Observational astronomy

Mayall telescope at Kitt Peak National Observatory

Observational astronomy is a division of the astronomical science that is concerned with getting data, in contrast with theoretical astrophysics which is mainly concerned with finding out the visible implications of physical models. It is the practice of observing celestial objects by using telescopes and other astronomical apparatus.

As a science, astronomy is somewhat hindered in that direct experiments with the properties of the distant universe are not possible. However, this is partly compensated by the fact that astronomers have a vast number of visible examples of stellar phenomena that can be examined. This allows for observational data to be plotted on graphs, and general trends recorded. Nearby examples of specific phenomena, such as variable stars, can then be used to infer the behavior of more distant representatives. Those distant yardsticks can then be employed to measure other phenomena in that neighborhood, including the distance to a galaxy.

Unaided eye

Prior to the discovery of the telescope, early observational astronomy relied upon the unaided eye and various instruments for measuring time and direction. Tycho Brahe is noted for his systematic observations of the heavens, and the data he collected was used by Johannes Kepler to build his laws of planetary motion.

The heavens have been regarded by humans for much of recorded history. Ancient stone structures were built as a means of measuring the passage of time based on the movements of the Sun. Constellations were specific patterns of stars in the sky that came to be associated with particular seasons on the Earth, as well as much lore and mythology.

The eye can also make other observations of the heavens without the use of a telescope. Ancient records recorded the occurrence of very bright stars (supernovae) that would suddenly appear in the sky, even being viewed during the daylight. There were also records of comets as portents of calamities, and shooting stars that crossed the sky. In modern times, meteorites are collected on the icy plains of Antarctica, and are studied to determine the properties of asteroids and even the surface of Mars.

Telescopes

Galileo Galilei was the first person to known to turn a telescope to the heavens and to record what he saw. Since that time, observational astronomy has made steady advances with each improvement in telescope technology.

A traditional division of observational astronomy is given by the region of the electromagnetic spectrum observed:

• Optical astronomy is the part of astronomy that uses specialized equipment to detect and analyze light in and slightly around the wavelengths that can be detected with the eyes (about 400 - 800 nm).
• Infrared astronomy deals with the detection and analysis of infrared radiation (wavelengths longer than red light). The most common tool is the telescope but with the instrument optimized for infrared. Space telescopes are also used to eliminate noise (electromagnetic interference) from the atmosphere.
• Radio astronomy detects radiation of millimetre to dekametre wavelength. The receivers are similar to those used in radio broadcast transmission but much more sensitive. See also Radio telescopes.
• High-energy astronomy includes X-ray astronomy, gamma-ray astronomy, and extreme UV astronomy, as well as studies of neutrinos and cosmic rays.

Optical and radio astronomy can be performed with ground-based observatories, because the atmosphere is transparent at the wavelengths being detected. Infrared light is heavily absorbed by water vapor, so infrared observatories have to be located in high, dry places or in space.

The atmosphere is opaque at the wavelengths used by X-ray astronomy, gamma-ray astronomy, UV astronomy and (except for a few wavelength "windows") far infrared astronomy, so observations must be carried out mostly from balloons or space observatories. Powerful gamma rays can, however be detected by the large air showers they produce, and the study of cosmic rays can also be regarded as a branch of astronomy.

Optical telescopes

For much of the history of observational astronomy, almost all observation has been performed in the visual spectrum. While the Earth's atmosphere is nearly completely transparent in this portion of the electromagnetic spectrum, most telescope work is still dependent on seeing conditions and is generally restricted to the night time. The seeing conditions depend on the depth, movement, and clarity of the air. Locations that are frequently cloudy or suffer from atmospheric turbulence restrict detailed observation. Likewise the presence of the full Moon can brighten up the sky, hindering observation of faint objects.

For observation purposes, the optimal location for an optical telescope is undoubtedly in outer space. There the telescope can make observations without being affected by the atmosphere. However, at present it remains costly to lift telescopes into orbit. Thus the next best locations are certain mountain peaks that have a high number of cloudless days and generally possess good atmospheric conditions. The peak of Mauna Kea, Hawaii and the Cerro Tololo in Chile both possess these properties, and so have attracted an assemblage of powerful observatories.

The darkness of the night sky is an important factor in optical astronomy. With the size of cities and human populated areas ever expanding, the amount of artificial lights at night has also increased. These lights produce a diffuse background illumination that makes observation of faint astronomical features very difficult without special filters. In a few locations such as the state of Arizona, this has led to campaigns for the reduction of light pollution. The use of hoods around street lights not only improves the amount of light directed toward the ground, but also helps reduce the light directed toward the sky.

Atmospheric effects can severely hinder the resolution of a telescope. Without some means of correcting for the blurring effect of the shifting atmosphere, telescopes larger than about 15-20 cm in aperture can not achieve their theoretical resolution. As a result, the primary benefit of using very large telescopes has been the improved light-gathering capability, allowing very faint magnitudes to be observed. However the resolution handicap has begun to be overcome by adaptive optics and other techniques, as well as the use of space telescopes.

Astronomers have a number of observational tools that they can use to make measurements of the heavens. For objects that are relatively close to the Sun and Earth, direct and very precise position measurements can be made against a more distant (and thereby nearly stationary) background. Early observations of this nature were used to develop very precise orbital models of the various planets, and to determine their respective masses and gravitational perturbations. Such measurements led to the discovery of the planets Uranus, Neptune, and Pluto. They also resulted in an erroneous assumption of a fictional planet Vulcan within the orbit of Mercury. (But the explanation of the precession of Mercury's orbit by Einstein is considered one of the triumphs of his general relativity theory.)

Other instruments

In addition to examination of the universe in the optical spectrum, astronomers have increasingly been able to acquire information in other portions of the electromagnetic spectrum. The earliest such non-optical measurements were made of the thermal properties of the Sun. Instruments employed during a solar eclipse could be used to measure the radiation from the corona.

Fully-steerable radio telescope in Green Bank, West Virginia.

With the discovery of radio waves, radio astronomy began to emerge as a new discipline in astronomy. The long wavelengths of radio waves required much larger collecting dishes, and later the development of the multi-dish interferometer to make sufficiently accurate radio maps of features. The development of the microwave horn receiver led to the discovery of the microwave background radiation associated with the big bang.

Radio astronomy has continued to expand its capabilities, even producing interferometers with baselines comparable to the size of the Earth. However, the ever-expanding use of the radio spectrum for other uses is gradually drowning out the faint radio signals from the stars. For this reason, in the future radio astronomy is likely to be performed from shielded locations, such as the far side of the Moon.

The last part of the twentieth century saw rapid technological advances in astronomical instrumentation. Telescopes were growing ever larger, and employing adaptive optics to partly negate atmospheric blurring. New telescopes were launched into space, and began observing the universe in the infrared, ultraviolet, x-ray, and cosmic ray parts of the electromagnetic spectrum. Orbiting instruments such as the Hubble Space Telescope produced rapid advances in astronomical knowledge, and reformed our understanding of the universe and its origins. New space instruments under development are expected to directly observe planets around other stars, perhaps even some Earth-like worlds.

In addition to telescopes, astronomers have begun using other instruments to make observations. Huge underground tanks have been built to detect neutrino emissions from the Sun and supernovae. Gravity wave detectors are being designed that may capture events such as collisions of massive objects such as neutron stars. Robotic spacecraft are also being increasingly used to make highly detailed observations of planets within the solar system, so that the field of planetary science now has significant cross-over with the disciplines of geology and meteorology.

Observation tools

The key instrument of nearly all modern observational astronomy is the telescope. This serves the dual purposes of gathering more light so that very faint objects can be observed, and magnifying the image so that small and distant objects can be observed. The optics used in a telescope have very exacting requirements which require great precision in their construction. Typical requirements for grinding and polishing a curved mirror, for example, require the surface to be within a fraction of a wavelength of light of a particular conic shape.

Large telescopes are housed in observatories, both to protect them from the weather and to stabilize the environmental conditions. The telescopes will typically have very precise equatorial mounts to allow them to compensate for the rotation of the Earth, and a variety of accessories that can be attached depending on the type of celestial object being observed. Modern telescopes and observatory domes are controlled by computers.

The photograph has served a critical role in observational astronomy for over a century, and only recently has it begun to be replaced by digital sensors such as CCDs and CMOS chips. Astrophotography requires highly sensitive photographic film that has been specially sensitized with chemicals so that they will record even the faintest sources of illumination.

Photographic images are permanent records that can be scrutinized with accurate instruments, eliminating sources of error produced during direct observations. They can also be compared with later photographs to search for changes. Photographs also allow for long exposure times, recording light sources that are too faint to be seen with the eye.

The blink comparator is an instrument that is used to compare two nearly identical photographs made of the same section of sky at different points in time. The comparator alternates illumination of the two plates, and any changes are revealed by blinking points or streaks. This instrument has been used to find asteroids, comets, and variable stars.

The position or cross-wire micrometer is an implement that has been used to measure double stars. This consists of a pair of fine, movable lines that can be moved together or apart. The telescope lens is lined up on the pair and oriented using position wires that lie at right angles to the star separation. The movable wires are then adjusted to match the two star positions. The separation of the stars is then read off the instrument, and their true separation determined based on the magnification of the instrument.

A vital instrument of observational astronomy is the spectroscope. The absorption of specific wavelengths of light by elements allows specific properties of distant bodies to be observed. This capability has resulted in the discovery of the element of helium in the Sun's emission spectrum, and has allowed astronomers to determine a great deal of information concerning distant stars, galaxies, and other celestial bodies. Doppler shift of spectra can also be used to determine the radial motion with respect to the Earth.

Early spectroscopes employed banks of prisms that would split the light into a broad spectrum. Later the grating spectrograph was developed, which reduced the amount of light loss compared to prisms. The spectrum can be photographed in a long exposure, allowing the spectrum of faint objects (such as distant galaxies) to be measured.

Stellar photometry came into use in 1861 as a means of measuring stellar colors. This technique measured the magnitude of a star at specific frequency ranges, allowing a determination of the overall color, and therefore temperature of a star. By 1951 an internationally standardized system of UBV-magnitudes (Ultraviolet-Blue-Visual) was adopted.

Photoelectric photometry using the CCD is now frequently used to make observations through a telescope. These sensitive instruments can record the image nearly down to the level of individual photons, and can be designed to view in parts of the spectrum that are invisible to the eye. The ability to record the arrival of small numbers of photons over a period of time can allow a degree of computer correction for atmospheric effects, sharpening up the image. Multiple digital images can also be combined to further enhance the image. When combined with the adaptive optics technology, image quality can approach the theoretical resolution capability of the telescope.

Filters are used to view an object at particular frequencies or frequency ranges. Multilayer film filters can provide very precise control of the frequencies transmitted and blocked, so that, for example, objects can be viewed at a particular frequency emitted only by excited hydrogen atoms. Filters can also be used to partially compensate for the effects of light pollution. Polarization filters can also be used to determine if a source is emitting polarized light, and the orientation of the polarization.

Observing

A variety of data can be observed for each star. The position coordinates locate the star on the sky using the techniques of spherical astronomy, and the magnitude determines its brightness as seen from the Earth. The relative brightness in different parts of the spectrum yields information about the temperature of the star, as well as certain properties of its photosphere. Photographs of the spectra allow the chemistry of the star to be examined.

Parallax shifts of the star against the background can be used to determine the distance. The radial velocity of the star and changes in its position over time (proper motion) can be used to measure its velocity relative to the Sun. Variations in the brightness of the star give evidence of instabilities in the star's atmosphere, or else the presence of an occulting companion. The orbits of binary stars can be used to measure the relative masses of each companion, or the total mass of the system. Spectroscopic binaries can be found by observing doppler shifts in the spectrum of the star and its close companion.

Stars of identical masses that are formed at the same time and under the similar conditions will typically have nearly identical observed properties. Observing a mass of closely associated stars, such as in a globular cluster, allows data to be assembled about the distribution of stellar types. These tables can then be used to infer the age of the association.

For distant galaxies, observations are made of the overall shape and properties of the galaxy, as well as the groupings in which they are found. Observations of certain types of variable stars and supernovae of known absolute magnitude in other galaxies allows the inference of the distance to the host galaxy. The expansion of space causes the spectra of these galaxies to be shifted, depending on the distance, and modified by the doppler effect of the galaxy's radial velocity. Both the size of the galaxy and its redshift can be used to infer something about the distance of the galaxy. Observations of large numbers of galaxies are referred to as surveys, and are used to model the evolution of galaxy forms.

See also

• List of observatories
• List of radio telescopes
• Optical telescopes
• Space observatory
• Timeline of telescopes, observatories, and observing technology