Impact case study database

The Department of Space and Climate Physics (also known as the Mullard Space Science Laboratory, or MSSL) at UCL includes an Instrument Science Group that develops high-specification cameras for use on scientific spacecraft and undertakes fundamental research in the field of imaging sensors, including CCDs. CCDs are optical sensors that take the form of a two-dimensional pixelated array. They have revolutionised the acquisition of image information; for example, modern digital cameras are based on devices of this kind. The success of scientific applications is often critically dependent on the precise response of CCDs in their expected environment. For space science applications, this environment is a satellite operating in the harsh conditions of space.

The MSSL group has worked closely with CCD manufacturer e2v Ltd, (T-e2v from 2017) for many years on a programme of CCD characterisation and modelling ( R1-R5). This partnership

involves the detailed design and manufacture of CCDs by T-e2v, followed by the scientific evaluation and characterisation of the devices in specialist facilities at MSSL. This characterisation is within the context of performance and process models, and includes aspects such as noise sources and electron mobility, diffusion and loss. Test data are simulated and interpreted to quantify underlying device properties, and device performance is estimated. The models that are used have been either developed at MSSL or adapted from those in the published literature. The results and evaluations from the research at MSSL informed T-e2v’s next generation of devices and their optimal use. The programme has so far involved the characterisation of more than 300 T-e2v’s CCDs.

The collaborative studies function within a virtuous circle in which research insights relating to the improvement of CCDs are shared and understanding builds from project to project. Insights that have resulted from the programme of research are in areas including the physical processes of the devices, such as electron mobility and diffusion, noise sources, linearity, electron loss mechanisms, sensitivity, temperature dependence and set voltage dependence; camera design optimisation; device specification; and CCD data interpretation (calibration) during data analysis. For example, the MSSL group showed how linearity of response could be increased beyond full-well capacity through pixel integration, and the point spread function dependency on wavelength gave an insight into the electron diffusion in the drift region of the CCDs.

Much of this CCD research was conducted as part of studies of future space mission concepts, of which there are many more than actual space missions, due to the way that technical risk is mitigated, and selections are made. Such studies are very comprehensive and are often competitive against other mission concepts. The lists of studies that MSSL has been involved with that have led to improved understanding of CCDs is presented in Table 1.

In many of the projects, a customer that is an expert in detector technology was involved, working with MSSL to optimise mission performance (for example, Lockheed Martin for GOES SXI ( R1) and Hinode FPP; and ESA for Eddington ( R2, R3), Euclid, Gaia ( R4, R5) and PLATO). The mode of collaborative working through customer(ESA)/supplier(T-e2v)/science groups (including MSSL) working groups developed for Gain proved highly effective for Euclid and has been adopted for PLATO (chaired by Professor Smith at MSSL).

For actual flight missions the emphasis was on device specification, device characterisation, identifying optimal operating conditions, screening and preparation for exploitation. Important developments that have formed part of this work include the examination of back illumination in CCDs that had been thinned to create extreme ultraviolet sensitivity, ( R1) and the evaluation of novel concept L3CCDs (a new type of CCD).

Table 1. Examples of studies in which MSSL has been involved with and that have led to improved understanding of CCDs.