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It 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
For detailed information on specific designs of reflecting
and refracting telescopes, see the main articles on Reflecting
telescopes and Refracting telescopes.
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.
Keplerian telescope, schematic
Telescopes which employ two convex lenses cause the image
to appear inverted. Terrestial 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 aR (in radians) is given by
aR = 1.22l / D,
where l is the wavelength and D is the aperture. For
visible light (l = 550rmnm), this equation can be rewritten:
aR = 138 / D.
Here, aR 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 aR. This is taken into account by the Dawes
limit
aD = 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.
• Enjoy the View - Use a Binocular
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 a 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.
See:
* Astronomy
• Telescopes
* List of optical telescopes
* Amateur telescope making
See also
• Monocular
• Binoculars
• Camera
Lens
• Zeiss
• Nikon
8x42 Monarch ATB
• Zhumell
7x50 Marine with Compass and Reticle
• Zhumell
Bring em Near Pirate Spyglass 25x30
• Swarovski
10x42 EL Binoculars
• Binocular
Cases
ptical, optcal, opitical, opitcal, optcial, optica,
This article is licensed under the GNU
Free Documentation License.
It uses material from the Wikipedia
article "Optical Telescope".
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Optical Telescope
