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The Euclid mission design
Giuseppe D Racca, Rene Laureijs, Luca Stagnaro, Jean Christophe Salvignol, Jose Lorenzo Alvarez, Gonzalo Saavedra Criado, Luis Gaspar Venancio, Alex Short, Paolo Strada, Tobias Boenke, Cyril Colombo, Adriano Calvi, Elena Maiorano, Osvaldo Piersanti, Sylvain Prezelus, Pierluigi Rosato, Jacques Pinel, Hans Rozemeijer, Valentina Lesna, Paolo Musi, Marco Sias, Alberto Anselmi, Vincent Cazaubiel, Ludovic Vaillon, Yannick Mellier, Jerome Amiaux, Michel Berthe, Marc Sauvage, Ruyman Azzollini, Mark Cropper, Sabrina Pottinger, Knud Jahnke, Anne Ealet, Thierry Maciaszek, Fabio Pasian, Andrea Zacchei, Roberto Scaramella, John Hoar, Ralf Kohley, Roland Vavrek, Andreas Rudolph, Micha Schmidt
TL;DR
Euclid addresses how dark energy, dark matter, and gravity shape cosmic geometry and structure evolution. It combines weak-lensing and galaxy-clustering probes with a stable wide-field spacecraft and dedicated data processing. The mission is designed for a six-year survey covering 15,000 deg2, while its reference survey remains subject to post-launch verification.
Problem
Euclid investigates the nature of dark energy, dark matter, and gravity through the geometry and evolution of cosmic structures.
Method
Euclid combines weak-lensing measurements of galaxy shapes with galaxy-clustering measurements from near-infrared spectroscopic redshifts.
Results
The as-designed system shows robust image-quality and radiometric margins, while spectroscopic purity is marginal and expected to improve with ground-processing maturation.
Takeaways & Limitations
The mission architecture provides a basis for implementation while emphasizing straylight and cleanliness control as important to spectroscopic performance.
Takeaways & Limitations
The reference survey is only a feasibility demonstration, and the final survey will follow launch and in-orbit performance verification.
Abstract
from arXiv · showhide
Euclid is a space-based optical/near-infrared survey mission of the European Space Agency (ESA) to investigate the nature of dark energy, dark matter and gravity by observing the geometry of the Universe and on the formation of structures over cosmological timescales. Euclid will use two probes of the signature of dark matter and energy: Weak gravitational Lensing, which requires the measurement of the shape and photometric redshifts of distant galaxies, and Galaxy Clustering, based on the measurement of the 3-dimensional distribution of galaxies through their spectroscopic redshifts. The mission is scheduled for launch in 2020 and is designed for 6 years of nominal survey operations. The Euclid Spacecraft is composed of a Service Module and a Payload Module. The Service Module comprises all the conventional spacecraft subsystems, the instruments warm electronics units, the sun shield and the solar arrays. In particular the Service Module provides the extremely challenging pointing accuracy required by the scientific objectives. The Payload Module consists of a 1.2 m three-mirror Korsch type telescope and of two instruments, the visible imager and the near-infrared spectro-photometer, both covering a large common field-of-view enabling to survey more than 35% of the entire sky. All sensor data are downlinked using K-band transmission and processed by a dedicated ground segment for science data processing. The Euclid data and catalogues will be made available to the public at the ESA Science Data Centre.
1. INTRODUCTION AND SCIENCE CASE
Euclid is an ESA cosmology mission designed to probe dark energy, dark matter, and gravity through cosmic geometry and structure evolution. Its science case centers on weak lensing and galaxy clustering, requiring a large, deep, stable survey.
- Science case: Euclid investigates dark energy, dark matter, and gravity by observing the Universe’s geometry and the evolution of cosmic structures.
- Cosmological probes: Weak lensing measures galaxy-shape distortions, while galaxy clustering uses near-infrared spectroscopy to measure galaxy redshifts and matter clustering.
- Key questions: The mission asks whether dark energy is a cosmological constant, an evolving field, or evidence that General Relativity or cosmological assumptions fail.
- Key questions: Euclid also targets the nature of dark matter, the absolute neutrino mass scale, relativistic species, and primordial density fluctuations.
- Survey requirements: The survey must cover 15,000 deg2, or 36% of the sky, image billions of galaxies to z~2, and operate for six years with a stable wide field of view.
2. MISSION ARCHITECTURE
Euclid’s architecture is shaped by survey speed, depth, precision, imaging quality, and mission duration. Its observing strategy combines a large field, constrained visibility, calibration activities, and optimized pointing sequences.
- Mission architecture: A 0.54 deg2 field of view and optimized survey strategy provide the speed needed to cover the sky, while pointing and thermal stability preserve image quality.
- Survey design: The SEL2 orbit and Sun-pointing constraints determine when sky regions are visible and shape the survey’s operational coverage.
- Survey design: Each field is observed in four dithered frames with simultaneous VIS and NISP exposures, followed by a slew to the next field.
- Survey design: ECTile optimizes pointing sequences under visibility, calibration, operational, and programmatic constraints using a merit function.
- Survey design: The optimized coverage is close to the theoretical maximum, although late-survey growth is small because most visible sky has already been observed.
- Survey design: The reference survey demonstrates feasibility only; the final survey will be delivered after launch and in-orbit performance verification.
3. SPACECRAFT DESIGN
Euclid’s spacecraft combines a Service Module supporting operations, power, thermal protection, data handling, and communications with a Payload Module built around a stable cold telescope and two instruments. The design targets wide-area, high-quality imaging through stringent thermal, dimensional, pointing, and image-quality requirements.
- 3.1 Service Module: The Service Module supports the payload through spacecraft subsystems, warm electronics, structural interfaces, a sunshield, and photovoltaic power generation.Its six side panels group telemetry and telecommand, attitude control, data management, power, payload, and FGS electronics.
- 3.1 Service Module: The architecture uses passive thermal control, protected 28 V power distribution, centralised command and data management, and dual X-/K-band telecommunications.The thermal design limits heat flux into the coldest NISP radiator to below 25 mW.
- 3.1 Service Module: The spacecraft stores 850 Gbit of daily science data in a 4 Tbit end-of-life mass memory and transfers files using CFDP.K-band is used for high-rate data downlink, while X-band supports telecommands, monitoring, ranging, and CFDP directives.
- 3.2 Payload Module: The Payload Module uses a 1.2 m Korsch SiC telescope to feed the VIS and NISP instruments.The telescope has a 0.47° field-of-view offset, 24.5 m focal length, and collecting area larger than 1 m².
- 3.2 Payload Module: The cold SiC telescope provides 0.4 µm/m/K thermal expansion and a 135 K instrument environment.Thermally decoupled interfaces and harness design minimise heat leaks from the Service Module and sunshield.
- 3.2 Payload Module: The PLM design achieves large margins on visible PSF ellipticity, FWHM, and NIR encircled-energy radius.These margins provide flexibility for relaxed image-motion requirements, including reduced pointing stability and greater tolerance to microvibrations.
4. THE EUCLID GROUND SEGMENT
The Euclid Ground Segment combines an ESA-managed Operational Ground Segment with a jointly managed Science Ground Segment for mission operations, instrument operations, data processing, survey definition, and archiving. Its distributed architecture addresses high data volumes, demanding communications, autonomous spacecraft operations, and integrated processing from telemetry to science products.
- 4. THE EUCLID GROUND SEGMENT: The Ground Segment consists of an ESA-managed OGS and a shared ESA–Euclid Consortium SGS responsible for operations, science processing, survey definition, and archiving.The OGS includes the Mission Operations Centre and ground-station network, while the SGS handles scientific data processing and instrument operations.
- 4. THE EUCLID GROUND SEGMENT: The ground segment is designed around challenges including approximately 850 Gbit/day downlinks during four-hour contacts and weather-sensitive 26 GHz K-band communications.X-band provides real-time telemetry, telecommands, and ranging, while CFDP supports file downlink and requires close onboard–ground coordination.
- 4.2 The Science Ground Segment: ESA’s SOC coordinates survey planning, initial consistency and quality checks, public data delivery, and the interface between scientific planning and mission operations.The SOC receives raw telemetry and auxiliary information from the MOC and provides observation-planning and instrument-commanding information in return.
- 4.2 The Science Ground Segment: The SGS processes data through ten logical, archive-connected Processing Functions spanning telemetry, calibrated imaging, spectra, external data, and high-level products.Functions include LE1, VIS, NIR, SIR, EXT, SIM, MER, SPE, PHZ, SHE, and LE3.
- 4.2 The Science Ground Segment: The SGS includes functions for simulation, data merging, spectroscopic and photometric redshifts, galaxy shapes, and high-level science products.MER aggregates multi-wavelength data, while LE3 computes Level 3 products from processed shape and redshift measurements.
- 4.2 The Science Ground Segment: Organisation Units develop and validate processing prototypes before transferring them with test harnesses to Science Data Centres for full pipeline implementation.Reviews and ICT or scientific challenges test technical feasibility, scalability, distributed operation, and consistency across SDCs.
5. SUMMARY OF MISSION PERFORMANCE
Mission-level performance requirements cover image quality, radiometric sensitivity, and spectroscopic purity and completeness. At Mission PDR, image-quality requirements were assessed from calculated PSFs across discrete field points, with requirements met at variable margins.
- Mission performance is evaluated across image quality, radiometric sensitivity, and spectroscopic purity and completeness.These requirement groups cover the complete VIS and NISP observing chain, channel sensitivity, line detection, and redshift measurement quality.
- Image Quality (IQ): VIS and NISP image quality use different criteria: weak-lensing shape measurements constrain VIS PSF morphology, while infrared photometry uses encircled energy.The distinction reflects the different scientific roles of the two channels.
- Image Quality (IQ): Complete-system PSF performance depends on telescope optics, residual pointing jitter, instrument optics, and detector response.The optical response also varies with wavelength, field position, and observed intensity, while alignment and manufacturing tolerances remain part of the performance assessment.
- Image Quality (IQ): At Mission PDR, calculated PSFs met image-quality requirements with variable margins across discrete field points.The assessment was performed at the 3σ level; ellipticity benefited from jitter that circularized the PSF, while ellipticity stability still requires final STOP and ground-processing confirmation.
Radiometric Performance
Euclid’s radiometric and spectroscopic assessments use signal, background, and spectra-extraction simulations to test survey sensitivity and redshift quality. The Mission PDR found good sensitivity margins but background-limited spectroscopic purity requiring further processing improvements.
- Radiometric Performance: Radiometric sensitivity is constrained by Zodiacal background and diffuse straylight, which vary across the survey and seasonally.A larger radiometric aperture increases collected background and reduces effective signal-to-noise ratio.
- Radiometric Performance: 15,000 deg2 can be observed at the required signal-to-noise ratio, with good margins across the evaluated Euclid channels.SNR maps used signal and background levels over the observable sky, and Table 9 summarizes channel performance for the best 15,000 deg2 areas.
- Spectroscopic performance: Spectroscopic purity is the fraction of measured redshifts that are correct, whereas completeness estimates the observable galaxies for which a redshift can be measured.The Mission PDR evaluation used current spectra-extraction algorithms and simulations for slitless spectroscopy.
- Spectroscopic performance: Spectroscopic purity is strongly limited by out-of-field straylight and Zodiacal light, while improved correction and extraction are expected to bring performance close to compliance.Survey pointing choices and cleanliness control are also being used to limit straylight effects.
- Overall Mission PDR assessment found robust image-quality and radiometric margins, with marginal purity expected to improve as ground processing matures.The result supported proceeding into Phase C/D while retaining focus on straylight and cleanliness control.
6. PROGRAMMATIC STATUS
Following mission approval, Euclid entered implementation with PLM development beginning in 2012 and selection of the industrial prime contractor in 2013. By the reported status, spacecraft subcontractors were progressing and the consortium had passed the preliminary design review.
- Euclid entered implementation after 2012 approval, beginning with PLM development and selecting Thales Alenia Space Italia as industrial prime contractor in July 2013.The PLM development was assigned to Airbus Defence and Space of Toulouse.
- At the time of writing, all SVM and PLM subcontractors were progressing, and the industrial consortium had passed its preliminary design review in July 2015.