Optical telescope
An optical telescope gathers and focuses light mainly from the visible part of the electromagnetic spectrum, to create a magnified image for direct visual inspection, to make a photograph, or to collect data through electronic image sensors.
There are three primary types of optical telescope :
- Refracting telescopes, which use lenses and less commonly also prisms
- Reflecting telescopes, which use mirrors
- Catadioptric telescopes, which combine lenses and mirrors
People use optical telescopes for outdoor activities such as observational astronomy, ornithology, pilotage, hunting and reconnaissance, as well as indoor/semi-outdoor activities such as watching performance arts and spectator sports.
History
The telescope is more a discovery of optical craftsmen than an invention of a scientist. The lens and the properties of refracting and reflecting light had been known since antiquity, and theory on how they worked was developed by ancient Greek philosophers, preserved and expanded on in the medieval Islamic world, and had reached a significantly advanced state by the time of the telescope's invention in early modern Europe. But the most significant step cited in the invention of the telescope was the development of lens manufacture for spectacles, first in Venice and Florence in the thirteenth century, and later in the spectacle making centers in both the Netherlands and Germany. It is in the Netherlands in 1608 where the first documents describing a refracting optical telescope surfaced in the form of a patent filed by spectacle maker Hans Lippershey, followed a few weeks later by claims by Jacob Metius, and a third unknown applicant, that they also knew of this "art".Word of the invention spread fast and Galileo Galilei, on hearing of the device, was making his own improved designs within a year and was the first to publish astronomical results using a telescope. Galileo's telescope used a convex objective lens and a concave eye lens, a design is now called a Galilean telescope. Johannes Kepler proposed an improvement on the design that used a convex eyepiece, often called the Keplerian Telescope.
The next big step in the development of refractors was the advent of the Achromatic lens in the early 18th century, which corrected the chromatic aberration in Keplerian telescopes up to that time, allowing for much shorter instruments with much larger objectives. Chester Moor Hall is credited with designing the first achromatic lens in 1729, which consisted of a concave crown and a convex flint lens. However, it was John Dollond who received the first patent after further development of the design.
For reflecting telescopes, which use a curved mirror in place of the objective lens, theory preceded practice. The theoretical basis for curved mirrors behaving similar to lenses was probably established by Alhazen, whose theories had been widely disseminated in Latin translations of his work. Soon after the invention of the refracting telescope, Galileo, Giovanni Francesco Sagredo, and others, spurred on by their knowledge that curved mirrors had similar properties to lenses, discussed the idea of building a telescope using a mirror as the image forming objective. The potential advantages of using parabolic mirrors led to several proposed designs for reflecting telescopes, the most notable of which was published in 1663 by James Gregory and came to be called the Gregorian telescope, but no working models were built. Isaac Newton has been generally credited with constructing the first practical reflecting telescopes, the Newtonian telescope, in 1668 although due to their difficulty of construction and the poor performance of the speculum metal mirrors used it took over 100 years for reflectors to become popular. Many of the advances in reflecting telescopes included the perfection of parabolic mirror fabrication in the 18th century, silver coated glass mirrors in the 19th century, long-lasting aluminum coatings in the 20th century, segmented mirrors to allow larger diameters, and active optics to compensate for gravitational deformation. A mid-20th century innovation was catadioptric telescopes such as the Schmidt camera, which uses both a lens and mirror as primary optical elements, mainly used for wide field imaging without spherical aberration.
The late 20th century has seen the development of adaptive optics and space telescopes to overcome the problems of astronomical seeing.
The electronics revolution of the early 21st century led to the development of computer-connected telescopes in the 2010s that allow non-professional skywatchers to observe stars and satellites using relatively low-cost equipment by taking advantage of digital astrophotographic techniques developed by professional astronomers over previous decades. An electronic connection to a computer is required to make astronomical observations from the telescopes. The digital technology allows multiple images to be stacked while subtracting the noise component of the observation producing images of Messier objects and faint stars as dim as an apparent magnitude of 15 with consumer-grade equipment.
Principles
The basic scheme is that the primary light-gathering element, the objective , focuses that light from the distant object to a focal plane where it forms a real image. This image may be recorded or viewed through an eyepiece, which acts like a magnifying glass. The eye then sees an inverted, magnified virtual image of the object.Image:Kepschem.png|thumb|Schematic of a Keplerian refracting telescope. The arrow at is a representation of the original image; the arrow at is the inverted image at the focal plane; the arrow at is the virtual image that forms in the viewer's visual sphere. The red rays produce the midpoint of the arrow; two other sets of rays produce its head and tail.
Inverted images
Most telescope designs produce an inverted image at the focal plane; these are referred to as inverting telescopes. In fact, the image is both turned upside down and reversed left to right, so that altogether it is rotated by 180 degrees from the object orientation. In astronomical telescopes the rotated view is normally not corrected, since it does not affect how the telescope is used. However, a mirror diagonal is often used to place the eyepiece in a more convenient viewing location, and in that case the image is erect, but still reversed left to right. In terrestrial telescopes such as spotting scopes, monoculars and binoculars, prisms or a relay lens between objective and eyepiece are used to correct the image orientation. There are telescope designs that do not present an inverted image such as the Galilean refractor and the Gregorian reflector. These are referred to as erecting telescopes.Design variants
Many types of telescope fold or divert the optical path with secondary or tertiary mirrors. These may be integral part of the optical design, or may simply be used to place the eyepiece or detector at a more convenient position. Telescope designs may also use specially designed additional lenses or mirrors to improve image quality over a larger field of view.Characteristics
Design specifications relate to the characteristics of the telescope and how it performs optically. Several properties of the specifications may change with the equipment or accessories used with the telescope; such as Barlow lenses, star diagonals and eyepieces. These interchangeable accessories do not alter the specifications of the telescope, however they alter the way the telescope's properties function, typically magnification, apparent field of view and actual field of view.Surface resolvability
The smallest resolvable surface area of an object, as seen through an optical telescope, is the limited physical area that can be resolved. It is analogous to angular resolution, but differs in definition: instead of separation ability between point-light sources it refers to the physical area that can be resolved. A familiar way to express the characteristic is the resolvable ability of features such as Moon craters or Sun spots. Expression using the formula is given by twice the resolving power over aperture diameter multiplied by the objects diameter multiplied by the constant all divided by the objects apparent diameter.Resolving power is derived from the wavelength using the same unit as aperture; where 550 nm to mm is given by:.
The constant is derived from radians to the same unit as the object's apparent diameter; where the Moon's apparent diameter of radians to arcsecs is given by:.
An example using a telescope with an aperture of 130 mm observing the Moon in a 550 nm wavelength, is given by:
The unit used in the object diameter results in the smallest resolvable features at that unit. In the above example they are approximated in kilometers resulting in the smallest resolvable Moon craters being 3.22 km in diameter. The Hubble Space Telescope has a primary mirror aperture of 2400 mm that provides a surface resolvability of Moon craters being 174.9 meters in diameter, or sunspots of 7365.2 km in diameter.
Angular resolution
Ignoring blurring of the image by turbulence in the atmosphere and optical imperfections of the telescope, the angular resolution of an optical telescope is determined by the diameter of the primary mirror or lens gathering the light.The Rayleigh criterion for the resolution limit is given by
where is the wavelength and is the aperture. For visible light in the small-angle approximation, this equation can be rewritten:
Here, denotes the resolution limit in arcseconds and is in millimeters.
In the ideal case, the two components of a double star system can be discerned even if separated by slightly less than. This is taken into account by the Dawes limit
The equation shows that, all else being equal, the larger the aperture, the better the angular resolution. The resolution is not given by the maximum magnification 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., on the summits of high mountains, on balloons and high-flying airplanes, or in space. Resolution limits can also be overcome by adaptive optics, speckle imaging or lucky imaging for ground-based telescopes.
Recently, it has become practical to perform aperture synthesis with arrays of 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 be used for imaging bright objects such as stars or measuring the bright cores of active galaxies.