The Gaia mission

Data reduction activities in the UK

Floor van Leeuwen, IoA

11 July 2005 Astrogrid meeting, IoA 2

Gaia: Design Considerations

• Astrometry (V < 20):– completeness to 20 mag (on-board detection) 109 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

Teff ~ 200 K, log g, [Fe/H] to 0.2 dex, extinction

11 July 2005 Astrogrid meeting, IoA 3

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

11 July 2005 Astrogrid meeting, IoA 4

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

11 July 2005 Astrogrid meeting, IoA 5

Payload and Telescope

SiC primary mirrors1.4 0.5 m2 at 99.4°

Superposition offields of view

SiC toroidalstructure

Basic anglemonitoring system

Combinedfocal plane (CCDs)

Rotation axis

11 July 2005 Astrogrid meeting, IoA 6

Astrometric Focal Plane

Along-scan star motion in 10 s

Total field: - active area: 0.64 deg2

- number of CCD strips: 20+ 110+40 - CCDs: 4500 x 1966 pixels - pixel size = 10 x 30 µm2

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

11 July 2005 Astrogrid meeting, IoA 7

Radial Velocity Measurement Concept

F3 giant S/N = 7 (single measurement)

S/N = 130 (summed over mission)

11 July 2005 Astrogrid meeting, IoA 8

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

11 July 2005 Astrogrid meeting, IoA 9

Proposed structure

• There will be 6 coordination units (CUs) taking responsibility of major fractions of the reduction tasks1. 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

11 July 2005 Astrogrid meeting, IoA 10

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 July 2005 Astrogrid meeting, IoA 11

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

11 July 2005 Astrogrid meeting, IoA 12

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)

11 July 2005 Astrogrid meeting, IoA 13

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 1012 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

11 July 2005 Astrogrid meeting, IoA 14

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

11 July 2005 Astrogrid meeting, IoA 15

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