extreme stability: some lessons from gaia - spacetec-cm · alcione mora et al. | extreme stability:...
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Issue/Revision: 1.0
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Status: Issued
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Extreme stability:some lessons from GaiaAlcione MoraESA-ESAC Gaia SOC
RIA-SpaceTec workshop. CSIC Madrid (Spain) 12/02/2016
With contributions from U. Bastian, M. Biermann, F. Chassat4, C. Fabricius,J. Hernandez, R. Kohley, L. Lindegren, E. Serpell, I. Serraller and W. van Reewen
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Alcione Mora et al. | Extreme stability: some lessons learned from Gaia | SpaceTec Madrid| 12/02/2016 | Slide 2
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Outline
Preliminary results. Gaia still to be fully understood!
1.Gaia stability: architecture
2.Miscellaneous effects: focus, contamination,
micrometeoroids and microclanks
3.The Gaia basic angle. Importance, measurement, in-
orbit results, correlations
4.Some lessons learned
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Alcione Mora et al. | Extreme stability: some lessons learned from Gaia | SpaceTec Madrid| 12/02/2016 | Slide 3
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1. Gaia stability: architecture
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Alcione Mora et al. | Extreme stability: some lessons learned from Gaia | SpaceTec Madrid| 12/02/2016 | Slide 4
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1. Gaia stability: architecture
Passive thermal stability: sun shield, thermal tent Constant 45º sun aspect angle constant irradiation No moving parts (gyros, reaction wheels, antenna steering, …) Payload on top of service module (SVM), isolated via bipods Thermal disturbances in SVM (computers, transponders, …)
Airbus DS
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Alcione Mora et al. | Extreme stability: some lessons learned from Gaia | SpaceTec Madrid| 12/02/2016 | Slide 5
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2. Miscellaneous effects: focus, contamination and micrometeoroids
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Alcione Mora et al. | Extreme stability: some lessons learned from Gaia | SpaceTec Madrid| 12/02/2016 | Slide 6
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2. Focus evolution
1. Astrometric focus: Cramér-Rao PSF sharpness (derivatives)
2. The focus evolves during the mission. The rate gets progressively smaller
1. Some hypotheses: glue shrinkage, hysteresis, water contamination
3. Mitigation: adaptive calibration, occasional refocus
FoV1. FoV1
FoV2. FoV2
A. Mora
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Alcione Mora et al. | Extreme stability: some lessons learned from Gaia | SpaceTec Madrid| 12/02/2016 | Slide 7
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2. Throughput loss
Monitoring of response by comparison to Tycho-2 photometry
First decontamination (FOV2)
Second decontamination
C.Fabricius, commissioning
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Alcione Mora et al. | Extreme stability: some lessons learned from Gaia | SpaceTec Madrid| 12/02/2016 | Slide 8
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2. Micrometeorids and microclanks
Micrometeorids: L2 is not perfect vacuum!● Attitude discontinuities expected and calibrated
Micro clanks: 1-2 mas micro clanks per minute 7.5 nm 20 Si atoms● Calibration in place. What if target precision were 10-100x higher?
8F. Van Leeuwen, commissioning
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Alcione Mora et al. | Extreme stability: some lessons learned from Gaia | SpaceTec Madrid| 12/02/2016 | Slide 9
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3. The Gaia basic angle
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Alcione Mora et al. | Extreme stability: some lessons learned from Gaia | SpaceTec Madrid| 12/02/2016 | Slide 10
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3. The Gaia basic angle
Γ
Γ basic angle
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Alcione Mora et al. | Extreme stability: some lessons learned from Gaia | SpaceTec Madrid| 12/02/2016 | Slide 11
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3. Basic angle variations
Gaia aims at global astrometry (reference frame, stellar motions and parallaxes) at µas accuracy
The basic angle needs to be stable (or known) to corresponding precision
Gaia is largely self-calibrating (calibration parameters estimated from observations)
Low frequency variations (f < 1 / 2Prot): eliminated by self-calibration
High frequency random variations
● Averaged during all transits, not so harmful
Systematic variations synchronized with spacecraft spin
● Only partially possible to eliminate by self-calibration
● Residual variations could create systematic errors in astrometric results
● Thus high-frequency variations need to be monitored by metrology
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Alcione Mora et al. | Extreme stability: some lessons learned from Gaia | SpaceTec Madrid| 12/02/2016 | Slide 12
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3. Basic angle measurement: BAM
One artificial fixed star per telescope needed● Collimated laser beams directed to the primary mirrors
● Gaia telescopes generate the image
Relative AL centroid displacement basic angle variation
Single CCD AL centroid location precision● AF: ∆yAL ~ 40 μas single transit bright star limit
● BAM: ∆yAL <0.5 μas in 10 min (differential measurement)– ~20s/frame ∆yAL,1frame < 2.7 μas. ~15x better than bright stars!!
Many photons and sharp LSF needed● Artificial stars are interferometric patterns
Better than LIGO or eLISA at low frequencies (0.02 – 1 mHz)
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Alcione Mora et al. | Extreme stability: some lessons learned from Gaia | SpaceTec Madrid| 12/02/2016 | Slide 13
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3. BAM working principle
BAM measurement: differential interference pattern centroid
Airbus DS
0.5 μas = 2.4 prad = 3.6 pm = 0.66 μfringeOn ground state of the art: 1 milli-fringe
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Alcione Mora et al. | Extreme stability: some lessons learned from Gaia | SpaceTec Madrid| 12/02/2016 | Slide 14
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3. BAM phase and period variations
Fringe phase periodic shift: Sun synchronous, ~1mas (nm stability!) Fringe phase discontinuities: several per day Fringe phase mid-long term evolution Fringe period variability
A. Mora, commissioning
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Alcione Mora et al. | Extreme stability: some lessons learned from Gaia | SpaceTec Madrid| 12/02/2016 | Slide 15
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3. BAM phase periodic component
Periodic signal preliminar Fourier analysis● 6-12 harmonics of rotation period: mas µas● Slow temporal evolution + plenty of data can be characterised
Model input for the AGIS solution
L. Lindegren, LL-105
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Alcione Mora et al. | Extreme stability: some lessons learned from Gaia | SpaceTec Madrid| 12/02/2016 | Slide 16
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A. Mora, commissioning
Sometimes related to on-board events
3. BAM phase discontinuities
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Alcione Mora et al. | Extreme stability: some lessons learned from Gaia | SpaceTec Madrid| 12/02/2016 | Slide 17
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One Day Astrometric Solution (ODAS): daily average (no periodicity) Some discontinuities are real. Long term evolution is not (but irrelevant)
3. BAM phase long term evolution
M. Biermann, FLS-033
ODAS
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Alcione Mora et al. | Extreme stability: some lessons learned from Gaia | SpaceTec Madrid| 12/02/2016 | Slide 18
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3. BAM period variability
E. Serpell, GAIA-ESC-TN-0066
Wavelength depends on laser temperatureand current
Power consumption depends on the sky!
• RVS LR-HR mode• VPU, PDHU power
Commissioning: ±0.005 K stability ~1/250,000
Now: ~1 mK~1/1,000,000 (no RVS LR mode)
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Alcione Mora et al. | Extreme stability: some lessons learned from Gaia | SpaceTec Madrid| 12/02/2016 | Slide 19
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3. BAM period variability
Laser temperature stability ~1 mK fringe period changes ~10-6 House keeping data affected by quantisation and time sampling
A. Mora
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Alcione Mora et al. | Extreme stability: some lessons learned from Gaia | SpaceTec Madrid| 12/02/2016 | Slide 20
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3. BAM fringe period vs position
1. Wavelet analysis: constant fringe period does not exist! NEAT, Theia2. Plane parallel fringe analysis works, but imperfect3. Changes are not homogeneous
A. Mora, commissioning
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Alcione Mora et al. | Extreme stability: some lessons learned from Gaia | SpaceTec Madrid| 12/02/2016 | Slide 21
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A. Mora, AMF-015
3. BAM (non-) Gaussianity
BAM pattern: more complex than pure Gaussian beam inteference● Wavefront errors unavoidable can be modeled?
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Alcione Mora et al. | Extreme stability: some lessons learned from Gaia | SpaceTec Madrid| 12/02/2016 | Slide 22
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3. BAM fit residuals
24 h slow component: related to downlink (transponder + PDHU) Peaks: follow sky density (galactic plane) SVM computers thermoelastic efffect
● Most SVM perturbations modify basic angle. Rule of thumb: 100 μas/K
Reverse scalefor temperature
Airbus DS
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Alcione Mora et al. | Extreme stability: some lessons learned from Gaia | SpaceTec Madrid| 12/02/2016 | Slide 23
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E. Serpell priv. comm.
3. BAM vs spin restart
Spin restart after safe mode: variations appear very soon: few min BAM signal: periodic + transient Expected if thermoelastic
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Alcione Mora et al. | Extreme stability: some lessons learned from Gaia | SpaceTec Madrid| 12/02/2016 | Slide 24
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3. Ultimate stability: Earth shadow
1. The Sun strongest perturbations mas (= nm)
2. Avoided in L2 Earth’s shadow + nuclear power
1. No RTGs in Europe. Environmental concerns
2. Expensive, low power/weight ratio. Research?
A. Mora, commissioning
NASA
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Alcione Mora et al. | Extreme stability: some lessons learned from Gaia | SpaceTec Madrid| 12/02/2016 | Slide 25
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4. Some lessons learned
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Alcione Mora et al. | Extreme stability: some lessons learned from Gaia | SpaceTec Madrid| 12/02/2016 | Slide 26
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4. Some lessons learned
Design for stability, but plan for instability:● State of the art in stability is mas, nm and mK● Future is µas, pm and µK. Models might not be ready● Do not play too much with the spacecraft. Routine is important
Telescope focus is not (that) stable refocus campaigns foreseen Water contamination obiquitous decontamination big perturbations Discontinuities in SVM and PLM micrometeoroids, thermal relaxation Minute changes in SVM important ensure constant power load HK is key high precision, resolution and, temporal frequency High precision metrology is essential and (never too) expenseve
● Calibration is difficult Simple models don't work● BAM data are an essential ingredient for Gaia data release 1
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Alcione Mora et al. | Extreme stability: some lessons learned from Gaia | SpaceTec Madrid| 12/02/2016 | Slide 27
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Thank you for your attention!
Stay tuned for data release 1!
http://www.esa.int/spaceinimages/Images/2015/08/Gaia_s_first_Hertzsprung-Russell_diagram