Ralf Kohley

Euclid: ESA's mission to map the geometry of the dark universe

Euclid is a space-borne survey mission developed and operated by ESA. It is designed to understand the origin of the Universes accelerating expansion. Euclid will use cosmological probes to investigate the nature of dark energy, dark matter and gravity by tracking their observational signatures on the geometry of the Universe and on the history of structure formation. The mission is optimised for the measurement of two independent cosmological probes: weak gravitational lensing and galaxy clustering. The payload consists of a 1.2 m Korsch telescope designed to provide a large field of view. The light is directed to two instruments provided by the Euclid Consortium: a visual imager (VIS) and a near-infrared spectrometer-photometer (NISP). Both instruments cover a large common field of view of 0.54 deg2, to be able to survey at least 15,000 deg2 for a nominal mission of 6 years. An overview of the mission will be presented: the scientific objectives, payload, satellite, and science operations. We report on the status of the Euclid mission with a foreseen launch in 2019.

The Euclid mission design

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.

Gaia: operational aspects and tests of Gaia Flight Model CCDs

ESAs cornerstone mission Gaia is planning to map 1% of the stellar population of our galaxy, around one thousand million objects, to micro-arcsecond accuracy. In addition to high precision astrometric information, prism dispersion optics will be used to provide multi-band photometry and a spectroscopic instrument provides information for deriving radial velocities. Gaias focal plane will be the largest ever flown to space comprising an almost Giga-pixel mosaic of 106 specially designed CCDs, the e2v technologies CCD91-72, operated synchronously in TDI mode. This paper will address some operational aspects of these detectors in the Gaia focal plane array and report on recent test results with respect to calibration needs.

The Euclid VIS CCD detector design, development, and programme status

The focal plane array of the Euclid VIS instrument comprises 36 large area, back-illuminated, red-enhanced CCD detectors (designated CCD 273). These CCDs were specified by the Euclid VIS instrument team in close collaboration with ESA and e2v technologies. Prototypes were fabricated and tested through an ESA pre-development activity and the contract to qualify and manufacture flight CCDs is now underway. This paper describes the CCD requirements, the design (and design drivers) for the CCD and package, the current status of the CCD production programme and a summary of key performance measurements.

How well can charge transfer inefficiency be corrected? A parameter sensitivity study for iterative correction.

Radiation damage to space-based Charge-Coupled Device (CCD) detectors creates defects which result in an increasing Charge Transfer Inefficiency (CTI) that causes spurious image trailing. Most of the trailing can be corrected during post-processing, by modelling the charge trapping and moving electrons back to where they belong. However, such correction is not perfect – and damage is continuing to accumulate in orbit. To aid future development, we quantify the limitations of current approaches, and determine where imperfect knowledge of model parameters most degrade measurements of photometry and morphology. As a concrete application, we simulate 1.5×109 “worst case” galaxy and 1.5×108 star images to test the performance of the Euclid visual instrument detectors. There are two separable challenges: If the model used to correct CTI is perfectly the same as that used to add CTI, 99.68 % of spurious ellipticity is corrected in our setup. This is because readout noise is not subject to CTI, but gets over-corrected during correction. Second, if we assume the first issue to be solved, knowledge of the charge trap density within ∆ρ/ρ= (0.0272±0.0005)%, and the characteristic release time of the dominant species to be known within ∆τ /τ = (0.0400 ± 0.0004)% will be required. This work presents the next level of definition of in-orbit CTI calibration procedures for Euclid.

A comparative study of charge transfer inefficiency value and trap parameter determination techniques making use of an irradiated ESA-Euclid prototype CCD

The science objectives of space missions using CCDs to carry out accurate astronomical measurements are put at risk by the radiation-induced increase in charge transfer inefficiency (CTI) that results from trapping sites in the CCD silicon lattice. A variety of techniques are used to obtain CTI values and derive trap parameters, however they often differ in results. To identify and understand these differences, we take advantage of an on-going comprehensive characterisation of an irradiated Euclid prototype CCD including the following techniques: X-ray, trap pumping, flat field extended pixel edge response and first pixel response. We proceed to a comparative analysis of the obtained results.

Euclid mission status

In June 2012, Euclid, ESAs Cosmology mission was approved for implementation. Afterwards the industrial contracts were signed for the payload module and the spacecraft prime, and the mission requirements consolidated. We present the status of the mission in the light of the design solutions adopted by the contractors. The performances of the spacecraft in its operation, the telescope assembly, the scientific instruments as well as the data-processing have been carefully budgeted to meet the demanding scientific requirements. We give an overview of the system and where necessary the key items for the interfaces between the subsystems.

The radiation environment at L2 as seen by Gaia

The radiation environment at L2 is of great importance to the science instruments of Gaia. Especially the non-ionising damage to the CCDs and the resulting increase in charge transfer inefficiency will ultimately limit the achievable science performance. With its launch in December 2013 for a nominal mission of 5 years Gaia is continuously collecting invaluable information of radiation effects on the 106 CCDs in the FPA from the analysis of the science data and dedicated calibration procedures. The paper shows first results and discusses the detected irradiation background with respect to predictions and reviews operational implications for the mission.

Laboratory simulation of Euclid-like sky images to study the impact of CCD radiation damage on weak gravitational lensing

Euclid is the ESA mission to map the geometry of the dark universe. It uses weak gravitational lensing, which requires the accurate measurement of galaxy shapes over a large area in the sky. Radiation damage in the 36 Charge-Coupled Devices (CCDs) composing the Euclid visible imager focal plane has already been identified as a major contributor to the weak-lensing error budget; radiation-induced charge transfer inefficiency (CTI) distorts the galaxy images and introduces a bias in the galaxy shape measurement. We designed a laboratory experiment to project Euclid-like sky images onto an irradiated Euclid CCD. In this way – and for the first time – we are able to directly assess the effect of CTI on the Euclid weak-lensing measurement free of modelling uncertainties. We present here the experiment concept, setup, and first results. The results of such an experiment provide test data critical to refine models, design and test the Euclid data processing CTI mitigation scheme, and further optimize the Euclid CCD operation.

Detector chain calibration strategy for the Euclid flight IR H2RGs

Euclid is an ESA mission to map the geometry of the Dark Universe with a planned launch date in 2021.1 Two primary cosmological probes, weak gravitational lensing and baryonic acoustic oscillations, are implemented through a VISible imager (VIS) and a Near-Infrared Spectrometer and Photometer (NISP).2 The ground characterization of the NISP Flight Sensor Chip Systems (SCS) followed by the pixel response calibration aims to produce all informations to correct and control the accuracy of the signal. This work reports on the ground characterization of the NISP detector chain. The detector and electrical effects are likely to generate statistical fluctuations and systematic errors on the final flux measurement. The analysis strategies to maintain the pixel relative response accuracy within 1% is proposed in this work. The Euclid NISP test ow is presented and the main concerns of the detector chain calibration, such as non-linearity, charge trapping and de-trapping are discussed on the basis of the analysis of the flight detectors characterization data.