The Gaia satellite was launched at the end of 2013 to collect data that will allow the determination of highly accurate positions, parallaxes, and proper motions for over one billion sources brighter than magnitude 20.7, from the solar system to our Milky Way and beyond. I will discuss the astrometric measurement concept and the design considerations and engineering challenges that follow. I will also review the problems faced in practice during the mission which however did not prevent its phenomenal success, as will be demonstrated with a few scientific highlights.
With its launch at the very end of 2013, ESA's astrometry satellite Gaia began its endeavor to compile astrometric and photometric measurements of at least one billion objects, as well as high resolution optical spectra of hundred million objects. The Gaia catalog therefore results in a wealth of coherently determined astrophysical parameters of these objects. After its extensive commissioning phase, Gaia entered the nominal mission phase in July 2014. The science ground segment, which is formed by the Gaia Data Processing and Analysis Consortium (DPAC), has since then started its operations. DPAC is a large, multi-national, science consortium which has to handle and process the dense and complex Gaia data stream. With its decentralized management and its distributed infrastructure, the Gaia DPAC is a remarkable undertaking. In this paper we will summarize some of the experiences of the DPAC facing the real Gaia data, compare this to the pre-launch expectations, and critically review the development phase.
The Gaia payload ensures maximum passive stability using a single material, SiC, for most of its elements. Dedicated metrology instruments are, however, required to carry out two functions: monitoring the basic angle and refocusing the telescope. Two interferometers fed by the same laser are used to measure the basic angle changes at the level of μas (prad, micropixel), which is the highest level ever achieved in space. Two Shack- Hartmann wavefront sensors, combined with an ad-hoc analysis of the scientific data are used to define and reach the overall best-focus. In this contribution, the systems, data analysis, procedures and performance achieved during commissioning are presented .
Gaia is an ESA mission due to be launched in 2013 and will be dedicated to astrometry. The attitude of the spacecraft
Gaia is an important part of the data processing of the mission because the astrometry will be calculated with respect to the
attitude. Therefore we need a very accurate characterisation of the attitude of the satellite during the observations in order
to get the best output from the mission.
We simulate the attitude of Gaia using the Dynamical Attitude Model (DAM). It is a simulation developed to achieve a
detailed understanding of the Gaia attitude and to provide realistic input data for testing the software pipeline. DAM takes
into account perturbations as well as internal hardware components controlling the satellite as the control system, sensors
and micro-Newton thrusters.
We study the errors in the simulated data, specifically the attitude reconstruction when fitting the Gaia reference attitude
with B-splines. We analyse the effect of different parameters and provide an estimation of the expected noise in the scientific
output of the mission due to the noise in the attitude reconstruction.
Wide-field multi-object spectroscopy is a high priority for European astronomy over the next decade. Most 8-10m
telescopes have a small field of view, making 4-m class telescopes a particularly attractive option for wide-field
instruments. We present a science case and design drivers for a wide-field multi-object spectrograph (MOS) with
integral field units for the 4.2-m William Herschel Telescope (WHT) on La Palma. The instrument intends to take
advantage of a future prime-focus corrector and atmospheric-dispersion corrector (Agocs et al, this conf.) that will
deliver a field of view 2 deg in diameter, with good throughput from 370 to 1,000 nm. The science programs cluster into
three groups needing three different resolving powers R: (1) high-precision radial-velocities for Gaia-related Milky Way
dynamics, cosmological redshift surveys, and galaxy evolution studies (R = 5,000), (2) galaxy disk velocity dispersions
(R = 10,000) and (3) high-precision stellar element abundances for Milky Way archaeology (R = 20,000). The multiplex
requirements of the different science cases range from a few hundred to a few thousand, and a range of fibre-positioner
technologies are considered. Several options for the spectrograph are discussed, building in part on published design
studies for E-ELT spectrographs. Indeed, a WHT MOS will not only efficiently deliver data for exploitation of
important imaging surveys planned for the coming decade, but will also serve as a test-bed to optimize the design of
MOS instruments for the future E-ELT.
ESA's Gaia mission aims to create a complete and highly accurate stereoscopic map of the Milky Way. The
stellar parallaxes will be determined at the micro-arcsecond level, as a consequence the measurement of the
stellar image location on the CCD must be highly accurate. The solar wind protons will create charge traps in
the CCDs of Gaia, which will induce large charge loss and distort the stellar images causing a degradation of
the location measurement accuracy. Accurate modelling of the stellar image distortion induced by radiation is
required to mitigate these effects. We assess the capability of a fast physical analytical model of radiation damage
effects called the charge distortion model (CDM) to reproduce experimental data. To realize this assessment
we developed a rigorous procedure that compares at the sub-pixel level the model outcomes to damaged images
extracted from the experimental tests. We show that CDM can reproduce accurately up to a certain level the test
data acquired on a highly irradiated device operated in time delay integration mode for different signal levels and
different illumination histories. We discuss the potential internal and external factors that contributed to limit
the agreement between the data and the charge distortion model. To investigate these limiting factors further,
we plan to apply our comparison procedure on a synthetic dataset generated through detailed Monte-Carlo
simulations at the CCD electrode level.
The European Space Agency's Gaia mission1 is scheduled for launch in 2012. It will operate at L2 for 5 years,
rotating slowly so that its two optical telescopes will repeatedly observe more than one billion stars. The resulting
data set will be iteratively reduced to solve for the relative position, parallax-distance and proper motion of every
observed star, yielding a three dimensional dynamical model of our galaxy. The focal plane contains 106 large
area silicon CCDs continuously operating in TDI mode at a line rate synchronised with the satellite rotation.2
One of the greatest challenges facing the mission is radiation damage in the CCDs which will cause charge
loss and image distortion. This is particularly severe because the large focal plane is difficult to shield and
because the launch will coincide with solar maximum. Despite steps taken to minimize the effects of radiation
(e.g. regular use of charge injection), the residual distortion will need to be calibrated during the pipeline data
processing. Due to the volume of data involved, this requires a trapping model which is physically realistic, yet
fast enough and simple enough to implement in the pipeline. The current prototype Charge Distortion Model
will be presented. This model was developed specifically for Gaia in TDI mode. However, an imaging mode
version has already been applied to other missions, for example, to indicate the potential impact of radiation
damage on the proposed Euclid mission.
The SINFONI instrument for ESO's VLT combines integral field spectroscopy and adaptive optics (AO). We discuss detailed simulations of the adaptive optics module. These simulations are aimed at assessing the AO module performance, specifically for operations with extended sources and a laser guide star. Simulated point spread function (PSF) images will be used to support scientific preparations and the development of an exposure time calculator, while simulated wavefront sensor measurements will be used to study PSF reconstruction methods. We explain how the adaptive optics simulations work, focusing on the realistic modelling of the laser guide star for a curvature wavefront sensor. The predicted performance of the AO module is discussed, resulting in recommendations for the operation of the SINFONI AO module at the