Jason Gow

VIS: the visible imager for Euclid

Euclid-VIS is the large format visible imager for the ESA Euclid space mission in their Cosmic Vision program, scheduled for launch in 2020. Together with the near infrared imaging within the NISP instrument, it forms the basis of the weak lensing measurements of Euclid. VIS will image in a single r+i+z band from 550-900 nm over a field of view of ~0.5 deg2. By combining 4 exposures with a total of 2260 sec, VIS will reach to deeper than mAB=24.5 (10σ) for sources with extent ~0.3 arcsec. The image sampling is 0.1 arcsec. VIS will provide deep imaging with a tightly controlled and stable point spread function (PSF) over a wide survey area of 15000 deg2 to measure the cosmic shear from nearly 1.5 billion galaxies to high levels of accuracy, from which the cosmological parameters will be measured. In addition, VIS will also provide a legacy dataset with an unprecedented combination of spatial resolution, depth and area covering most of the extra-Galactic sky. Here we will present the results of the study carried out by the Euclid Consortium during the period up to the Critical Design Review.

Determination of in situ trap properties in CCDs using a "single-trap pumping" technique

The science goals of space missions from the Hubble Space Telescope through to Gaia and Euclid require ultraprecise positional, photometric, and shape measurement information. However, in the radiation environment of the space telescopes, damage to the focal plane detectors through high-energy protons leads to the creation of traps, a loss of charge transfer efficiency, and a consequent deterioration in measurement accuracy. An understanding of the traps produced and their properties in the CCD during operation is essential to allow optimization of the devices and suitable modeling to correct the effect of the damage through the postprocessing of images. The technique of “pumping single traps” has allowed the study of individual traps in high detail that cannot be achieved with other techniques, such as deep level transient spectroscopy, whilst also locating each trap to the subpixel level in the device. Outlining the principles used, we have demonstrated the technique for the A-center, the most influential trap in serial readout, giving results consistent with the more general theoretical values, but here showing new results indicating the spread in the emission times achieved and the variation in capture probability of individual traps with increasing signal levels. This technique can now be applied to other time and temperature regimes in the CCD to characterize individual traps in situ under standard operating conditions such that dramatic improvements can be made to optimization processes and modeling techniques.

Mitigating radiation-induced charge transfer inefficiency in full-frame CCD applications by 'pumping' traps

The charge transfer efficiency of a CCD is based on the average level of signal lost per pixel over a number of transfers. This value can be used to directly compare the relative performances of different structures, increases in radiation damage or to quantify improvements in operating parameters. This number does not however give sufficient detail to mitigate for the actual signal loss/deference in either of the transfer directions that may be critical to measuring shapes to high accuracy, such as those required in astronomy applications (e.g. for Gaia’s astrometry or the galaxy distortion measurements for Euclid) based in the radiation environment of space. Pocket-pumping is an established technique for finding the location and activation levels of traps; however, a number of parameters in the process can also be explored to identify the trap species and location to sub-pixel accuracy. This information can be used in two ways to increase the sensitivity of a camera. Firstly, the clocking process can be optimised for the time constant of the majority of traps in each of the transfer directions, reducing deferred charge during read out. Secondly, a correction algorithm can be developed and employed during the post-processing of individual frames to move most of any deferred signal back into the charge packet it originated from. Here we present the trap-pumping techniques used to optimise the charge transfer efficiency of p- and n-channel e2v CCD204s and describe the use of trap-pumped images for on-orbit calibration and ground based image correction algorithms.

Technology advancement of the CCD201-20 EMCCD for the WFIRST coronagraph instrument: sensor characterization and radiation damage

Abstract. The Wide Field InfraRed Survey Telescope-Astrophysics Focused Telescope Asset (WFIRST-AFTA) mission is a 2.4-m class space telescope that will be used across a swath of astrophysical research domains. JPL will provide a high-contrast imaging coronagraph instrument—one of two major astronomical instruments. In order to achieve the low noise performance required to detect planets under extremely low flux conditions, the electron multiplying charge-coupled device (EMCCD) has been baselined for both of the coronagraph’s sensors—the imaging camera and integral field spectrograph. JPL has established an EMCCD test laboratory in order to advance EMCCD maturity to technology readiness level-6. This plan incorporates full sensor characterization, including read noise, dark current, and clock-induced charge. In addition, by considering the unique challenges of the WFIRST space environment, degradation to the sensor’s charge transfer efficiency will be assessed, as a result of damage from high-energy particles such as protons, electrons, and cosmic rays. Science-grade CCD201-20 EMCCDs have been irradiated to a proton fluence that reflects the projected WFIRST orbit. Performance degradation due to radiation displacement damage is reported, which is the first such study for a CCD201-20 that replicates the WFIRST conditions. In addition, techniques intended to identify and mitigate radiation-induced electron trapping, such as trap pumping, custom clocking, and thermal cycling, are discussed.

Assessment of space proton radiation-induced charge transfer inefficiency in the CCD204 for the Euclid space observatory

Euclid is a medium class European Space Agency mission candidate for launch in 2019 with a primary goal to study the dark universe using the weak lensing and baryonic acoustic oscillations techniques. Weak lensing depends on accurate shape measurements of distant galaxies. Therefore it is beneficial that the effects of radiation-induced charge transfer inefficiency (CTI) in the Euclid CCDs over the course of the 5 year mission at L2 are understood. This will allow, through experimental analysis and modelling techniques, the effects of radiation induced CTI on shape to be decoupled from those of mass inhomogeneities along the line-of-sight. This paper discusses a selection of work from the study that has been undertaken using the e2v CCD204 as part of the initial proton radiation damage assessment for Euclid. The experimental arrangement and procedure are described followed by the results obtained, thereby allowing recommendations to be made on the CCD operating temperature, to provide an insight into CTI effects using an optical background, to assess the benefits of using charge injection on CTI recovery and the effect of the use of two different methods of serial clocking on serial CTI. This work will form the basis of a comparison with a p-channel CCD204 fabricated using the same mask set as the n-channel equivalent. A custom CCD has been designed, based on this work and discussions between e2v technologies plc. and the Euclid consortium, and designated the CCD273.

Charge-coupled devices for the ESA Euclid M-class mission

The European Space Agency has funded e2v’s development of an image sensor for the visible instrument in the Euclid space telescope. Euclid has been selected for a medium class mission launch opportunity in 2020. The project aims to map the dark universe with two complementary methods; a galaxy red-shift survey and weak gravitational lensing using near infrared and visible instruments. The baseline for the visible instrument was to be the CCD203-82, which has been successfully flown on NASA’s Solar Dynamics Observatory. However, to optimise the device for Euclid, e2v have designed and manufactured the CCD273-84. This device has a higher-responsivity lower-noise amplifier, enhanced red response, parallel charge injection structures and narrower registers which improve low signal charge transfer efficiency. Development models for Euclid have been manufactured with a thinner gate dielectric than standard for improved tolerance to ionising radiation. This paper describes the imager sensor in detail and focuses on the novel aspects of the device, package and interface.

Optimization of Device Clocking Schemes to Minimize the Effects of Radiation Damage in Charge-Coupled Devices

The European Space Agency Euclid mission aims to answer the question of how the universe originated through the mapping of the dark Universe. One method to investigate this geometry is to measure subtle changes in ellipticity using image sensors such as the charge-coupled device (CCD). However, the radiation environment in space plays a major part in the performance of CCD-based camera systems. When placed in space, a CCD becomes damaged by the radiation environment, and this can lead to a “smearing” of the charge, acting to change the ellipticity, and therefore, one must be able to separate the changes in ellipticity caused by radiation damage from those the mission aims to measure. To this end, the radiation-damage-induced shape changes require an in-depth investigation such that optimized operation can be achieved. A Monte Carlo simulation is being used to predict this impact, backed by experimental data from a detector formerly baselined for the mission. During the experimental study, an investigation was undertaken into the serial readout of the CCD to demonstrate an approach toward performance optimization through a consideration of the trap species involved. A change in the clocking scheme was found to result in a factor of 3 reduction in charge transfer inefficiency.

Multi-level parallel clocking of CCDs for: improving charge transfer efficiency, clearing persistence, clocked anti-blooming, and generating low-noise backgrounds for pumping

A multi-level clocking scheme has been developed to improve the parallel CTE of four-phase CCDs by suppressing the effects of traps located in the transport channel under barrier phases by inverting one of these phases throughout the transfer sequence. In parallel it was apparent that persistence following optical overload in Euclid VIS detectors would lead to undesirable signal released in subsequent rows and frames and that a suitable scheme for flushing this signal would be required. With care, the negatively biased electrodes during the multi-level transfer sequence can be made to pin the entire surface, row-by-row, and annihilate the problematic charges. This process can also be extended for use during integration to significantly reduce the unusable area of the detector, as per the clocked anti-blooming techniques developed many years ago; however, with the four-phase electrodes architecture of modern CCDs, we can take precautionary measures to avoid the problem of charge pumping and clock induced charge within the science frames. Clock induced charge is not all bad! We also propose the use of on-orbit trap-pumping for Euclid VIS to provide calibration input to ground based correction algorithms and as such a uniform, low noise background is require. Clock induced charge can be manipulated to provide a very suitable, low signal and noise background to the imaging array. Here we describe and present results of multi-level parallel clocking schemes for use in four-phase CCDs that could improve performance of high precision astronomy applications such as Euclid VIS.

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.

Proton irradiation of a swept charge device at cryogenic temperature and the subsequent annealing

A number of studies have demonstrated that a room temperature proton irradiation may not be sufficient to provide an accurate estimation of the impact of the space radiation environment on detector performance. This is a result of the relationship between defect mobility and temperature, causing the performance to vary subject to the temperature history of the device from the point at which it was irradiated. Results measured using Charge Coupled Devices (CCD) irradiated at room temperature therefore tend to differ from those taken when the device was irradiated at a cryogenic temperature, more appropriate considering the operating conditions in space, impacting the prediction of in-flight performance. This paper describes the cryogenic irradiation, and subsequent annealing of an e2v technologies Swept Charge Device (SCD) CCD236 irradiated at −35.4°C with a 10 MeV equivalent proton fluence of 5.0 × 108 protons centerdot cm−2. The CCD236 is a large area (4.4 cm2) X-ray detector that will be flown on-board the Chandrayaan-2 and Hard X-ray Modulation Telescope spacecraft, in the Chandrayaan-2 Large Area Soft X-ray Spectrometer and the Soft X-ray Detector respectively. The SCD is readout continually in order to benefit from intrinsic dither mode clocking, leading to suppression of the surface component of the dark current and allowing the detector to be operated at warmer temperatures than a conventional CCD. The SCD is therefore an excellent choice to test and demonstrate the variation in the impact of irradiation at cryogenic temperatures in comparison to a more typical room temperature irradiation.