Andrew D. Holland

Evaluation of the Athena/WFI instrumental background

The Wide Field Imager (WFI) is one of two focal plane instruments of the Advanced Telescope for High-Energy Astrophysics (Athena), ESA’s next large X-ray observatory, planned for launch in the early 2030’s. In the aimed orbit, a halo orbit around L2, the second Lagrange point of the Sun-Earth system the radiation environment, mainly consisting of solar and cosmic protons, electrons and He-ions, could affect the science performance. Furthermore as additional contribution the unfocused hard X-ray background is taken into account. It is important to understand and estimate the expected instrumental background and to investigate measures, like design modifications or analysis methods, which could improve the expected background level in order to achieve the challenging scientific requirement of < 5×10−3 cts/cm2/keV/s. For that purpose, the WFI background working group is investigating possible approaches, which will also be subject to technical feasibility studies. Finally an estimate of the WFI instrumental background for a proposed combination of design optimization and background rejection algorithm is given, showing that WFI is compliant with science background requirements.

The Off-plane Grating Rocket Experiment (OGRE) system overview

The Off-plane Grating Rocket Experiment (OGRE) is a sub-orbital rocket payload that will make the highest spectral resolution astronomical observation of the soft X-ray Universe to date. Capella, OGRE’s science target, has a well-defined line emission spectrum and is frequently used as a calibration source for X-ray observatories such as Chandra. This makes Capella an excellent target to test the technologies on OGRE, many of which have not previously flown. Through the use of state-of-the-art X-ray optics, co-aligned arrays of off-plane reflection gratings, and an X-ray camera based around four Electron Multiplying CCDs, OGRE will act as a proving ground for next generation X-ray spectrometers.

Optical design of the Off-plane Grating Rocket Experiment

The Off-plane Grating Rocket Experiment (OGRE) is a soft X-ray spectroscopy suborbital rocket payload scheduled for launch in Q3 2020 from Wallops Flight Facility. The payload will serve as a testbed for several key technologies which can help achieve the desired performance increases for the next generation of X-ray spectrographs and other space-based missions: monocrystalline silicon X-ray mirrors developed at NASA Goddard Space Flight Center, reflection gratings manufactured at The Pennsylvania State University, and electron-multiplying CCDs developed by the Open University and XCAM Ltd. With these three technologies, OGRE hopes to obtain the highest-resolution on-sky soft X-ray spectrum to date. We discuss the optical design of the OGRE payload.

WFIRST coronagraph detector trap modeling results and improvements

The WFIRST coronagraph is being designed to detect and characterize mature exoplanets through the starlight reflected from their surfaces and atmospheres. The light incident on the detector from these distant exoplanets is estimated to be on the order of a few photons per pixel per hour. To measure such small signals, the project has baselined the CCD201 detector made by e2v, a low-noise and high-efficiency electron-multiplying charge-coupled device (EMCCD), and has instituted a rigorous test and modeling program to characterize the device prior to flight. An important consideration is detector performance degradation over the proposed mission lifetime due to radiation exposure in space. To quantify this estimated loss in performance, the project has built a detector trap model that takes into account detailed trap interactions at the sub-pixel level, including stochastic trap capture and release, and the deferment of charge into subsequent pixels during parallel and serial clocking of the pseudo-two-phase CCD201 device. This paper describes recent detector trap model improvements and modeling results.

The CIS115: a CMOS sensor qualified for optical imaging in the Jovian environment (Conference Presentation)

The European Space Agency’s (ESA’s) Jupiter Icy Moon Explorer will spend 8 years transiting to the Jovian environment after launching from French Guiana in 2022. The spacecraft’s 10 scientific instruments, including a high resolution optical imager called JANUS, will explorer the Jovian system for a mission duration of 3 years studying the icy surfaces of Ganymede, Callisto and Europa and atmosphere of Jupiter. Using the combination of a 13 slot filter wheel and a back-illuminated CMOS image sensor, the JANUS camera will perform colour mapping and imaging at wavelengths between 350 nm and 1064 nm and resolutions of up to 10 m/pixel resolution during a Ganymede orbital phase. The CIS115 is a rolling shutter image sensor from Teledyne-e2v that has been selected for JANUS. It is back-illuminated and anti-reflection coated in order to optimise detection efficiency in its 3 MPixel imaging area. Its 4T architecture reduces the dark current in the pinned photodiode collecting area to approximately 13 pA/cm^2 at 20˚C and allows the device to be operated with correlated double sampling for a readout noise performance of 5 electrons rms. In preparation for its use in JANUS, the CIS115 has undergone a thorough qualification programme, including exposure to ionising and non-ionising radiation levels of up to 200 krad(Si) and 2x10^10 protons/cm^2 (10 MeV equivalent), and a single event effect test campaign. The CIS115 device qualification is now complete and results from the radiation test campaigns are being used to predict the expected performance at various phases of the mission as radiation damage is accumulated in the sensor. Dark current is the primary performance characteristic that has been observed to degrade with irradiation, and predicting the device’s performance at the end of life allows the maximum operating temperature of the detector to be set and justified. Additionally, behaviour observed during the qualification testing has led to optimised readout schemes that reduce the device image lag performance across the dynamic range to below the 0.1% level.

The operational characteristics and potential applications of a low voltage EMCCD in a CMOS process

The Electron Multiplying Test Chip 1 (EMTC1) was developed with the aim of creating a device which could produce superior Electron Multiplication (EM) gain at a greatly reduced voltage. An EM gain exceeding 3% per stage has been recorded for a relatively low voltage (~13.0V) from two recently developed pixel structures. An electro-optical characterisation of the EMTC1 is presented focusing on charge transfer via experimental and simulation results aiming to provide insight into the transfer and multiplication process. The Charge Transfer Inefficiency (CTI) is analysed with the aim of providing a greater understanding of the charge transfer process. Light starved applications such as Earth observation and automated inspection are known to benefit from Time Delay Integration (TDI) and electron multiplication. Though traditionally implemented in CCDs, implementing TDI in CMOS technology can lead to an increase of functionality, higher readout speeds and reduced noise. This paper presents a discussion of the implication of these results on the potential applications of this sensor.

Image lag optimisation in a 4T CMOS image sensor for the JANUS camera on ESA's JUICE mission to Jupiter (Conference Presentation)

The CIS115, the imager selected for the JANUS camera on ESA’s JUICE mission to Jupiter, is a Four Transistor (4T) CMOS Image Sensor (CIS) fabricated in a 0.18 µm process. 4T CIS (like the CIS115) transfer photo generated charge collected in the pinned photodiode (PPD) to the sense node (SN) through the Transfer Gate (TG). These regions are held at different potentials and charge is transferred from the potential well under PPD to the potential well under the FD through a voltage pulse applied to the TG. Incomplete transfer of this charge can result in image lag, where signal in previous frames can manifest itself in subsequent frames, often appearing as ghosted images in successive readouts. This can seriously affect image quality in scientific instruments and must be minimised. This is important in the JANUS camera, where image quality is essential to help JUICE meet its scientific objectives. This paper presents two techniques to minimise image lag within the CIS115. An analysis of the optimal voltage for the transfer gate voltage is detailed where optimisation of this TG “ON” voltage has shown to minimise image lag in both an engineering model and gamma and proton irradiated devices. Secondly, a new readout method of the CIS115 is described, where following standard image integration, the PPD is biased to the reset voltage level (VRESET) through the transfer gate to empty charge on the PPD and has shown to reduce image lag in the CIS115.

Soft x-ray imaging with a newly designed large-area CCD (Conference Presentation)

SMILE (Solar wind Magnetosphere Ionosphere Link Explorer) is a joint venture between the European Space Agency (ESA) and the Chinese Academy of Sciences (CAS). The mission will study the dynamic interaction between the solar wind and the Earth’s magnetosphere. Two of the instruments, namely the Soft X-ray Imager (SXI) and the Ultra-Violet Imager (UVI), will observe northern aurora and the boundary of the magnetosphere simultaneously, a first for space-missions. To complement these remote observations, in-situ measurements of the solar wind ion distribution as well as measurements of the strength of magnetic fields will be attained via the Light Ion Analyser (LIA) and the Magnetometer (MAG) respectively. Together, these four instruments will provide a complete picture of the interactions between the solar wind and the response of the Earth’s magnetosphere, which is the main driver of space-weather. The SXI will be used to observe and image Solar Wind Charge eXchange (SWCX) that occurs at the interface between the solar wind and the Earth’s magnetosphere. At this location, ions in the solar wind interact with neutrals in the Earth’s exosphere leading to the production of soft X-rays with key emission lines at energies between 0.1-2 keV. The SXI will use a wide angle silicon micro pore optic to focus the incoming X-rays onto a focal plane of two large area Charge-Coupled Devices (CCDs) from Teledyne-e2v. The CCD for SXI is designated the CCD370, with 4510x4510 pixels of 18 µm, which will be read out with 6x6 on-chip binning to give an effective pixel size of 108 µm square. The binning improves charge transfer efficiency and energy resolution, and allows the pixel area to be divided into asymmetric frame and store regions of the device for frame-transfer operation. The CCD370 design includes a range of modifications from its predecessor (the CCD270 from the PLATO mission), with the goal of optimising it for imaging soft X-rays; a supplementary buried channel in the parallel region, a narrowed serial channel width, and increased output amplifier responsivity will improve the low signal sensitivity and charge transfer efficiency. Here, the CCD370 performance in the SXI telescope will be presented, predicted from the first measurements using the laboratory SXI CCD characterisation camera and CCD270s. The improvements expected from design changes that optimise the SXI CCDs for soft X-ray detection, and plans for pre-flight calibration and radiation damage testing will be described.

Photon counting EMCCD developments for the WFIRST coronagraph (Conference Presentation)

A photon counting camera based on the Teledyne-e2v CCD201-20 electron multiplying CCD (EMCCD) is being developed for the NASA WFIRST coronagraph, an exoplanet imaging technology development of the Jet Propulsion Laboratory (Pasadena, CA) that is scheduled to launch in 2026. The coronagraph is designed to directly image planets around nearby stars, and to characterize their spectra. The planets are exceedingly faint, providing signals similar to the detector dark current, and require the use of photon counting detectors. Red sensitivity (600-980nm) is preferred to capture spectral features of interest. EMCCDs are baselined both as science and wavefront sensors in the coronagraph in order to simplify the system architecture. We are engaged in a test program to characterize the performance of the EMCCD in the required modes, as well as in a technology development program with Teledyne-e2v to ruggedize the sensors for use in space. In this paper we will summarize our progress, program status, and plans for flight development.