Telescope Design And Selection

essential. Nothing will kill your enthusiasm for stargazing faster than a poor-quality telescope with a shaky mount that provides blurry, wiggling images.

Large refractors are used where image quality and resolution are most important, as for viewing surface details of the Moon and planets or for observing double star systems.

Giant reflectors are used where aperture is most important, as to probe the faintest, most distant objects. They are easier to build and are more cost effective than refractors. Folded optics reduce the physical length of huge reflectors, so they can be housed inside smaller domes than refractors. The primary mirror is supported from behind so it does not sag under gravity as large lenses do.

Astronomers design ever-larger telescopes and new observing techniques to increase light collection and improve resolving power (Figure 2.15).

Structure Keck Telescope

Figure 2.15. The Keck Telescope in Hawaii, U.S., uses a segmented-mirror design for optical and infrared research. Computer controls precisely align 36 hexagonal mirrors, each about 2 m (6 feet) across and 7.5 cm (3 inches) thick, to form one surface and function as a huge 10-m (33-foot) mirror. Keck I and twin Keck II can operate independently or together. ►www2.keck.hawaii.edu^

Figure 2.15. The Keck Telescope in Hawaii, U.S., uses a segmented-mirror design for optical and infrared research. Computer controls precisely align 36 hexagonal mirrors, each about 2 m (6 feet) across and 7.5 cm (3 inches) thick, to form one surface and function as a huge 10-m (33-foot) mirror. Keck I and twin Keck II can operate independently or together. ►www2.keck.hawaii.edu^

The newest telescopes have lighter-weight monolithic mirrors, cast as a single piece, or segmented mirrors, mosaics of individual mirrors (Figure 2.15), that are used both independently or in combination, with a computer control system. Multiple telescopes, more than one primary mirror, send light collected by all primaries to a central focus where it is combined to image as a single gigantic virtual mirror.

The largest operating telescope in the southern hemisphere is the European Southern Observatory's 16-m Very Large Telescope (VLT). ►www.eso.org.paranal^ The VLT has a multiple-mirror design utilizing four linked 8.2-m telescopes that were named in the local Mapuche language.

Most of the world's biggest telescopes have interesting visitor Web sites ►http://astro.nineplanets.org/bigeyes.html^ and self-guided tours for the public (Table 2.2).

TABLE 2.2 Major Optical Telescopes in the World

(m)

Observatory Location

Description

Giant Magellan Telescope*

21.4

Las Campanas La Serena, Chile

Seven 8.4-m primary mirrors—six off-axis surrounding the seventh central on axis; resolving power of a 24.5-m primary mirror

Gran Telescopio Canarios

10.4

Roque de los Muchachos Canary Islands, Spain

Segmented mirror of 36 hexagonal components based on Keck design

Keck I Keck II

10.0

W. W. Keck Mauna Kea, HI

Two mirrors of 36 segments each; 85 meters apart; usable as optical interferometer (Figure 2.15)

South African Large Telescope (SALT)

10

S. African Astronomical Sutherland, S.A.

Multinational, hexagonal mirror array, fixed elevation, based on Hobby-Eberly design

Hobby-Eberly

9.2

MacDonald Mt. Fowlkes, TX

Spherical segmented mirror, fixed elevation mounting; spectroscopy only

Large Binocular Telescope

8.4

LBT Corporation Mt. Graham, AZ

Two 8.4-m mirrors on one mount give light gathering of an 11.8-m and resolution of a 22.8-m mirror

Subaru Telescope

8.3

Japan Nat'l Astronomy Mauna Kea, HI

Lightweight (22.8t) meniscus primary mirror 20-cm thick; active support

Very Large Telescope

8.2

European Southern Cerro Paranal, Chile

Four separate 8.2-m telescopes: Antu (Sun), Kueyen (Moon), Melipal (Southern Cross), Yepun (Venus) or combined as a 16.4-m aperture

Gillet Gemini North Gemini South

8.0

Gemini« Mauna Kea, HI Cerro Pachon, Chile

Multinational optical/infrared twins offer unobstructed coverage of both northern and southern skies

MMT

6.5

Mount Hopkins, AZ

Lightweight primary mirror: concave front plate, flat back plate, honeycomb pattern of glass ribs in between

Walter Baade Landon Clay

6.5

Las Campanas La Serena, Chile

Twins Magellan I and II observe large parts of sky simultaneously

*Under construction.

a Facilities of the National Optical Astronomy Observatories (NOAO), headquartered in Tucson, AZ.

Large telescope performance is dramatically improved by new techniques. Adaptive optics adjust the mirrors to correct for rapid, hundredths-of-a-second distortions due to turbulence in Earth's atmosphere. Active optics correct for minute- or hour-long mirror-shape distortions due to gravity, temperature drifts, and wind.

What are the main advantages of an optical telescope over the unaided eye?

Answer: Superior light-gathering power and resolution. A telescope can also be equipped to record light over a long period of time.

Was this article helpful?

0 0
Telescopes Mastery

Telescopes Mastery

Through this ebook, you are going to learn what you will need to know all about the telescopes that can provide a fun and rewarding hobby for you and your family!

Get My Free Ebook


Post a comment