Pointing stability of SOT against the microvibration

1
Pointing stability of SOT against the
microvibration

• K.Ichimoto and Solar-B Team
• SOT17 2006.4.17-20

2
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)

OTA
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.
3
Test configurations
Two complimentary configurations were adopted.
S/C hanged up by springs
630nm tunable laser

Transmissibility of microvib. is measured with
accelerometers on M1/M2 of OTA. Less
environmental noise.
4
Microvibration transmissibility measurement with
accelerometers on OTA

• s Accelerometers on M1/M2 to detect shift and
  tilt.
• Sinusoidal force and torque injected at the
  locations of MW and IRUs in MTM, and the
  response of OTA pointing was measured as a
  function of frequency at each location.
• Net pointing error (fgt20Hz) are calculated by
  using the component disturbance data.
• MW and IRU are run in FM test to evaluate the SOT
  pointing error.

5
Pointing error measurement with optical sensors
Optical layout
FPP
FG-CCD
CT-CCD
180oBS
Insertion pipe
Acc. sensors
Image plane
PSD
Data logger
Pointing error is measured by position sensitive
detector (PSD, 3kHz) and FPP/CT camera (580Hz)
(only in FM).
6
Test history
Components disturbance measurement (IRU/MW/PMU)
Transmissibility 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
Evaluation of SOT pointing error and feedback to
the flight hardware
7
Example of microvibration transmissibility
spectrum from IRU-A to M1 (red) and M2 (blue)
tilt. FEM prediction (dots) fails to reproduce
the transmissibility
8
SOT requirement on image stability 0.09 (3s)
0.042 (0-p)
(sinusoidal jitter)
(requirement)
Strehl degradation due to optical error
Strehl degradation due to image jitter
psf with sinusoidal jitter, l 390nm
0.09 (3s)
9
Momentum wheels
Example of data power spectrum density of M2
tilt against MW-A spin rate
10
Momentum wheels
Suitable windows of spin rate of MW were
identified.
X
Hanged up config. Acc. data
Requirement 0.03arcsec (rms)
Y
Operational spin range of MWs is decided to be
2800100rpm so that disturbance of MW will not be
a significant cause of SOT pointing error.
11
IRU-A
Optical measurement in FM
PSD-X
PSD-Y
12
IRU-B
Optical measurement in FM
PSD-X
PSD-Y
13
IRU-A and B
Total pointing error integrated for f gt20Hz.
For IRU-A, the observed disturbance level is much
smaller (lt1/30) 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.
14
FPP wheels
NFI filter wheel
BFI filter wheel
15
XRT wheels
XRT filter wheel-1
XRT filter wheel-2
16
XRT VLS
EIS coarse mirror
Since operation of EIS coarse mirror mechanism is
very rare, we do not care..
17
Record of OTA pointing error induced by mission
mechanisms. (by the PSD sensor from continuous
rotation measurement, unitarcsec rms)
requirement 0.03 (1s) 0.014 (0-p)
18
Pointing disturbance caused by XRT-VLS shutter
(requirement)
Strehl degradation due to image jitter
Strehl degradation due to optical error
Disturbance level of XRT-VLS ? Final Strehl 0.59
Final Strehl will be 0.42 _at_390nm.
19
Point spread function under the sinusoidal jitter
For sinusoidal jitter 009 (3s) is equivalent to
0.042 (0-p)
XRT VLS 0.1 (0-p)
requirement
20

• Summary
• After extensive experiments, we expect that the
• requirement 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 orbit?
• 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?