6/11/2012 Building on NEAT concept - M. Gai - INAF-OATo 1

Building on NEAT conceptM. Gai – INAF-OATo

(a) Extension of science case

(b) Payload implementation options

[aspects not necessarily coincident]

6/11/2012 Building on NEAT concept - M. Gai - INAF-OATo 2

Proposed science topic: Solar System dynamics

Implementation:

relative astrometry of Solar System objects vs. reference stars

Goals:

• very high precision ephemerides of planets [deep space navigation]

- Fundamental Physics tests – gravitation theories, PPN ,

Context:

Best current ephemerides from IMCCE (INPOP) and JPL

- ranging data on Jupiter expected from JUNO (2017), JUICE (2033)

- ranging data on Saturn from Cassini adequate to current precision

6/11/2012 Building on NEAT concept - M. Gai - INAF-OATo 3

Hints on Solar System planet observation feasibility

NEAT scale: EFL = 40 m s 5"/mm

…thus mapping 50 mas on the sky over a 10 µm pixel

Planet typical size: 10" ÷ 40" Npx 200 ÷ 800 pixel

… larger individual CCD required, ≥ 1 k pixel

Magnitude dilution: 2.5 log Npx2 = 11.5 ÷ 14.5 mag

…planets readable as not-too-bright stars

Requirement: readout of significant areas [binning feasible]

Minor planets: also feasible, smaller size, comparable brightness

6/11/2012 Building on NEAT concept - M. Gai - INAF-OATo 4

Photon limited astrometric performance [approximate]

Planets: diameter / SNR few µas

Stars: diffraction size / SNR few µas

…for intermediate magnitude stars, 1 hour total exposure

Systematic errors:

Planets: illumination / mass distribution uncertainty < 1 mas TBC

Stars: Gaia catalogue error few ten µas

Planet model improvement required planetary science

Expected improvement on ephemerides: one – two orders of magnitude

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Operation requirements

Ephemerides approach: global “Grand Fit” of Solar System

Many measurements over a few years to cover a reasonable fraction of orbits

Side benefit: average systematic errors and astrophysical noise

Targets: Jupiter, Saturn, Uranus, Neptune, …?

Expected impact on observing time: 5% to 10%

6/11/2012 Building on NEAT concept - M. Gai - INAF-OATo 6

Payload option: Multiple Field Superposition

Rationale:

look for reference stars over significant area,

with limited size focal plane

Concept:

Field Of View FOV 2 pivoting around

FOV 1, roughly centred on science target

Field size: dRdR

Area covered: 2πRdR

6/11/2012 Building on NEAT concept - M. Gai - INAF-OATo 7

Field multiplicity: 2 to 4

Field size: ~0º.1

2 fields strip width 0º.1

4 fields strip width 0º.3

Expectation on reference stars:

• Larger number

• Brighter magnitude

6/11/2012 Building on NEAT concept - M. Gai - INAF-OATo 8

Rationale for moderate size focal plane detector

Impact of primary ionising radiation reduced significantly even with simple shielding [few mm Al]

Radiator design adapted to match shielding requirements

Fully static detector design

Target/reference acquired by satellite pointing

Possible implementation:

• 33 mosaic

• 2k2k CCDs

• 10 to 15 m pixels (with focal length tuning)

6/11/2012 Building on NEAT concept - M. Gai - INAF-OATo 9

Multiple Field Superposition Implementation

4-way beam combiner on a common telescope from GAME concept [M. Gai, SPIE 8445-37]

Caution: incomplete beam superposition along the optical path

static beam walk (metrology / calibration)

OK for relative measurement differential astrometry

6/11/2012 Building on NEAT concept - M. Gai - INAF-OATo 10

Reference star availability assessment

Data from GSC II catalogue

Target selected for magnitude V ≤ 10 mag total 1,967 objects

Reference selected for magnitude V ≤ 12 mag total 320,446 objects

Sky region avoiding 3º around

poles and zero declination for

simpler numerical implemen-

tation [1,934 targets;

317,640 reference stars]

6/11/2012 Building on NEAT concept - M. Gai - INAF-OATo 11

Referenced targets: fraction and density

Radius Base Angle

2 fields: radius ≥ 1º for probability >90% of at least one reference star

4 fields: > 90% at radius ≥ 0º.5;

more reference stars

radius = 2º

% targets with ≥ 1 reference star

Histogram: # reference stars / target

6/11/2012 Building on NEAT concept - M. Gai - INAF-OATo 12

Referenced targets: magnitude of reference stars

radius = 2ºfield = 0º.3

radius = 2ºfield = 0º.1

Histogram of the three brightest reference stars for each target in the sample

1 mag brighter reference stars using 4 fields (0º.3 strip width) vs. 2 (0º.1)

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Conclusions

NEAT science case may include Solar System dynamics

Reasonable impact on

•detector requirements (larger field of individual CCD)

•observing time allocation (≤10%)

Technical options

Multiple field superposition:

•improve on density / brightness of reference stars for any target

•simpler, smaller detection system

•benefit for shielding against ionising radiation