CCD Edge

CCD-based camera is the imaging tool of choice at professional observatories. Because today’s CCD cameras are affordable, even amateurs can now purchase and use them. In fact, the amateur today, with modest equipment, can routinely image objects which were once photographed only in professional observatories equipped with huge and expensive telescopes.

The CCD camera has enabled the amateur astronomer to capture esthetically-pleasing images of faint galaxies, nebula, star clusters, and other distant celestial objects. More important, amateurs are now making significant scientific contributions with their CCD-based imaging systems. All of this is possible because the CCD detector, which is the core of the CCD camera, has many advantages over other types of light detectors.

The CCD detector has many advantages that give the CCD imager an edge when capturing images of distant, faint astronomical objects. The major advantages of CCD-based cameras are their light gathering efficiency, linear behavior, cooled operation, digital images, and immediate image availability.

The CCD detector is a highly-efficient collector of photons. Quantum efficiency (QE) is the measure of a detector’s efficiency in detecting photons. More specifically, QE is the percentage of photons that are converted into a usable signal. Whereas the naked eye and emulsion film have a QE of about 1%, today’s CCD detectors have a QE of 50-85% in the visible spectrum and have a much broader spectral response than other digital or video cameras. The CCD detector’s high QE makes it possible to image an object with shorter exposures than required when using a film camera. There is less time for problems with the imaging system to degrade the final image if shorter exposures are used.

The linear characteristic of the CCD detector makes it the tool of choice for scientific imaging. The signal collected is proportional to the luminosity of the imaged objects. For scientific research, this predictability is important. For example, linearity is critical to the science of photometry. Photometry is the science of determining an object’s brightness. The CCD camera is widely used by both professionals and amateurs for photometry. In fact, the availability and affordability of today’s CCD cameras have enabled amateurs, with modest equipment, to do serious scientific research and to make their own discoveries. The linear characteristic of the CCD detector also makes it possible to remove much of the image noise.

Modern, cooled, CCD cameras do more than make it possible to remove noise. Their cooled operation directly reduces a major contributor to noise: thermal electrons. Cooling the CCD detector makes it possible to take longer images before the pixels become saturated. When CCD pixels become saturated, the excess electrons can spill into adjacent pixels causing extended bright spots in the image. This is called blooming. Cooling the detector helps to eliminate this problem by reducing the accumulation of thermal electrons. Since digital and video cameras are not cooled, noise caused by thermal electrons can seriously compromise image quality as the exposure time increases. They are not, therefore, as desirable for imaging faint objects as is the cooled CCD camera.

We live in the digital age, and the CCD camera is a tool for the times. Coupled with a personal computer, the CCD camera is a natural for astronomical imaging. Because the CCD camera produces a digital image, the images are immediately available for display, processing, or analysis. If the image is not of the desired quality, another can be taken at once. Moreover, feedback from short images can aid the imager in finding, framing, and focusing astronomical objects. In view of all of the advantages of the CCD detector and of the CCD-based camera, it is no surprise that the CCD camera has revolutionized imaging the sky. Both professional and amateur astronomers now use this tool at their telescopes. For the amateur, obtaining images of faint and distant objects–without the use of huge and expensive telescopes–is now possible. Discoveries of new objects, such as supernova, comets, and asteroids are not unusual among amateur astronomers, thanks to the CCD camera. Sky & Telescope and Astronomy routinely publish spectacular CCD images that would not have been possible a few years ago. The CCD-based camera is not only the professional’s edge; it’s the amateur’s edge, too.