1. The Gaia mission Data reduction activities in the UK Floor van Leeuwen, IoA

2. 11 July 2005Astrogrid meeting, IoA2 Gaia: Design Considerations Astrometry (V < 20): –completeness to 20 mag (on-board detection)  10 9 stars –accuracy: 10-20  arcsec at 15 mag (Hipparcos: 1 milliarcsec at 9 mag) –scanning satellite, two viewing directions  global accuracy, with optimal use of observing time –principles: global astrometric reduction (as for Hipparcos) Radial velocity (V < 16-17): –application: third component of space motion, perspective acceleration dynamics, population studies, binaries spectra: chemistry, rotation –principles: slitless spectroscopy using Ca triplet (848-874 nm) Photometry (V < 20): –astrophysical diagnostics (5 broad + 14 medium-band) + chromaticity   T eff ~ 200 K, log g, [Fe/H] to 0.2 dex, extinction

3. 11 July 2005Astrogrid meeting, IoA3 The main dates ITT for industry has been issued, responses due in October Final choice of contractor early 2006, start phase B2 Contract already issued for CCD manufacturing (e2V in the UK) Launch date: End of 2011 Mission: Mid 2012 till end of 2017 Mission products public: around 2020

4. 11 July 2005Astrogrid meeting, IoA4 Hardware and software The payload and spacecraft will be built by one industrial consortium, under control of ESA Data reduction will be largely the responsibility of the European scientific community Both aspects are highly complex and very demanding –Accuracies aimed at –Number of objects targeted –Complexity of the connectivity of the data: only through the connectivity can absolute parallaxes be obtained

5. 11 July 2005Astrogrid meeting, IoA5 Payload and Telescope SiC primary mirrors 1.4  0.5 m 2 at 99.4° Superposition of fields of view SiC toroidal structure Basic angle monitoring system Combined focal plane (CCDs) Rotation axis

6. 11 July 2005Astrogrid meeting, IoA6 Astrometric Focal Plane Along-scan star motion in 10 s Total field: - active area: 0.64 deg 2 - number of CCD strips: 20+ 110+40 - CCDs: 4500 x 1966 pixels - pixel size = 10 x 30 µm 2 Sky mapper: - detects all objects to 20 mag - rejects cosmic-ray events Astrometric field: - readout frequency: 55 kHz for AF2-10 - total detection noise: 5-6 e - for AF2-10 Broad-band photometry: - 5 photometric filters FoV2 FoV1

7. 11 July 2005Astrogrid meeting, IoA7 Radial Velocity Measurement Concept F3 giant S/N = 7 (single measurement) S/N = 130 (summed over mission)

8. 11 July 2005Astrogrid meeting, IoA8 Data reductions Data Analysis Coordination Committee (DACC) –Large-scale organisation of data reduction tasks over interested institutes in Europe –Ensure proper backing in manpower and hardware for the proposed developments –Ensure that all tasks are covered Creation of the Gaia Data Analysis Consortium –Expected spring-2006 –The AO will be issued and will effectively formalising the structure proposed by the DACC

9. 11 July 2005Astrogrid meeting, IoA9 Proposed structure There will be 6 coordination units (CUs) taking responsibility of major fractions of the reduction tasks 1.System architecture 2. Simulations 3.Core processing, Global Iterative Solution 4.Shell processes 5.Photometry 6.Spectroscopy Each CU covers several Development Units (DUs) –Identify the major developments to be covered by the CU –A DU will often be associated with only one or two institutes

10. 11 July 2005Astrogrid meeting, IoA10 Development units Each development unit will cover a number of WPs A DU has some freedom to propose and experiment with solutions for its tasks –All proposed solutions need the approval of the CU management –All solutions need to be ready within the deadlines set by the project Some DUs will have strong links with (former) working groups, others will have to be created to cover all needs of the project

11. 11 July 2005Astrogrid meeting, IoA11 Reporting structure Gaia Science team –Data Analysis Consortium Executive (CU managers) (DACE) –CU Management –DU Management –WP leaders Because of the small sizes of teams, a CU manager will also often be a DU manager and a WP leader Coordination of activities between CUs are the responsibility of the DACE The CU managers are also members of the Gaia Science team, to which they report their progress

12. 11 July 2005Astrogrid meeting, IoA12 UK involvements Main responsibility (manager) on photometry CU (IoA) –Covers main reduction and calibration –Local photometry data base –Science alerts –Variability Major involvement in the spectroscopy CU (MSSL) –Disentanglement of spectra –Radial velocities Crucial involvement in the core processing CU –CCD calibrations (Leicester, Brunel)

13. 11 July 2005Astrogrid meeting, IoA13 The size of the problem Photometry –Mission consists of an average of 85 field transits for 1 billion stars –Each field transit consists of 16 CCD transits, giving a total of 1.365 10 12 measurements over a period of 5 years –Per day: 748 million observations –Compare: Hipparcos total was 13 million observations –Processing needs to be at least at 10 times real data rate, else we can’t do the necessary iterations –Processing requirements: of the order of 10 billion observations for around 625 million different objects per day: need to be reduced, stored etc

14. 11 July 2005Astrogrid meeting, IoA14 eScience challenges I/O arrangements for extracting 10 billion observations per day, and updating the data for 625 million objects per day –Commercial data bases are not designed for this level of use –Simpler designs are needed, causing considerably less overheads Processing power –High levels of parallelisation (farming out) over large numbers of processors –CPU and memory requirement of individual tasks are modest, but the number of tasks is very high –Needs very-well-organised coordination procedures

15. 11 July 2005Astrogrid meeting, IoA15 The team The UK team will start with a relatively small team of astronomers –Work on algorithm definitions for the various tasks –Provide interfaces with he relevant working groups Gradually professional programmers will be brought in to lead the coding –Total team size for the last three years before launch will be approximately 14 people, spread over Cambridge (6-7), MSSL (2-3), Edinburgh (2), Leicester + Brunel (2-3) Guidance on system design from CU1 (syst.architecture) –Ensures compatibility across the CUs