Optical telescope

An optical telescope is a telescope which is used to gather, and focus light, for directly viewing a magnified image, making a photograph, etc. The term is used especially for a monocular with static mounting for observing the sky. Handheld binoculars are common for other purposes.

Light is made up of photons, and professional telescopes concentrate the light onto electronic detectors which collect the photons. There are two primary types of optical telescope: reflectors (which use mirrors) and refractors (which use lenses). In addition there are compound telescopes such as the Maksutov telescope and the Schmidt-Cassegrain telescope.

How it works

The basic scheme is that the primary light-gathering element, the objective ( objective lens (1) or concave mirror), focuses light from a distant object (4) to a focal plane where it forms a real image (5). This image is viewed through an eyepiece (2), which acts like a magnifying glass. The eye (3) sees a magnified virtual image (6) at a large distance.

Telescopes which employ two convex lenses cause the image to appear inverted. Terrestrial versions of such telescopes and binoculars employ prisms (e.g. Porro prisms) or a relay lens between objective and eypiece to invert the image once more. Thus, an upright image appears in the eypiece.

Many types of telescope fold the optical path with secondary or tertiary mirrors. These may be integral part of the optical design ( Cassegrain reflector and similar types), but also serve for making the telescope more compact and placing the eyepiece or detector at a more convenient position. On large telescopes these additional mirrors are often used to provide improved image quality over a larger field of view.

Angular resolution

Ignoring blurring of the image by turbulence in the atmosphere ( atmospheric seeing) and optical imperfections of the telescope, the angular resolution of an optical telescope is determined by the width of the objective, termed its " aperture." The Rayleigh criterion for the resolution limit α_R (in radians) is given by

α_R = 1.22λ / D,

where λ is the wavelength and D is the aperture. For visible light (λ = 550rmnm), this equation can be rewritten:

α_R = 138 / D.

Here, α_R denotes the resolution limit in arcseconds and D is in millimeters. In the ideal case, the two components double stars can be split even if separated by slightly less than α_R. This is taken into account by the Dawes limit

α_D = 116 / D.

It should be noted that the resolution is NOT given by the maximum magnification (or "power") of a telescope. Telescopes marketed by giving high values of the maximum power often deliver poor images.

For large ground-based telescopes, the resolution is limited by atmospheric seeing. This limit can be overcome by placing the telescopes above the atmosphere, e.g., space telescopes, balloon telescopes and telescopes on high-flying airplanes ( Kuiper Airborne Observatory, SOFIA) or by adaptive optics or speckle imaging for ground-based telescopes.

Recently, it has become practical to perform aperture synthesis with arrays optical telescopes. Very high resolution images can be obtained with groups of widely-spaced smaller telescopes, linked together by carefully-controlled optical paths, but these interferometers can only used for imaging bright objects such as stars or measuring the bright cores of active galaxies. Example images of starspots on Betelgeuse can be seen here.

Focal length and f-ratio

The focal length determines how wide an angle the telescope can view with a given eyepiece or size of a CCD detector or photographic plate. The f-ratio of a telescope is the ratio between the focal length and the aperture (i.e., diameter) of the objective. Thus, for a given aperture (light-gathering power), low f-ratios indicate wide fields of view. Wide-field telescopes are used to track satellites and asteroids, for cosmic-ray research, and for surveys of the sky. It is more difficult to reduce optical aberrations in telescopes with low f-ratio than in telescopes with larger f-ratio.

Light-gathering power

The light-gathering power of an optical telescope is directly related to the diameter of the objective lens or mirror. Note that the area of a circle is proportional to the square of the radius. A telescope with a lens which has a diameter three times that of another will have nine times the light-gathering power. Larger objectives gather more light, and more sensitive imaging equipment can produce better images from less light.

Research telescopes

Nearly all large research-grade astronomical telescopes are reflectors. Some reasons are:

• In a lens the entire volume of material has to be free of imperfection and inhomogeneities, whereas in a mirror, only one surface has to be perfectly polished.
• Light of different colors travels through a medium other than vacuum at different speeds. This causes chromatic aberration.
• There are technical difficulties involved in manufacturing and manipulating large-aperture lenses. One of them is that all real materials sag in gravity. A lens can only be held by its perimeter. A mirror, on the other hand, can be supported by the whole side opposite to its reflecting face.

The size of optical telescopes increased steadily in the 20th century, doubling between the 1910s and the 1940s, and doubling again between the late 1940s and the 1990s. The largest current telescopes are the 11m SALT and Hobby-Eberly telescopes and the 10.4m Gran Telescopio Canarias.

In the 1980s a number of technological improvements were made which created a new generation of telescopes. These advances included the creation of multi-mirror telescopes and the invention of cheap personal computers which could control the mirrors. Another major advanced was the invention of rotating furnaces in which centrifugal force would shape a telescope mirror to close to its final shape.

Names of types:

• Binoculars are just two monoculars mounted side-by side with adjustments to let both be used. A major practical advantage of these telescopes is not magnification, so much as a brighter field of view at dusk and dawn. Monoculars and binoculars with built-in compasses are used by army artillery units and ships to navigate by triangulating from topographic (shore) features. Hand-held telescopes are limited by hand-shaking to about 7 power. The brightest-field, best-magnifying practical monocular is about 7x50.