Encyclopedia of the Solar System 2nd ed

724 Encyclopedia of the Solar System

FIGURE 5 Schematic of different telescope mounts: (a)
equatorial, (b) alt-az, (c) azimuth-only, (d) fixed. The Hale 5.1-m
telescope was the last large telescope to be built with an
equatorial mount. The equatorial mount has one axis aligned to
the rotation axis of the Earth. (Note: there are many types of
equatorial mounts. The Hale telescope uses a type known as the
horseshoe equatorial mount.) All fully steerable large telescopes
utilize the alt-az mount, such as the Keck, Gemini, VLT, and
Subaru telescopes (see Table 1). In the alt-az mount, the azimuth
axis points to the zenith with a perpendicular altitude axis. Two
large telescopes built specially for spectroscopy use the
azimuth-only mount—the Hobby-Eberly and the South African
Large Telescope. The telescope moves only in azimuth and is
fixed in declination. The only large telescope to date that uses a
fixed mount (the telescope points only to the zenith) is the Large
Zenith Telescope, and it uses a liquid mercury mirror.

the high-precision alignment at visible wavelengths pre-

sented formidable obstacles. Fortunately, the problems

of fabricating segmented mirrors and aligning them were

solved. The hexagonal mirror segments have a thickness of

75 mm, and so the aspect ratio of the 10-m primary is 133

and the total weight of the glass required is 14.4 tons, about

the same weight as the 5-m Hale telescope. Another novel

approach uses two 8.4-m primary mirrors on a single struc-

ture as in the Large Binocular Telescope (Fig. 6d). A third

approach involves building a telescope with a fixed vertical

elevation. Stars move past the prime focus and are tracked

for a limited time. This approach has limitations but is

much less expensive to build. Two projects (the Hobby-

Eberly Telescope and the South African Large Telescope)

have adopted this design to achieve 9-m class telescopes at

about 15–20% of the cost of an equivalent alt-az telescope.

An even less expensive approach is to simply stare at the

zenith with a liquid mercury mirror as demonstrated by

the Large Zenith Telescope.

FIGURE6a Hale 5.1-m telescope. The last large telescope to

be built in the “classical style” with an equatorial mount, a

culmination of about 280 years of development of the reflecting

telescope.©C2005 Gigapxl Project

Large telescopes generally employ one of three differ-

ent types of primary mirror fabrication. These are (1) Seg-

mented mirrors. Each segment is figured appropriately and

all segments are aligned so as to act as a single mirror.

(2) Thin meniscus mirror using low expansion glass. Such

mirrors are made as thin as possible to be light weight and

to have a short thermal time constant (thus coming into

equilibrium with the atmospheric temperature quickly).

(3) Thick honeycomb mirror using borosilicate glass. The

advantage of using borosilicate glass instead of low expan-

sion glass is that the former is much cheaper. The disad-

vantage of borosilicate glass is that the mirror temperature

needs to be controlled more carefully. All of these types of

primary mirror fabrication approaches have been proven

successful. Column (7) in Table 1 shows the type of mirror

used.

All large telescopes use active optics to control the shape

of the primary mirror. Active optics is the slow adjust-

ment of a mirror to correct aberrations in the image. These