The Charge-Coupled Device: Revolutionizing How Astronomers See the Universe
After 50 Years of Development, What’s on the Horizon?
22 April 2024
The Dark Energy Camera (pictured above), mounted at the prime focus of the Víctor M. Blanco 4-meter Telescope at the U.S. National Science Foundation Cerro Tololo Inter-American Observatory, was the largest CCD in the world when it entered service in 2012, containing a total of 74 CCDs with 570 megapixels in total.
Have you ever looked at a remarkably detailed image of a galaxy, nebula or molecular cloud and wondered how the light from that object got from interstellar space to your screen? The answer is via the power of a light-capturing chip called a charge-coupled device (CCD). These small but mighty pieces of equipment, with their ability to collect photons — the fundamental particles of light — and convert them into a digital signal, are instrumental in numerous technologies. From world-class telescopes to the phone in your pocket, CCDs play a crucial role in how humans capture, interact with and study the Universe.
The creation of the CCD was a landmark event in the evolution of science and technology, heralding the beginning of the digital imaging revolution. In October 1969 at Bell Laboratories, physicists Willard S. Boyle and George E. Smith embarked on a mission to find a more affordable way of storing bits of data in computers, which at the time was done using magnetic bubble memory. This led to the conception of the silicon-based CCD. Initially aimed at enhancing computer memory, they soon discovered the material's sensitivity to light, revealing its potential for capturing images.
The pace at which the CCD prototype came to fruition was astonishing; Smith recalls that drafting the initial concept took no more than an hour, and in less than a week they had assembled a working prototype, marking the swift birth of a technology that would transform the landscape of astronomy and digital imaging.
Understanding CCDs and their function
A CCD consists of a thin silicon wafer placed on top of a rectangular grid of pixels, each typically 20–30 microns wide — less than half the width of the average human hair. These pixels, numbering from hundreds to thousands per row, underneath the silicon layer are crucial for the device's image capturing capability. A typical CCD can range in size from 1 to 7.5 centimeters across, and when several are placed together in a mosaic they can act as one large light capturing device. For example, Vera C. Rubin Observatory’s LSST Camera, the largest digital camera ever built for astronomy, boasts 189 CCDs with 3200 megapixels in total.
When light is focused onto the CCD, photons strike the light sensitive silicon layer, dislodging electrons that are then collected in the pixels below, similarly to raindrops falling into a bucket. The number of electrons accumulated by each pixel is proportional to the intensity of light striking it, allowing the device to record images based on variations in light intensity. The signal is then passed through an analog-to-digital converter before being interpreted by a computer and displayed on a screen as a reconstructed image.
Use in astronomy
One of the earliest uses of CCD technology was its transformative application to astronomy. Historically, the challenge in astronomy has been capturing detailed images of faint and distant celestial objects. Astronomers previously depended on silver-coated photographic plates, an inefficient and time-consuming method of converting incoming photons into valuable astronomical data.
A landmark moment occurred in 1976 when Jim Janesick from NASA’s Jet Propulsion Laboratory and Brad Smith from the University of Arizona used a CCD detector to capture images of Jupiter, Saturn, and Uranus with the 61-inch Telescope on Mt. Bigelow, Arizona. The CCD’s clear superiority over photographic plates was quickly recognized, leading to their swift embrace by the field of astronomy.
CCDs offered remarkable advantages over photographic plates, such as exceptional low-light performance, a wider spectral range and the ability to quickly convert photons to electrons. These advances revolutionized astronomy by facilitating immediate data analysis and enabling practical space-based observations. The benefits afforded by CCDs led to their quickly replacing photographic plates and established their widespread use in digital photography.
CCD technology brought about a new era in optical electronics and digital imaging, being quickly integrated into various technologies. CCDs were central to the advent of the first digital and handheld video cameras and are still used in modern smartphones. In medical imaging they enhanced diagnostics with their integration into digital X-ray, mammography, and fluoroscopy devices.
CCDs also improved light detection measurements in scientific instruments such as spectrographs and microscopes and played a pivotal role in document digitization via optical character recognition systems. Furthermore, their application in the initial barcode scanners revolutionized inventory and retail management.
The next generation of CCDs
A project is underway at NSF NOIRLab to deploy an advanced kind of CCD, called a skipper CCD, for the SOAR Telescope, which is operated by the U.S. National Science Foundation Cerro Tololo Inter-American Observatory in Chile. Skipper CCDs are designed with normal CCD architecture but with the capability to achieve extremely low readout noise. This is done by reading the pixel charge multiple times during the readout process and then averaging the charges read. This reduces the electrical bandwidth of the measured charge to almost direct current, eliminating the CCD’s intrinsic noise and making it possible to detect even fainter objects.
The project has developed a four-skipper mosaic for the SOAR Telescope’s Integral Field Spectrograph. In March and April 2024 the team conducted tests of the skipper mosaic and successfully achieved extremely low readout noise. This is the first on-sky demonstration of the new-generation skipper CCD that has achieved such a feat, pointing toward significant advances in the way astronomers capture and study the Universe.
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