Imaging detectors are at the heart of many of the most challenging space instruments. Whether it is the need to ‘see’ far off galaxies, or to help predict the Earth’s weather, the ability to capture an image is an essential prerequisite for a significant proportion of the space missions currently in operation or development.

The majority of detectors for the visible, ultraviolet and near infrared wavelengths are based upon Silicon Charge Coupled Device technology. This technology, although similar in principle to that used for consumer cameras, is optimised for the highest radiometric performance for each specific instrument requirement, and of course designed to be robust in the environment of space.

The requirements for such detectors are complex, and often competing. The early stages of an instrument design has to engage in many trade-offs between the various aspects of the system and the detector, which to be effective involves the detector designer and manufacturer from the start.

Complications (nearly) always arise from the fact that the imaging detector has interface and performance requirements for the optical parts of an instrument, and the electronic and thermal parts. The cleanliness, optical properties and geometric positioning requirements are optical in nature, but the thermal, electrical and reliability requirements are those of a semiconductor component. We therefore have to deal with the problems of most of the designers involved in the instrument.

When silicon imager first began to be accepted as viable for use in spacecraft (around the early 1980s), the initial reaction of the “product assurance” community was that they should be considered as integrated circuits, and therefore subject to the same qualification requirements, but with additions specific to the optical nature of the components. It rapidly became apparent however that this approach was sound in practice, but unacceptable in many cases for cost and schedule reasons. As the state of the art progressed and the products became more exotic, the cost of manufacturing large quantities of samples and subjecting them to extensive testing programs was unsustainable, and a more specific, image sensor-focused quality regime has emerged. The details of this approach are different for the world’s major space agencies; but the core principles are similar, leaning more on extensive characterization and evaluation of base technology, with detailed environmental test activities on a relatively limited number of samples for a specific mission imager design. Even then, the provision of such bespoke imaging detectors into an operational space instrument can cost millions (of Dollars, Pound or Euros). Depending upon the complexity of the design and the model philosophy, it can also take several years to go from the initial design concept to the delivery of the completed, verified flight models for integration.

The needs of the space imaging community, whether it is for space science and astronomy, or earth observation applications are always such as to drive the capability of the technology. This, therefore always results in the common dilemma between the need to fly the next best thing, and wanting to only use technology with proven heritage and reliability. It has also meant that now detectors can look much more like optical components than semiconductor chips that happen to make pictures. This is most obviously the case for the large focal planes in space based observatories, such as the Hubble Space Telescope, the Solar Dynamics Observatory, the Kepler exo-planet finder, or the ESA Gaia Astrometry mission. The programs all involved the development of custom detectors with specific optimizations to the performance and architecture to suit the instrument requirements. They have very different mechanical configurations, which is due to the different approaches taken to the focal plane design by the system teams. As a detector supplier, e2v listens to these different requirements and constructs a program of manufacture and verification to optimize the reliability verification, cost and schedule, while maintaining the best possible imaging performance.

The Hubble Space Telescope Wide Field Camera 3, which apart from enabling a massive amount of astronomical research, also has the benefit of giving stunning images. The CCD imagers that make this possible are the CCD43-62, originally designed in the 1990s, but finally launched on the fourth service mission in 2009. These devices are optimized for ultraviolet sensitivity. Two detectors, each with 2048 x 4096 pixels are butted together to form the instrument’s focal plane. 

Figure 1. The Hubble Space Telescope Wide Field Camera 3 focal plane.

Figure 2. One of the first released Wide Field Camera 3 images.

The NASA Kepler mission is succeeding in discovering many new planets around other stars in our galaxy. This has a large and complex focal plane, constructed by Ball Aerospace of Boulder, Colorado, using specifically developed CCDs from e2v. Figure 3 shows the basic array, which is assembled into pairs, and has lenses added to make a module, shown in figure 4. These modules are then assembled into the complete focal plane, figure 5, which contains 42 of the large area CCDs. 

Figure 3. Kepler CCD in handling jig.

Figure 4. Kepler CCD Pair assembled into a module.

Figure 5. Kepler focal plane.

The Solar Dynamics Observatory (SDO) is a NASA mission, launched in February 2010. It carries several instruments to observe the sun in a variety or wavebands. For this program e2v developed a large area 4096 x 4096 pixel CCD, with optimizations to the processing for the different wavebands of interest. These ranged from the extreme ultraviolet to the visible waveband. SDO has been hugely successful in its first year of operation, with one instrument alone, the AIA (Solar Atmospheric Imaging Assembly), taking over 23 million images of the sun. is shown in figure 6.

Figure 6. An example of the e2v CCD203 used in SDO instruments.

Figure 7. SDO image of the sun.

The design, manufacture, test and qualification of imaging detectors for use in space instrumentation is challenging on many levels, from the purely technical to the logistic and communication issues that arise from dealing with multinational projects. The satisfaction however of seeing the results of these efforts in new discoveries and the spectacular images more than makes up for the day to day frustrations.