mirr (no cont.rotation)

PSD-X Neg. Neg. Neg.

PSD-Y Neg. Neg. Neg.

IRU-A (110-120Hz)

PSD-X 0.0015 0.0012 0.0011

PSD-Y 0.0011 0.0015 0.0014

IRU-B1/2 (150-160Hz)

PSD-X 0.0066 0.0064 0.0081

PSD-Y 0.0070 0.0059 0.0076

Record of OTA pointing error induced by mission mechanisms.　　 (by the PSD sensor from continuous rotation measurement, unit=arcsec rms)

requirement = 0.03” (1 = 0.014” (0-p)

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Pointing disturbance caused by XRT-VLS shutter

Disturbance level of XRT-VLS Final Strehl ~ 0.59

Strehl degradation due to image jitter

Strehl degradation due to optical error

(requirement)

Final Strehl will be ~ 0.42 @390nm.

60 < f < 200Hz

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Point spread function under the sinusoidal jitter

requirementXRT VLS ~ 0.1 (0-p)”

For sinusoidal jitter 009” (3) is equivalent to 0.042” (0-p)

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Summary:After extensive experiments, we expect that therequirement of SOT pointing stability is to be satisfied.

Issues to be addressed after launch:- What is the real disturbance level of the XRT-VLS in orb

it?

- How frequently we need the visible image of XRT for data co-alignment?

- Shall we stop the IRU-B (which is redundant) during the nominal operation with IRU-A?