Telescope -
Cont'd
Research telescopes
Most large research telescopes can operate as either a cassegrain
telescope (longer focal length, and a narrower field with higher
magnification) or newtonian telescope (brighter field). They have a
pierced primary, a newtonian focus, and a spider to mount a variety
of replaceable secondaries.
Harlan J. Smith Telescope at
McDonald Observatory, Texas
A new era of telescope making was inaugurated by the MMT, a
synthetic aperture composed of six segments synthesizing a mirror
of 4.5 metres diameter. Its example was followed by the Keck
telescopes, a synthetic-aperture 10 m telescope.
The current generation of telescopes being constructed have a
primary mirror of between 6 and 8 meters in diameter (for
ground-based telescopes). In this generation of telescopes, the
mirror is usually very thin, and is kept in an optimal shape by an
array of actuators (see active optics). This technology has driven
new designs for future telescopes with diameters of 30, 50 and even
100 metres.
Initially the detector used in telescopes was the human eye. Later,
the sensitized photographic plate took its place, and the
spectrograph was introduced, allowing the gathering of spectral
information. After the photographic plate, successive generations
of electronic detectors, such as the charge-coupled device (CCDs),
have been perfected, each with more sensitivity and
resolution.
Current research telescopes have several instruments to choose
from: imagers, of different spectral responses; spectrographs,
useful in different regions of the spectrum; polarimeters, that
detect light polarization, etc.
In recent years, some technologies to overcome the bad effect of
atmosphere on ground-based telescopes were developed, with good
results. See tip-tilt mirror and adaptive optics.
The phenomenon of optical diffraction sets a limit to the
resolution and image quality that a telescope can achieve, which is
the effective area of the Airy disc, which limits how close we may
place two such discs. This absolute limit is called Sparrow's
resolution limit. This limit depends on the wavelength of the
studied light (so that the limit for red light comes much earlier
than the limit for blue light) and on the diameter of the telescope
mirror. This means that a telescope with a certain mirror diameter
can resolve up to a certain limit at a certain wavelength, so if
you want more resolution at that very wavelength, you have to build
a wider mirror.
Famous optical telescopes
-- The Hubble Space Telescope is in orbit outside of the Earth's
atmosphere to allow for observations not distorted by refraction,
in this way they can be diffraction limited, and used for coverage
in the ultraviolet (UV) and infrared.
Hubble Space Telescope as seen
from the Space Shuttle Discovery.
-- The Very Large Telescope (VLT) is currently (2002) the record
holder in size, with four telescopes each 8 metres in diameter. The
four telescopes, belonging to ESO and located in the Atacama desert
in Chile, can operate independently or together.
-- There are many plans for even larger telescopes. One of them is
the Overwhelmingly Large Telescope or OWL, which is intended to
have a single aperture of 100 metres in diameter.
-- The 200 inch (5.08 m) Hale telescope on Palomar Mountain is a
conventional research telescope that was the largest for many
years. It has a single borosilicate (Pyrex) mirror that was
famously difficult to construct. The mounting is also unique, an
equatorial mount that is not a fork, yet permits the telescope to
image near the north celestial pole.
-- The 100 inch (2.54 m) Hooker Telescope in the Mount Wilson
Observatory was used by Edwin Hubble to discover galaxies, and the
redshift. The mirror was made of green glass by Saint-Gobain. It is
now part of a synthetic aperture array with several other Mt.
Wilson telescopes, and is still useful for advanced research.
-- The 1.02 m Yerkes Telescope (in Wisconsin) is the largest
aimable refractor in use.
-- The 0.76 m Nice refractor (in France) that became operational in
1888 was at that time the world's largest telescope. This was the
last time the most powerful operationnal telescope in the world was
located in Europe. It was outperformed one year later by the 0.91 m
refractor at the Lick Observatory.
-- The largest refractor ever constructed was French. It was on
display at the 1900 Paris Exposition. Its lens was stationary,
prefigured so as to sag into the correct shape. The telescope was
aimed by the aid of a Foucault sidérostat, which is a movable plane
mirror with a 2 m diameter, mounted in a large cast-iron frame. The
horizontal tube was 60 m long and the objective had 1.25 m in
diameter. It was a failure.
Other meanings
-- There is a constellation called Telescopium.
-- Old-type telescopes were often made from tube sections that slid
inside each other for easier stowage when not being used. As a
result, anything that can lengthen and shorten in this way is said
to be telescopic
(adjective), and the process of shortening in this way is called
telescoping (verb).
Issue #89
