Optical Detectors

Type of physical science: Astronomy; Astrophysics

Field of study: Observational techniques

Optical detectors are instruments that record light received from a source. A detector normally includes optics that shape and manipulate a beam of light for projection against the actual sensing surface, and the associated electronics to deliver the signal for interpretation. Detectors fall into two general categories: photographic emulsions and electronic sensors.

Overview

Optical detectors are instruments that sense or record images and data from visible and near-visible light collected and focused by telescopes or other devices. Since the electromagnetic spectrum is continuous--from radio waves to γ rays--the definition of "visible" or "optical" is somewhat arbitrary because only a portion of this continuous spectrum interacts readily enough with matter to give rise to organs sensitive to it. "Visible" means light that can be sensed by the human eye, the first instrument humans used to detect light and to discern the color and position of stellar objects and the one emulated in building early detectors. Generally, "visible" is defined as spanning a wavelength range from 400 nanometers (violet) to 700 nanometers (red).

The exact cutoffs vary with the individual. Nevertheless, man-made detectors have broader spectral responses than the human eye, so optical astronomy is defined by light admitted by the earth's "optical window," from 295 nanometers (ultraviolet, set by oxygen absorption) to 1,100 nanometers (infrared, set by water absorption). Nearly 100 percent of light in this band passes through the atmosphere and thus is available to astronomers on the ground. Even this definition is becoming blurred, though, with the use of space-based telescopes that can see the entire spectrum without filtration.

While one may think in terms of telescopes as being the only optical system used in astronomy, the detectors themselves often have quite complex optical systems to manipulate, then deliver a beam of light to where it causes a change in the electrical or chemical state of the detector material. This change is recorded in some fashion and then analyzed to produce information about the object that emitted the light. What is known as a detector will vary with the context of the discussion. For the telescope operator, it will be whatever box the investigator brings. For the investigator, it is the sensor material at the end of the optical path within the box.

An important consideration in designing detectors is an understanding of the photoelectric effect and its related phenomena. The photoelectric effect establishes the minimum energy level that a photon (discrete particle of light) must have to liberate an electron from an atom. If the incident photons are too low in energy (long wavelength), then no electrons will be freed, regardless of the intensity. Above the minimum energy, electrons may be raised in energy, then fall as they release new photons (sometimes at lower-energy levels), or be emitted altogether by the atom. The requisite wavelength will vary with the type of material being used and whether it has been electrically charged.

The human eye, the first detector used behind the telescope, uses the photoelectric effect in its retinal cells. While human vision is a marvelous phenomenon, however, it is inefficient. It has a quantum efficiency of only 1 percent (that is, 99 percent of the incident photons are undetected). Further, the interpretation of images through the brain and hand is highly subjective, even with trained observers, and the retina is virtually incapable of building a time exposure.

Photography provided the first means of preserving the scene presented by a telescope.

Photography uses light-sensitive emulsions on a stable base. Light striking the emulsion will cause a change in the chemical state of the materials. When the emulsion is developed, the change in the emulsion is fixed so that it resists further changes, and the unexposed areas are washed away. The result is a pattern whose density matches the intensity of the light projected upon it. Astronomical photography often uses glass plates because of their stability, but plastic-base films are popular where a series of pictures is to be taken and there is no convenient way to change plates.

Film has a quantum efficiency of 1 to 4 percent and is sensitive to a spectral range spanning roughly 300 to 800 nanometers (the higher efficiencies and broader spectral range are bought at the cost of sophisticated chemistry to expand the film's slight advantage over the eye).

Color films are actually layers of three emulsions (to correspond with human three-color vision), so astronomical photography normally uses black-and-white film combined with filters that pass only a selected band of the light spectrum. Color pictures are generated by projecting two or more black-and-white images through the appropriate filters onto color film.

The chief advantage of film is that it can be exposed for a long period of time--literally all night long--to make an image. Once developed and fixed, the film plate provides a stable record of the sky that can be compared even years later.

Electronic sensors have revolutionized astronomy by providing a means of directly converting light from stars into electronic signals that can be analyzed as they are received. The first useful electronic sensors were photomultiplier tubes, which enclose a series of electrically charged plates in a vacuum tube. A single photon striking the first plate will cause a cascade of electrons that increases as each subsequent plate is hit and thus produces a measurable electrical current that can be directly correlated to the trickle of light from a faint star. Variations of the photomultiplier tubes are photon counters that can record individual photon arrivals. Television cameras also can be used, but they suffer from "bloom" and "smearing" (easily seen when a camera pans past a bright light) that can distort images. Further, they are electronically "noisy" and their sensitivity is not as good as film, so the use of television in astronomy has been quite limited.

Solid-state imaging devices improved many astronomical techniques in the 1970's and 1980's. In particular, the charge-coupled device (CCD) was rapidly adopted by all astronomers.

The CCD may be considered an electronic analog to the human retina. It contains a two-dimensional array (like boxes on graph paper) of light sensors that are read out by a computer. After an image has been recorded, the first row of detectors is read out. Each cell dumps its charge into the next cell, from one end to the next, and the register reads the level of the charge and sends it out as a digital signal. When the first row is completed, the second row is read, and so on. This process can be accomplished in a millisecond or so, depending on the design. The process has been compared to recording the rainfall on a plantation by laying out a grid of buckets that tip over in sequence to pass water down the line to a gauge. Like the buckets, though, a little of the charge can be left from one cell to another and can cause a linelike smear in a CCD image. The CCD itself must also be exposed to a strong light source to raise all the cells to the same charge level so that each CCD starts from the same point for the next exposure.

When used in astronomy, CCD cameras usually have shutters so that the exposure time can be set precisely and the image read without blurring.

Two chief advantages of CCDs are their quantum efficiency and spectral range. The CCD quantum efficiency ranges from 10 to better than 90 percent, depending on the design of the device and the region of the spectrum being sampled. Although the quantum efficiency is not equal across the spectrum, the response of each cell in a CCD can be measured and adjustments factored into its observations (even dead spots in the CCD can be compensated for by scene-averaging techniques). The CCD spectral range is approximately 300 to 1,100 nanometers, which roughly matches the "optical window." CCDs also have a much greater dynamic range than the human eye or film. Therefore, they can record the intensity of faint and bright objects that may be side by side. Film exposures have to be set for bright objects at the expense of faint ones (or the reverse), so multiple exposures may be required to bring out all the details in a scene.

The first commercial CCDs were simple, low-resolution devices composing an array of only one hundred rows by one hundred columns of picture elements (or, 100 x 100 pixels). It was invented for the AT&T Picturephone but quickly found applications in military cameras and other systems where small size was critical. By the late 1970's, the market had expanded to arrays of 320 x 512 pixels (to match broadcast television) and 512 x 512 pixels. In astronomy, the major advance came from Texas Instruments, which was able to produce 800 x 800 pixel CCDs. By the late 1980's, Tektronix was delivering CCDs with 2,048 x 2,048 pixels. In addition, CCDs have also been available in linear designs (that is, a single strip of detectors) that are useful in spectroscopy.

In spectroscopy, additional optics are needed to take light from a select point in the sky and direct it to a dispersion grating or through a prism. In either case, the different photons are reflected (gratings) or refracted (prisms) according to their wavelength. This new beam of light may then be focused upon a strip of film or an electronic detector may be moved in an arc to measure intensity along its length. In the early 1970's, linear CCDs were popular as "one-dimensional" spectral detectors. These detectors are largely being replaced by two-dimensional arrays. An interesting application in spectroscopy uses a slit to block all but a selected wavelength of light, then refocuses the light projected on an image of the scene in that one wavelength. If one tilts the grating with respect to the slit, one can scan different wavelengths past the detector and build a catalog of images in several wavelengths.

Applications

Film and electronic detectors have been widely used to great advantage in astronomy both for imaging and spectroscopy. Film was the first to be used and continues to enjoy wide use in several areas. Despite its slow speed and narrow sensitivity (compared to electronics), film is still the best detector for producing images of large areas of the sky. In essence, film plates can be manufactured to match the size of any telescope focal plane (36 x 36 centimeters is the largest standard), and film is the easiest way to record large quantities of data, although much of it is in analog form (that is, the densities of the emulsion that must be measured). Film is used in Schmidt cameras to make all-sky surveys or photograph rich starfields. Film is also used on suborbital and manned astronomy missions, where the data return to Earth is limited. The Spacelab 2 mission, for example, used film cameras in a solar telescope designed to make a detailed "white light" movie of activities on the surface of the sun and to measure magnetic fields. The volume of data that would have to be transmitted to the ground was beyond the capability of the spacecraft. Suborbital rockets and balloons, which are valuable tools for placing telescopes (briefly) above the atmosphere, lack virtually any data transmission capability, so film must be used to record the scene. Nevertheless, many of these films are subjected to analysis by electronic sensors on the ground after a human eye has selected areas of interest.

From the 1960's through the 1980's, most space-based astronomy involved those regions of the spectrum that are obscured by the atmosphere; the first optical astronomy telescope, launched in 1990, was the Hubble Space Telescope, which subsequently experienced technical problems. The Hubble carries eight instruments mounted behind its primary mirror.

Three of these instruments are fine guidance sensors that provide precision pointing instructions; the other five are science instruments that represent many of the basic types now in use: wide field/planetary and faint object cameras, high-resolution and faint object spectrometers, and the high-speed photometer. Only the wide field/planetary camera uses CCDs (eight in total) because the technology was still new and risky at the time its design was proposed in 1977. The camera operates in two modes to produce images of the planets or starfields. In either case, the incoming light is split by a pyramid prism into four miniature Cassegrain telescopes, which refocus the light onto one CCD behind each telescope. The four images, each 800 x 800 pixels in size, can be reconstructed into a single image 1,600 x 1,600 pixels (about sixteen times the information in a single conventional television picture). Three sets of filter wheels allow the investigator to select the colors that will be observed and thus build highly detailed, multicolored images of a scene. The other Hubble instruments use vacuum tube detectors common to the early 1970's. The faint object camera is designed to capture details of faint, distant objects in the universe using an image intensifier in front of a television camera. The two spectrometers are similarly complementary, producing spectra of bright or faint objects. The high-resolution spectrometer uses CCDs, while the faint object spectrometer uses Digicon photomultiplier tubes that count the incoming photons. The photometer allows precision measurements of changes in brightness over periods of seconds or months.

The speed of CCDs in creating images is allowing scientists to compensate for atmospheric distortion and enhance the resolution of ground-based telescopes. Since very short integration times are possible with bright objects, a CCD can be read several times per second.

The distortion in the scene can be measured and an algorithm (or rule) applied somewhat like an electronic lens to compensate for the atmosphere's motion. A series of images can then be combined to produce a better record of the object's shape or its position relative to other stars.

Another advantage of CCD cameras over earlier devices is that they are small and light enough that several instruments can be mounted on a telescope, and they require comparatively less servicing. With proper housing, they can be used with minimal retests from night to night.

Many applications of CCDs involve spectroscopy, since the CCD can provide fine measurements quickly. In spectroscopy, the incident light is spread by a prism or grating (sometimes several of these or combinations of both) so that individual wavelengths fall at different points on the focal plane. Various optical techniques are used to magnify and refocus light, but the basic effect is the same as passing sunlight through a prism to get a rainbow. Once the light falls on the detector face, it can be measured and a graph quickly produced. Many linear CCDs have been used in this fashion, but in the 1980's astronomers started using two-dimensional arrays to take the spectra of several objects at once. An innovative approach to this method uses a metal plate that is punctured with the ends of the fiber-optic lines (essentially quartz threads) in positions that match a starfield to be studied. The optical fibers act as a light pipe carrying the light from the telescope proper to a slit shining on a spectral grating, which focuses the light onto a CCD. In effect, a random array of stars is made to line up, so thirty to forty spectra can be recorded at once. Alternately, a slit can be scanned across an extended object (like a galaxy) and the spectra of each point on the slit taken so that the object is measured in dozens of wavelengths at once.

One highly attractive feature of CCDs is the ability to take the image directly from the chip and run it into a computer for detailed analysis. Since each pixel on the CCD becomes a digitized number, it can be manipulated directly by the computer while preserving the integrity of the original data recording. For example, a computer can be commanded to show details of an object having a narrow brightness range while leaving out the others, or to map brightness contours, or to show details of a bright galactic core and of the faint arms. Multiple images taken through different filters can be overlaid, registered, and combined as false-color images to enhance details that might escape the eye. This is especially useful where a CCD was used in the spectral mode and gathered several hundred channels of data at one time.

Context

Optical detectors have made optical astronomy possible as a highly productive and quantitative science by providing a means for objectively measuring the radiation emitted by the stars. Although space-based astronomy ultimately may displace ground-based observatories, optical astronomy will always be the centerpoint of astronomy as a whole because it is through the optical window that humans first perceived the universe and through which all data must be converted and passed so that it may be understood.

Optical astronomy got its real start in the 1600's when Galileo manufactured his own telescope and pointed it at the heavens. His discovery of mountains and valleys on Earth's moon and of smaller moons orbiting Jupiter inspired generations of observers, who quickly discovered the rings of Saturn and many other phenomena.

For its first two hundred years, optical astronomy depended heavily upon the human eye as the only detector, with the brain and hand as the only interpreters and recorders of data.

The development of the spectroscope in the early 1800's allowed astronomers to start their first chemical assays of the stars and showed them to be made of the same materials that are found on Earth. These lines, however, had to be traced or drawn by hand.

The advent of photography sparked a radical change in astronomy as it allowed long-duration exposures to be made and for the image to be recorded objectively. Most important, photography allowed the imaging of extended, diffuse objects whose central bodies astronomers had been able to see only dimly, and for the spectrum to be imaged for more accurate and leisurely analysis. Where telescopes had expanded humankind's concept of the solar system and showed the planets to be worlds in their own right, film plates allowed the discovery of the universe itself. Long exposures revealed many nebulas to be island universes (galaxies) and showed the earth to be situated in one such galaxy. Further, these exposures also revealed faint (and therefore, distant) galaxies and clusters of galaxies in what had once been thought to be empty space. Over the decades, these findings had a strong impact on cosmology and led to a reformulation of an understanding of how the universe was formed. Photography also showed many other nebulas to be clouds of expanding or contracting gas, thus offering vital clues about the death and birth (respectively) of stars and how matter is distributed through the universe.

The application of spectroscopy to optical astronomy also had a major impact in that the chemistry of the stars and planets could be assayed by the distinct signature created by the light it emitted or absorbed. This allowed the discovery of helium in the solar atmosphere before its discovery on Earth and led to the understanding of nuclear fusion as the energy source of stars. The most striking contribution of optical spectroscopy was the measurement of the recession velocity of stars based on how their spectra were mysteriously shifted toward the red end of the spectrum. Edwin Powell Hubble determined that this phenomenon was caused by the star or galaxy moving away from the observer, that all stars or galaxies were moving outward (confirming Albert Einstein's prediction), and that the degree of reddening corresponded to the distance from the earth and thus to the number of years the light had been traveling. Hubble was able to establish that the age of the universe is some 12 to 14 billion years.

The advent of electronic detectors, both for imaging and spectroscopy, has allowed astronomers to improve the precision of their measurements and thus to refine the constants they apply to calculations. Improved quantum efficiency of detectors has given new life to old observatories since they now detect more of the light (up to 90 percent versus 4 percent in the 1950's) delivered by the telescope. Electronic systems have also been applied to photographic images since they now allow more precise measurements of emulsion densities and thus can bring up details that would escape the eye.

Principal terms

CHARGE-COUPLED DEVICE (CCD): a microchip with a two-dimensional array of light sensors that can be read by computer

NANOMETER: a billionth of a meter; a unit of measurement for ultra-small dimensions such as light wavelengths

PHOTOELECTRIC EFFECT: the phenomenon that governs the minimum amount of energy (quanta) required to raise an atomic particle to the next energy level

QUANTUM EFFICIENCY: the percentage of incident radiation that is detected or converted to a useful purpose by a device

SPECTROSCOPY: the study of the intensity of light at specific wavelengths; instruments are spectrographs or spectrometers

TELESCOPE: a device that permits detailed inspection of a distant object

Bibliography

Field, George. THE SPACE TELESCOPE. Chicago: Contemporary Books, 1989. Description of the origins of the Hubble Space Telescope and the problems of ground-based and space-based astronomy. Includes descriptions of how optics and detectors work.

Kaufmann, William J., III. UNIVERSE. New York: W. H. Freeman, 1985. College-level introductory text that covers astronomy. Readable; includes descriptions of astrophysical questions and their relationships.

Krisciunas, Kevin. ASTRONOMICAL CENTERS OF THE WORLD. Cambridge, England: Cambridge University Press, 1988. Survey of the major ground-based observatories and a description of their capabilities.

McLean, Ian S. ELECTRONIC AND COMPUTER-AIDED ASTRONOMY: FROM EYES TO ELECTRONIC SENSORS. Chichester, England: Horwood, 1989. A survey of the history and applications of electronics and astronomy. While much of the book is a technical survey, several chapters provide a general history and explanation of CCDs.

Tucker, Wallace. THE COSMIC INQUIRERS: MODERN TELESCOPES AND THEIR MAKERS. Cambridge, Mass.: Harvard University Press, 1986. Well-written descriptions of modern observatories and the scientific questions that led to their construction.

Hubble Space Telescope