Gaia will map the structure and unravel the formation history of our
Galaxy through a stereoscopic census. It will measure parallaxes and
proper motions for every object in the sky brighter than magnitude 20
— amounting to 1 billion stars, galaxies, quasars and solar system
objects. The astrometric accuracy will be 12–25 μas, depending on
colour, at 15th magnitude and 100–300 μas at 20th magnitude.
Multi-colour photometry will be obtained for all objects by means of
low-resolution spectrophotometry between 330 and 1000 nm. In addition
radial velocities with a precision of 1–15 km/s will be measured for
all objects to 17th magnitude. Gaia is scheduled for launch in late 2011
and the processing of the data will start immediately thereafter. The
catalogue is expected to be completed in 2020.
Gaia is an ESA mission and the prime contractor for the development, building, and testing of the spacecraft and payload is EADS-Astrium. The on-ground data processing will be undertaken by the Gaia Data Processing and Analysis Consortium (DPAC). More information on Gaia can be found at the links below. The following sections describe the Dutch contributions to the Gaia mission preparations.
The last part of the optical train of the Gaia telescopes is shown on
the right together with the focal plane containing the detectors for the
three instruments of Gaia. The photometric instrument consists of two
low-resolution fused-silica prisms dispersing all the light entering the
field of view. The Blue Photometer (BP) operates in the wavelength range
330–680 nm; the Red Photometer (RP) covers the wavelength range
640–1000 nm.
The next image on the left is a simulation of a crowded
stellar field as observed by Gaia's photometric instruments. The image of
each star is smeared into a little spectrum. From these dispersed images
one dimensional BP and RP spectra have to be extracted. Simulated versions
of the latter are shown in the image below. The two panels show BP (left)
and RP (right) spectra for a range of main sequence stars (O5, black
lines, to M5, red lines). From these spectra all sources observed by Gaia
can be classified and parametrized in terms of astrophysical parameters.
The group in Leiden (Paola Marrese, Giorgia Busso, Anthony Brown) will
develop the data processing algorithms to extract the BP and RP spectra
from the raw dispersed images. In addition algorithms for deriving colour
parameters describing the source SEDs will be developed. The main
challenges are the treatment of CTI effects (see below), the calibration
of the PSF, and the disentangling of overlapping images in crowded fields.
Algorithms for the latter task are developed in collaboration with groups
at the INAF observatories in Rome and Teramo. Scott Trager in Groningen
is involved in the ground based spectrophotometry of standard stars, which
will be used to put the photometric measurements on a physical scale.
A major issue for Gaia is the increasing effect of charge transfer
inefficiency (CTI) caused by radiation damage to the CCDs accumulated
during the mission. Solar wind particles passing through the CCDs create
`traps' which can hold electrons in the same position on the CCD for a
while before releasing them again. This has two effects on a PSF image
which is clocked through the (4500) CCD lines during time-delayed
integration operation: (1) a fraction of the signal charge will be lost
and (2) The PSF will be distorted into an asymmetric shape. The image
below illustrates these effects (highly exaggerated).
The flux losses will cause a decrease in the precision of all Gaia's
measurements. For the astrometric data the PSF distortion will lead to
systematic errors in the centroid measurement of the images from the
astrometric field in the focal plane. A major complication is that the CTI
effects are highly variable with the pixel illumination history. This is
illustrated in the image here below. The stars travel from left to right
across the focal plane leaving charge trails behind them (electrons
released after having been trapped). These trails will affect the CCD
state (amount of traps already filled) seen by following stars. Hence the
CTI effects on a given image depend on the previous images that crossed
the focal plane. In addition there will be periodic charge injections
(vertical lines in the image) which are used to `reset' the state of the
CCD (by filling traps) but they leave their own charge trails which will
affect the stellar images and the background in the measurements.
The PhD research of Thibaut Prod'homme is focused on the empirical and theoretical modelling of CTI in astronomical CCDs. The results will be incorporated into the radiation damage mitigation algorithms for the Gaia data processing.
The research of Daniel Risquez focuses on the incorporation of the
detailed physical understanding of the dynamics of a continuously rotating
space platform into the attitude modelling for space astrometric missions
such as Hipparcos and Gaia. The highest quality for the attitude modelling
will be needed to reach the astrometric goals of the Gaia mission, as the
reconstructed attitude provides the reference frame relative to which the
astrometric measurements are obtained. The results of this study will be
used in understanding the dynamics of the satellite once it is in
operation, and to incorporate, in as far as possible, that understanding
in the core astrometric processing for the Gaia mission.
As an example of the kinds of effects studied, the plot on the left shows the expected torque acting on the Gaia satellite due to solar radiation. Photons from the sun impact on the sunshield and transfer angular momentum to the spinning satellite, thus changing the orientation of its spin axis if nothing is done to compensate. The torque is calculated over one spin period (6 hours, 'Scan Phase' shows the Sun's position with respect to Gaia's spin axis). Six different sunshields are analysed, with different surface roughness and overall misalignments.
Like Hipparcos Gaia will observe the sky in two directions simultaneously,
separated by the basic angle of 106.5°. The astrometric accuracy is
highly dependent on the stability of this angle. Gaia will be equipped
with a Basic Angle Monitoring (BAM) system, to monitor this angle with a
precision of 0.5 μas. The BAM consists of two laser interferometers.
Two pairs of parallel laser bundles are sent to the two telescopes, which
create two interference patterns on a detector. If the basic angle varies,
the interference
patterns will shift.
TNO is responsible for the BAM Opto Mechanical Assembly (OMA). With the BAM, an Optical Path Difference (OPD) as small as 1.5 picometers RMS can be measured. To fulfil the stability requirements for such accurate OPD measurements, the entire BAM OMA is constructed from Silicon Carbide (SiC). Ultra stable cryogenic mounts have been developed for mirrors and beam splitters. The BAM OMA shall have an extremely small wave front error, less than 25 nm RMS over the entire optical path. TNO has developed processes for the polishing of SiC mirrors to very low surface errors.
TNO is also responsible for the wavefront sensor (WFS) which will be used
after the launch of Gaia to monitor the wave front errors of the two Gaia
telescopes mounted on the Gaia satellite. The image shows the
Qualification Model of the Gaia Wavefront Sensor. The WFS system is based
on the `Shack Hartmann' principle and must have have low optical
aberrations itself. Gaia will operate over a broad wavelength and in
cryogenic conditions (450 to 900 nm wavelength band and 130 to 200 K
operation temperature). For these boundary conditions a temperature
independent solution of Invar was selected, with fused silica optics, with
the least number of dispersive elements in the design.
The activities describes above are or have been funded by NWO, NOVA, and the Marie Curie research training network ELSA (European Leadership in Space Astrometry). Figure credits: EADS-Astrium (Gaia instruments), C. Carreau (Spacecraft), A. Short (CTI figures), TNO (BAM, WFS). More information can be found at:
Below other astrometric missions are listed which are currently under development by NASA and JAXA. Both mission are highly complementary to Gaia, and will benefit from the reference frame established by Gaia. SIM Lite will find planets of much smaller mass than Gaia can and JASMINE will be able to map the inner regions of the Galaxy better as it can see further through the dust. Nano-JASMINE is a precursor to JASMINE, aiming at an astrometric accuracy of several milliarcseconds for the brightest stars in the sky. The Gaia AGIS data processing system, which will carry out the core astrometric data reduction, may also be used in the Nano-JASMINE data processing.