sait 2008m. gai - inaf-oato1 game: gamma astrometric measurement experiment space-time curvature...

SAIt 2008 M. Gai - INAF-OATo 1

GAME: Gamma Astrometric Measurement Experiment

Space-time curvature parameter

Light deflection close to the Sun

Apparent star position variation

Small mission class

M. Gai

INAF-Osservatorio Astronomico di Torino


• Scientific rationale

• The GAME concept

• Implementation highlights

• Status and next steps

Outline of talk:


Goals: Fundamental physics and cosmology

Scientific rationale of GAMEScientific rationale of GAME

Review of basic framework:

• The classical tests on General Relativity

• Spacetime curvature in General Relativity in Parametrised Post-Newtonian (PPN) formulation

• Current experimental findings on

•Potential cosmological implications of

• Science bonus


A space mission in the visible range to achieve

•long permanent artificial eclipses

•no atmospheric disturbances, low noise

Method: repeated measurement of star angular separation

The GAME concept The GAME concept


• Observation sequence

• Fully differential measurement

• Precision on image location

• Systematic error control: beam combiner

• Fizeau-like interferometer solution

• PSF of the phased array

• Elementary astrometric performance

• Photon limited mission performance

Key GAME concepts


Implementation highlights Implementation highlights

• Payload layout

• Telescope configuration

• Detector

• Baffling

• Metrology

• Mission profile


Status and next stepsStatus and next steps

Proposal submitted to FP7 IDEAS call (Feb. 2008)

Next presentations: SPIE, COSPAR

Preparation of proposals for next calls: ASI, MIUR, …

Realistic simulations of observations and baffling

Lab tests on telescope + BC feasibility / criticalities

Lab test on metrology and baffling performance

System level tests on sky of representative GAME mock-up

Dissemination among scientific community to build support to the GAME concept


The current GAME teamINAF-OATo: M. Gai, M.G. Lattanzi,S. Ligori,

L. Corcione, M.T. Crosta, A. Vecchiato, G. Massone, D. Loreggia, D. Gardiol, A. Sozzetti, D. Bonino, D. Busonero

INRIM: M. Bisi, F. Bertinetto, F. Alasia, P. Cordiale

Industry: Thales Alenia Space; Altec

Collaborations: A. Tartaglia (PoliTo); S. Capozziello (Univ.

NA)


GAME

OVER

[PRESENTATION]


1. Perihelion precession of planetary orbit Mercury

2. Light deflection by massive objects Sun

Eddington’s parameter

3. Gravitational redshift / blueshift of light

“Modern” tests:

Gravitational lensing; Equivalence principle;

Time delay of electromagnetic waves (Shapiro effect);

Frame dragging tests (Lense-Thirring effect);

Gravitational waves; Cosmological tests (cosmic background)

Classical tests of general relativity [Einstein, 1916]


0 1 2 3 4 5 60

0.5

1

1.5

Distance from Sun centre [degs]

De

flect

ion

an

gle

[arc

sec]

1cos1

cos122

Dc

GM

Light deflection around massive objects

1".74 at Solar limb 8.4 rad

Apparent variation of star position depending on light bending due to the gravitational field of the Sun spacetime curvature


Repeated through XX century

Precision achieved: ~10%

Limiting factors:

•Need for natural eclipses

•Atmospheric turbulence

Value Year Authors

1.66 ± 0.19 1973TMET

1.98 ± 0.46 1961Schmeidler

2.17 ± 0.34 1959Schmeidler

1.70 ± 0.10 1952van Biesbroeck

2.01 ± 0.27 1947van Biesbroeck

2.73 ± 0.31 1936Mikhailov

2.24 ± 0.10 1929Freundlich & al.

1.77 ± 0.40 1922Dodwell & al.

1.98 ± 0.16 1920Dyson & al.

Dyson - Eddington - Davidson experiment (1919):

First test of General Relativity by light deflection nearby the Sun

Short exposures, high background

Large astrometric noise

[A. Vecchiato et al., MGM 11 2006]


Cassini

Radio link delay timing, /~1e-14

(similarly for Viking, VLBI: Shapiro delay effect, i.e. “temporal” component)

Alternative experiment results

Hipparcos

Different observing conditions: global astrometry, estimate of full-sky level deflection on survey sample

Precision achieved: 1e-4 ~ 2e-5

Earth

Sun

Cassini

[B. Bertotti et al. Nature 2003]


Cosmology: Dark Matter, Dark Energy


[Capozziello & Francaviglia Gen. Relativ Gravit 40, 2008 and references therein]


f(R) gravitation fits experimental data


f(R) gravitation within Solar System

Local measurements cosmological constraints


Additional science topics

Light deflection effects due to oblate and moving planets • Monopole and quadrupole terms of asymmetric mass

distribution

Close encounters between Jupiter and selected quasars and stars • Speed of gravity; link between dynamical reference

system and ICRF

Upper limits on masses of massive planets and brown dwarfs• Nearby (d < 30-50 pc), bright (4 < V < 9) stars, orbital

radii 3-7 AU

Time resolved photometry on transiting exo-planet systems • Constraints on additional companions: mass, period,

eccentricity

Observation in / through inner part of Solar System • NEO orbits and asteroid dynamics (a few close

encounters) • Circumsolar environment transient phenomena • Mercury’s orbit perihelion precession determination


Observation sequence

Measurements epochs: E1, E2 to modulate deflection on fields F1, F2 (Sun action on/off)

Deflection measurement

Calibration fields, 0


2;12,12;22;11;21;1 EEFFEFEFEFEF

2;14,32;42;31;41;3 EEFFEFEFEFEF

Star separation variation: deflection + instrument [base angle]

Calibration fields affected mainly by instrument variations

Fully differential measurement

Basic equations referred to stars in Fields 1, 2, 3, 4; Epochs 1, 2

Base Angle: hardware separation between observing directions


Precision on image location

SNRX

1

4

NSNR

: Standard deviation of image location

: Effective wavelength of observation

Signal to Noise Ratio photons, RON, background

N: Number of photons collected

: Instrumental factor of degradation (geometry, sampling, …)

X: Root Mean Square size of the aperture

can be a small fraction of pixel/image size

DiffractionStatistics

[MG et al., PASP 110, 1998]


Image displacement BC mirror tilt only: 1 as x 1 m = 5 pm

Base angle materialised in one component / degree of freedom

Systematic error control: beam combiner

[MG et al., Meas. Sci. Tech 10, 1999]

HIPPARCOS-like split mirror


Basic technical requirements

Sufficient angular resolution and photometric SNR

increase telescope size

[collect more photons and concentrate them more effectively]

Sufficient rejection of high Solar (background + diffracted) flux

reduce telescope size; increase baffling

[shield line of sight from sources at narrow angle]

Conflicting requirements!


A solution: Fizeau-like interferometer

Pupil masking: set of elementary apertures cut on telescope pupil

F1

F2

Top / bottom: pointed to different lines of sight, separated by Base Angle

BA

7-aperture Phased Array

Elementary aperture: 412 cm

Aperture spacing: 4 cm

External diameter: 0.57 m

Internal diameter: 0.30 m

Geometry to be optimised vs. astrometry / background


PSF of the phased array

Along ecliptic (LowRes) [mas]

Acr

oss

ecl

iptic

(H

iRe

s) [m

as]

sqrt (PSF)

-2000 -1000 0 1000 2000

-2000

-1500

-1000

-500

0

500

1000

1500

20000.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

-2000 -1000 0 1000 20000

0.2

0.4

0.6

0.8

1

x [mas]

Inte

nsi

ty

Monochromatic, = 650 nm

Image size: ~4 arcsec

Central peak: ~200 mas FWHM

[MG & Cancelliere, MNRAS 377, 2007]


Bright case:

100 s exposures, close to Sun

Background: 15.5 mag per square arcsec

Faint case: 500 s exposures, away from Sun

Background: 19.5 mag per square arcsec

Elementary astrometric performance

Band: _0 = 650 nm, = 120 nm


Photon limited mission performance

Precision on one 15 mag star: < 1 mas

1 million stars required to achieve 1 as cumulative

1e-6 equivalent precision on

Observing time required: 20 + 20 days

[on average Galactic plane stellar density from GSCII]

Observation in Galactic center direction: higher density

1e-7 equivalent precision on (3 + 3 month)

[Vecchiato et al., IAUS 248, 2007]


PM: Pupil Mask

FM1: Folding Mirror 1

FM2: Folding mirror 2 (Beam Combiner)

TM1: Telescope Primary

TM2: Telescope Secondary

CCD: Detector Camera

Volume of payload: ~1.51.51 m

Mass budget: 120 kg + baffling structure

Payload layout


Telescope configuration

Ritchey-Chrétien, 5 mirrors, EFL = 20.6 m, visibility > 95% over 77 arcmin field; distortion < 1e-4

[Loreggia et al., IAUS 248, 2007]


[MG et al., A&A 367, 2001]

E2V CCD42-80

Back illuminated, high performance CCD sensor

Format: 2k4k, 13.5 m pixels

Two devices required for 77 arcmin field, sampling ~5 pixels/peak

A possible CCD detector


Baffling

Pupil masking on subsequent folding mirrors

Observed field (solid) transmitted by facing surfaces, forbidden directions (dashed) shielded by pupil masks AND side baffles


Base Angle Metrology

Metrology: considered to complement on-sky calibration


Stowed in launcher fairing

Operating configuration – Side view

Operating configuration – Top viewTelemetry

requirements OK with Kiruna and Fucino ground stations

Mission profilePolar orbit, 1500 km elevation no eclipse

Spacecraft: ~Simbol-X

Lifetime: 2(+2) years


GSCII star counts along ecliptic plane

Astrometric performance depends on actual number, brightness and spectral type of observed targets


0 1 2 3 40

0.5

1

1.5


De

flect

ion

an

gle

[arc

sec]

0 50 100 150

10-2

10-1


De

flect

ion

an

gle

[arc

sec]

and GAIA (1-10 mas)

Measurement range

of GAME (0.1 – 1 arcsec)

Observations are set in region of high amplitude deflection


GAME concept validation on-sky

Observations close to full Moon: realistic performance on diffused background; set-up of pointing strategy


Baffling concept validation in lab

Validation of rejection performance predictions


E2V CCD42-80 Quantum Efficiency

Spectral band optimised on detector response AND sunlight rejection performance

sait 2008m. gai - inaf-oato1 game: gamma astrometric measurement experiment space-time curvature...

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