Observational astronomy

However, this is partly compensated by the fact that astronomers have a vast number of visible examples of stellar phenomena that can be examined.

Optical and radio astronomy can be performed with ground-based observatories, because the atmosphere is relatively transparent at the wavelengths being detected.

Likewise the presence of the full Moon can brighten up the sky with scattered light, hindering observation of faint objects.

These artificial lights produce a diffuse background illumination that makes observation of faint astronomical features very difficult without special filters.

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 at visible wavelengths.

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.

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.

Interferometer arrays produced the first extremely high-resolution images using aperture synthesis at radio, infrared and optical wavelengths.

Orbiting instruments such as the Hubble Space Telescope produced rapid advances in astronomical knowledge, acting as the workhorse for visible-light observations of faint objects.

Neutrino astronomy is motivated by the possibility of observing processes that are inaccessible to optical telescopes, such as the Sun's core.

Gravitational wave detectors are being designed that may capture events such as collisions of massive objects such as neutron stars or black holes.

[5] 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.

For example, if the temperature is different from one side of the telescope to the other, the shape of the structure changes, due to thermal expansion pushing optical elements out of position.

Domes are often opened around sunset, long before observing can begin, so that air can circulate and bring the entire telescope to the same temperature as the surroundings.

To prevent wind-buffet or other vibrations affecting observations, it is standard practice to mount the telescope on a concrete pier whose foundations are entirely separate from those of the surrounding dome and building.

Until the advent of computer controlled drive mechanisms, the standard solution was some form of equatorial mount, and for small telescopes this is still the norm.

The photograph has served a critical role in observational astronomy for over a century, but in the last 30 years it has been largely replaced for imaging applications by digital sensors such as CCDs and CMOS chips.

Specialist areas of astronomy such as photometry and interferometry have utilised electronic detectors for a much longer period of time.

Glass plates are still used in some applications, such as surveying,[citation needed] because the resolution possible with a chemical film is much higher than any electronic detector yet constructed.

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.

Later the grating spectrograph was developed, which reduced the amount of light loss compared to prisms and provided higher spectral resolution.

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.

The position coordinates locate the object on the sky using the techniques of spherical astronomy, and the magnitude determines its brightness as seen from the Earth.

Observing a mass of closely associated stars, such as in a globular cluster, allows data to be assembled about the distribution of stellar types.