Scientific Preparations for GAIA:

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Scientific Preparations for GAIA
Astrometric observations and accuracy
by Lennart Lindegren, Lund Observatory

• Goals
• Means
• Some open questions
• Accuracy assessment

What needs to be done?
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Astrometry goals

• detect and observe all objects with G ? 20 per
  beam (0.02 arcsec2)
• measure the basic astrometric parameters for
  all stars
• - mean sky parallax error
• provide an accurate and globally consistent
  reference frame
• measure additional parameters for double and
  multiple stars
• measure instantaneous positions for other
  objects (asteroids etc)
• provide imaging capability for complex or
  diffuse objects
• allow re-analysis of observations for any
  object
• fully integrated with photometric and
  radial-velocity data analysis

? 4 ?as at G 20
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Astrometry means

• continuously scanning satellite
• two astrometric telescopes with a basic angle
  of 106? between optical axes
• thermal and optomechanical stability to few ?as
  through design
• large aperture (1.7 0.7 m2) gives 400 e/s at
  G 20
• large field of view (0.8? ? 0.7?) for
  integration time and scan overlap
• low optical distortion and aberrations
  (three-mirror telescope)
• 0.08 arcsec diffraction image fully sampled (F
  50 m, pixel 9 ?m)
• 1 s integration at G 15 gives precision 200
  ?as (0.05 ?m 0.005 pixel)
• high throughput (on average 20,000 objects are
  observed in parallel)
• special devices basic-angle monitor WFE
  sensor for mirror alignment

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Payload configuration
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Sky scanning principle
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Astro telescope (1 of 2 identical)
focal length 50 m (1 ?as 0.24 nm)
primary mirror 1.7 ? 0.7 m2
adjustable secondary mirror (5 d.o.f)
total wavefront error (budget) 36 nm RMS
flat focal plane 0.7 ? 0.6 m2 0.8 ? 0.7 deg2
next slide
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Focal plane layout (1 of 2 identical telescopes)
detect, confirm, determine rate
astrometric measurements (no filter)
filter photometry
scan of detected object
2.2 ? 1.3 arcsec at lower resolution
0.2 ? 0.9 arcsec at full resolution
star motion 120 arcsec/s
250 CCDs 2780 ? 2150 pix 9 ?m ? 27 ?m
0.49 m 0.56 deg 17 s
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Line-spread function, sampled patches and noise
(illustration)
Over the mission, some 1600 to 3100
patches (elementary observations) are collected
for every object
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Simulation of patches collected over 2 years
4 ?
30 ? 30 arcsec
30 ? 30 arcsec
2 years scanning in a high-density area (Baades
Window, l 1.3, b 3.8 ).
Typical low-density area (b ? 60 ). Mean number
of detected objects 0.2
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• Patch analysis for a simple point source
• astrometry (centroiding, location estimation)
• photometry (flux measurement, amplitude
  estimation)

Samples are converted to counts with
known noise properties (Poisson) Model
location (centroid)
estimating the location and amplitude (known
LSF)
from other samples
calibration
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Estimating the location and amplitude
(centroiding)
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• Attitude
• defines the celestial (ICRS) orientation of the
  telescope reference frame
• proposed representation quaternion
  in cubic splines (GAIA-LL-35)
• some basic algorithms have been worked out and
  tested (GAIA-LL-34.2)

• Calibrations
• geometric calibrations
• define the reference point of each CCD column
  in telescope frame
• (also as function of spectral energy
  distribution chromaticity)
• photometric calibrations
• define the transformation from G to amplitude
  for each CCD column
• PSF calibrations
• define the distribution of counts relative the
  adopted centroid
• (as function of colour index, position in
  field, and time)

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Chromaticity Example of centroid positions for
monochromatic LSF
Obtained by fitting a gaussian with standard
width s
Field point 1 WFE 39 nm RMS
1 mas
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Astrometry Synthesis
Local fitting applicable to well-behaved single
stars (core processing)
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Interaction astrometry photometry
In general, astrometry and photometry must be
considered jointly
sum
primary
Example A binary with projected separation 25
mas and ?m 1 mag is resolved by LSF fitting of
the two components
secondary
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Astrometry Synthesis
Global fitting applicable to arbitrary sources
(shell processing)
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Some open questions

• optical modelling
• polarization, effect of coatings, is
  Fraunhofer theory sufficient?
• use of wavefront sensor?
• use of basic angle monitor?

• CCD properties
• MTF, sub-pixel structure, charge trapping
  (CTI), ...

• satellite dynamics
• perturbations, control dynamics, pointing
  jitter, micrometeorites, ...

• on-board detection, data sampling, timing and
  compression

• calibration requirements (geometric,
  photometric, PSF)

• general procedures for robust estimation

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Accuracy assessment

• Objectives
• to predict the final mission accuracies by
  taking into account
• all identified error sources
• to assist in making trade-offs between
  conflicting requirements

• Accuracy model
• semi-analytical, complemented with ad hoc
  simulations
• evolving as studies progress
• clear and up-to-date documentation maintained

• Policy questions
• open access to documentation, assumptions,
  results (?)
• maintain several models, e.g. conservative
  versus realistic (?)
• practical organisation of work

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Accuracy assessment things to consider (1/2)
Source absolute flux extinction sky
background double/multiple stars solar-system
objects Satellite environment thermal radiation
pressure solar wind particle radiation micrometeo
rites real-time attitude determination attitude
control (FEEP)
Instrument mirror alignment and
deformation polishing errors optical
transmittance straylight diffraction
spikes filter characteristics CCD characteristics
geometry, QE, MTF, CTI, ... electronic chain
characteristics discretization and
compression source detection (ASM) TDI tracking
and windowing distortion and rate
errors chromaticity
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Accuracy assessment things to consider (2/2)
Mission scanning law mission length data
gaps source distribution (Galaxy model) data
saturation timing satellite orbit orbit
determination barycentric ephemerides error
margins
Data analysis reference systems detection
probabilities false detections centroiding
algorithm calibration model attitude
model astrometric error propagation outlier
treatment covariance estimation spin
harmonics large-scale frame distortion small-scale
correlations parallax zero point link to
extragalactic frame