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
6/11/2012 Building on NEAT concept - M. Gai - INAF-OATo 5
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)
6/11/2012 Building on NEAT concept - M. Gai - INAF-OATo 13
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
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