Astronomers have embraced silicon image sensors in the form of charge coupled devices (CCDs) since they were first invented, according to e2v. In fact, CCD inventors received a 2009 Nobel Prize in physics for the device that has famously delivered images from the Hubble Space Telescope. Virtually all ground-based observatories and many space telescopes now use this sort of image sensor for recording images from X-ray to near IR wavelengths.
Silicon image sensors considerably exceed the sensitivity of the original photographic plate and allow direct electronic image capture and the means to send electronic frames remotely. This technology has enabled many aerospace applications including scientific image recording, auto-guiding activities (such as the International Space Station ATV), and very high sensitivity spectrographs for measuring some of the universe’s most distant objects.
One example of the use of a CCD sensor is within the NASA Kepler spacecraft, which is currently returning data in the search for Earth-like extra-terrestrial planets. Distant planets are exceptionally hard to detect, and Earth-sized ones can only be effectively detected by using a space-based telescope.
The Kepler mission was designed just to do this one job very well and has a set of 42 large-area CCDs in a mosaic focal plane. This is the largest focal plane so far used in space for optical imaging and delivers 95 Megapixel image data as it observes an area of sky with over 100,000 stars. It is an ultraprecise photometer that measures the tiny decrease in a star’s brightness that occurs when a planet crosses in front of it.
Since its launch in 2009, Kepler has found thousands of candidate exo-planets. As the mission continues the data set will grow, and after further painstaking analysis the Kepler team expect to dramatically increase the number of known Earth-like planets. So far, 16 confirmed exo-planets have been announced.
The ambitious ESA GAIA mission is designed for a different purpose. When launched in early 2013, it will produce a 3-D map of around 1 billion objects in the galaxy more precisely than ever before, says e2v. This scope of measurement can only be performed in the clarity of space with an ultrastable instrument, which is designed to operate for five years.
The focal plane exceeds the size of Kepler with 106 CCDs designed for astrometric, photometric, and radial velocity measurements to record the positions and motions of 1 billion stars throughout its mission. This will deliver the most detailed map of the Milky Way ever recorded to aid in the understanding of star formation and evolution of our galaxy.
The sensors operate synchronously in TDI (time-delay-integrate) mode within a near Gigapixel mosaic. The individual sensors are placed in a precision high-stability silicon carbide package within each focal plane. Extreme care is taken in the sensor and instrument design to allow measuring stars with a positional accuracy down to 10 micro-arcseconds (for perspective, that is about the size of a U.S. dime on the moon).
The proposed ESA PLATO mission is currently being studied to offer an order-of-magnitude improvement over the Kepler mission. This mission has the aim of determining the conditions for planet formation and the emergence of life.
When launched in 2017-2018, PLATO will use an even larger set of 136 silicon sensors to deliver nearly 3 Gigapixels of image data and will further expand knowledge of the universe over its 5-year mission. The instrument concepts that are currently being studied and the heritage and performance of silicon image sensors mean that this mission is considered viable even with such a large set of sensors.
When supplying custom sensors for astronomy and space applications, the design of the sensor package can be as challenging as that of the silicon sensor itself. Much semiconductor design effort is expended to ensure the highest levels of device performance, which is complemented by the provision of precise, stable, and space-qualified packages.
The example missions described above include close-butted packages (for high focal plane filling efficiency), precision surface flatness (to maintain focus), design for cryogenic temperatures, and many other features to facilitate use in these demanding applications.
So far, most high-performances imaging systems have used the silicon CCD because of excellent sensitivity and well-established aerospace heritage. However, the active pixel sensor (APS, or CMOS imager) is widely known for examples such as cell phone cameras and has advantages in some applications. Specialist image sensor suppliers, including e2v, usually now include the APS device in their portfolio. These devices can be backthinned to provide similar sensitivity to that of CCDs.
This type of device uses less power and can have considerable embedded functionality. In particular, it is often the sensor of choice where high frame rates (combined with large pixel formats) are important. It is now possible to design Megapixel APS devices running at 1000 frames per second with sensitivity levels comparable to that of CCDs.
Expect more such devices to come into use within the next decade, although for many applications the CCD is still expected to have an important role.
This article was written for Aerospace Engineering by Paul Jorden, Technical Specialist, High Performance Imaging, e2v.