Optical astronomy

Optical astronomy was the first form of astronomical observation. It relied on the human eye as an instrument capable of detecting light and discerning color and location. In modern science, the term optical astronomy refers to observations made using visible and near-visible wavelengths of light, even if not made directly with the human eye.

Overview

Optical astronomy encompasses all astronomical observations that can be made using visible light and adjacent wavelengths of the electromagnetic spectrum. This broad area includes highly detailed studies of intensity, polarization, spectral features, and other visible and near-visible light aspects. Originally done with the naked eye and later by looking through telescopes, optical astronomy evolved to the point where observations were made photographically during most of the twentieth century and digitally during the twenty-first century.

Gamma, X, ultraviolet, visible, infrared, and radio waves are all electromagnetic radiation forms. They differ only in wavelength, frequency, photon energy, and the instruments used to detect them. The wavelength range to which the human eye is sensitive, called visible light, spans only a small part of the entire electromagnetic spectrum, from violet light, at about 400 nanometers (10-9, or one-billionth of a meter), to red light, at about 700 nanometers. This is the same wavelength range that the Sun emits most intensely, which is no coincidence; human eyes evolved to react to the dominant wavelength range of sunlight. Furthermore, the Earth’s atmosphere blocks much of the electromagnetic spectrum from reaching the ground. One of the two major wavelength ranges that can penetrate the atmosphere and reach the ground produces the so-called optical window; the atmosphere is almost entirely transparent, between about 300 nanometers in the ultraviolet and 1,000 nanometers in the infrared, and partially transparent out to about 10,000 nanometers. (The other range is the radio window, between one centimeter and ten meters.) Modern optical astronomy is considered to span the range from approximately 300 nanometers (near-ultraviolet) to about 1,100 nanometers (near-infrared), based on the optical window and the available detectors and instruments.

The optical systems that astronomers use are designed to deliver a beam of light to the surface of a particular detector or sensor, where it causes a change in the electrical or chemical state of the sensor material. This change is then recorded in some fashion and analyzed to produce information about the object that emitted the light. The two basic types of optical telescopes, refractors and reflectors, make use of two properties of light: it can be refracted (bent) when it passes through a transparent medium (a lens), and it can be reflected at the surface of a material (a mirror). The first telescopes developed in the early 1600s were refractors using lenses. However, reflectors using mirrors are more versatile, allowing a variety of optical paths to focus light. (Because reflection is the same at all wavelengths without regard to color, the same basic layout of visible light reflectors is used in most nonvisible-radiation detectors, including radio, infrared, ultraviolet, and X-ray telescopes. The principal differences have to do with the materials used for the mirrors and the detectors employed for different parts of the spectrum.) Some optical telescopes actually use a combination of mirrors and lenses to focus light into an image. In spectroscopy, additional optics are needed to take light from a chosen object and disperse it by wavelength into a spectrum. This is done using a prism (a wedge-shaped piece of glass) or a diffraction grating (which has lots of fine grooves scribed into the surface of some material). In either case, different wavelengths are bent in different amounts. The spread-out spectrum can then be focused on a photographic or electronic detector.

The human eye was the first detector used behind the telescope. The development of photography provided a means of preserving telescopic images for later study. Astronomical photographs usually were made on glass plates coated with photographic emulsion because of the stability of glass. Still, plastic-based films were sometimes employed where a series of pictures were to be taken, and there was no convenient way to change plates. Until the 1980s, photography was the method of choice for most observations. Later, digital electronic detectors came into wide use because they could record images more quickly than photography could. The charge-coupled device (CCD) is an electronic analog to the retina in the eye. A CCD is an array (like squares on graph paper) of little light sensors called picture elements (or pixels) that a computer reads. CCDs provide a direct measure of the intensity of the visible light (or other form of radiation) striking each pixel and produce images stored in and can be directly manipulated by computers.

Until 1990, optical astronomy was largely conducted from the surface of the Earth since space-based astronomy concentrated on regions of the electromagnetic spectrum that could not be detected from the ground. Although several small optical telescopes were carried on suborbital and short orbital platforms, optical astronomy did not truly move into space until 1990, when the Hubble Space Telescope (HST) was launched into Earth orbit. Above the atmosphere, it can obtain much sharper images than from the ground, where atmospheric turbulence always blurs images (a phenomenon astronomers refer to as “seeing”), at least to some extent. The HST has provided a wealth of high-resolution images after correcting initial post-launch optical problems.

Applications

Optical astronomy observations can be divided into several techniques: imagery, photometry, spectroscopy, and polarimetry. These, in turn, are applied to studying various sorts of objects.

Imagery provides pictures of objects, showing their locations, shapes, and other features. Some of the first measurements in optical astronomy were made with micrometers attached to telescope eyepieces so visual observers could measure accurate positions for constructing star charts. (Earlier charts were made using naked-eye pointing devices.) Photography and electronic imaging provide a means of recording images that can be studied and measured later, revealing details that cannot be seen with the eye peering through a telescope.

Photometry, literally light measure or measurements of brightness, was also performed initially with the eye but was highly subjective; it was difficult to form a standard of brightness based on human perception. Then, photographic and electronic systems became available. With photographic emulsions, brightness could be measured objectively by the degree of exposure recorded. With solid-state photon counters and other electronic sensors, the instantaneous brightness of an object can be measured at any time, and quantitative comparisons can be made with measurements taken on different nights or among different objects. Brightness variations too quick or subtle for the eye to detect also became measurable. Both photographic and electronic photometry measurements can be made using all available light or limited to some particular wavelength range using specific colored filters.

Spectroscopy involves dispersing the light by wavelength using a prism or diffraction grating. It allows astronomers to determine a broad range of attributes of an object. Transparent gases produce emission line and absorption line spectra with distinct bright emission or dark absorption lines at particular wavelengths. In contrast, solids, liquids, and opaque gases produce continuous emission or reflection spectra (all wavelengths present with the intensity varying continuously by wavelength). The pattern of emission or absorption lines is determined by the chemical elements present in the transparent gas and its temperature. The variation of intensity with wavelength in a reflection spectrum from a solid or liquid can be compared to the reflection spectra of laboratory samples to determine the probable surface material of the source. The variation of intensity with wavelength in a continuous emission spectrum closely follows a theoretical curve called the Planck distribution for thermal radiators; the wavelength at which the intensity reaches a maximum indicates the surface temperature of the source. The motion of the source toward or away from the observer (or detection device) along the line of sight shifts the entire spectrum and all its features to shorter or longer wavelengths due to the phenomenon known as the Doppler effect. If the source moves toward the observer, all the wavelengths it emits or reflects compress and the spectrum shifts toward shorter (bluer) wavelengths, called a blueshift. If the source moves away from the observer, all the wavelengths it emits or reflects stretch, and the spectrum shifts to longer (redder) wavelengths, called a redshift. The amount of the spectral shift depends on the speed of approach or recession. The intensity of magnetic fields can be measured by Zeeman splitting, in which pairs or triplets of spectral lines appear where a single line would be expected.

Polarimetry measures the linear or circular polarization of light. Polarization indicates certain conditions at the emission source or along the light path. The wave model treats light as associated electric and magnetic fields (hence the name electromagnetic radiation) that oscillate in two perpendicular planes as light propagates. It may be unpolarized (with no particular orientation to the planes of oscillation), linearly polarized (with the two planes of oscillation keeping the same orientation), or right or left circularly polarized (with the two planes of oscillation rotating completely around the light path through each wave cycle). Most light is emitted unpolarized but can become polarized by reflecting off a surface or being scattered by dust particles (which polarizes sunlight in the atmosphere). Light can also be polarized when emitted in a strong magnetic field (such as produces the Zeeman effect in spectra) or when emitted by electrons trapped in a magnetic field (synchrotron radiation). Most polarimetry is conducted with detectors that use the property of birefringence, which retards light waves vibrating at right angles to other light waves. Such detectors employ certain crystals that exhibit birefringence, or Pockels cells, which become birefringent in response to an electric current.

Context

After millennia of having nothing more to use than the unaided eye, optical astronomy got a real boost in the 1600s when Galileo manufactured his own refracting telescopes and used them to study the sky. His discovery of mountains and valleys on Earth’s Moon, spots on the Sun, moons orbiting Jupiter, and many stars too faint to be seen without a telescope (including those that constitute the hazy white band of light called the Milky Way) spurred further telescopic observations and more discoveries. Sir Isaac Newton designed the first reflecting telescope, although problems in fabricating and silvering the mirrors caused reflectors to lag behind refractors in size and capability for many years. (In the twenty-first century, all large optical telescopes that are built are reflectors.)

Even after the introduction of the telescope, for 250 years, more optical astronomy relied on the human eye as the only detector and the brain and hand as the only interpreters and recorders of data. The advent of photography sparked a radical change in astronomy. The development of dry photographic plates in the late 1800s allowed long-duration exposures to be made for imaging faint, extended, diffuse objects whose central bodies astronomers had been able to view only dimly by eye. Photography also recorded images objectively for later study by any who were interested. The advent of electronic detectors in the 1980s made recording light even more efficient. The improved quantum efficiency of electronic detectors has given new life to old observatories, allowing them to record much more of the light (up to 60 to 70 percent now versus 1 percent photographically in the 1950s) delivered by their telescopes.

The application of spectroscopy to optical astronomy had a significant impact in that the chemistry of astronomical sources could be assayed by the distinct patterns of bright or dark lines in the spectra of the light they emitted or reflected. In the early 1800s, Joseph von Fraunhofer discovered that the solar spectrum had distinct black lines, leading to the discovery that each chemical element emits or absorbs specific wavelengths of light, thus providing a powerful tool for analyzing chemical compositions. This led to the discovery of helium in the solar atmosphere before its discovery on Earth. The most striking contribution of optical spectroscopy was the discovery by Edwin Powell Hubble of the expansion of the universe based on how most galaxy spectra were shifted toward longer wavelengths (the red end of the visible spectrum). He found that spectral redshifts of galaxies were correlated with their distances, with the farther galaxies having larger redshifts. He deduced that this redshifting was due to galaxies moving away from each other with speeds proportional to their distances apart—space is expanding.

Optical astronomy also provided the first observational confirmation of a prediction of general relativity with measurements (during a solar eclipse) showing that the Sun slightly bends starlight passing close by its edge. This effect has been seen even more dramatically with the discovery of gravitational lensing by galaxies that bend the light of more distant objects to create multiple images.

The observational techniques of optical astronomy can be applied to study a wide range of astronomical objects—such as planets, stars, nebulae, and galaxies. Optical astronomy has served as the foundation for all the other branches of observational astronomy that deal with longer or shorter wavelength ranges, and it continues to interact with them as astronomers try to identify optical counterparts of new objects found in the radio, infrared, ultraviolet, X-ray, and gamma-ray spectral regions. Significant advances have been made in observing all parts of the electromagnetic spectrum because as new technology makes progress possible in one wavelength range, there is increased effort to bring the others up to par with it. Optical astronomy continues to serve as the linchpin for astronomy, and the future of optical astronomy remains bright.

Bibliography

Chaisson, Eric, and Steve McMillan. Astronomy Today. 9th ed. Addison-Wesley, 2018.

Field, George. The Space Telescope. Contemporary Books, 1989.

Fraknoi, Andrew, David Morrison, and Sidney Wolff. Voyages to the Stars and Galaxies. Brooks/Cole-Thomson Learning, 2006.

Freedman, Roger A. Universe: Stars and Galexies. 6th ed. W. H. Freeman, 2019.

Hirsh, Richard F. Glimpsing an Invisible Universe: The Emergence of X-Ray Astronomy. Cambridge University Press, 1983.

Kitchin, C. R. Astrophysical Techniques. 7th ed. Adam Hilger, 2021.

Krisciunas, Kevin. Astronomical Centers of the World. Cambridge University Press, 1988.

McLean, Ian S. Electronic Imaging in Astronomy: Detectors and Instrumentation. Springer, 2008.

"Optical Astronomy." ESA Hubble, esahubble.org/wordbank/optical-astronomy. Accessed 20 Sept. 2023.

Schneider, Stephen E., and Thomas T. Arny. ISE Pathways to Astronomy. 6th ed. McGraw-Hill, 2020.

Wall, J. V., ed. Optics in Astronomy. New York: Cambridge University Press, 1993.