722 Encyclopedia of the Solar System
2. Advances in the Construction of Large
Telescopes and in Image Quality
The Hale 5.1-m telescope went into operation in 1949.
It represented the culmination of continual telescope
design improvements since the invention of the reflecting
telescope by Newton in 1668. The basic approach was to
scale up and improve design approaches that were used
previously. Figure 4 shows the increase in telescope aper-
ture with time. After the completion of the Hale telescope,
astronomers recognized that building larger telescopes
would require completely new approaches. Simple scaling
of the classical techniques would lead to primary mirrors
that would be too massive and an observatory (including
the dome enclosure) that would be too costly to build.
Since the 1990s, a number of ground-breaking approaches
have been tried, and the barrier imposed by classical
telescope design has been broken. Table 1 shows a list of
telescopes with apertures greater than 5 meters. Some of
the telescopes listed in Table 1 are still under development.
Major technical advances that have led to the develop-
ment of large telescopes include:
FIGURE 4 Increase in telescope area with time. Only the area
of the largest telescopes at each time period is shown, so this
indicates the envelope of maximum telescope area as a function
of year. The time for the telescope area to double is about 26
years from the invention of the telescope in 1608 to the current
year. However the doubling time has decreased from about 1900
to the present. The solid line shows a doubling of telescope
aperture about every 19 years. The next jump in aperture size is
likely to be in the range of 20â50 meters. For comparison the
square symbol shows a 30-m class telescope in the year 2020, and
this indicates an even shorter doubling time. The increase in
telescope area is due to advances in telescope construction
technology and the willingness of society to bear the costs. How
much longer can this increase in telescope area continue on the
ground? (See Racine 2004, Pub. Astron. Soc. Pacific, vol. 116,
p. 77) for data on the growth of telescope aperture with time.)
(1) Advances in computer-controlled hardware al-
lows correction for flexure of the primary mirror. This has
permitted thinner mirrors to be used, reducing the mass
of the mirror and the total mass of the telescope. For ex-
ample, the mass of the ESO Very Large Telescope 8.2-m
primary mirror is 23 tons with an aspect ratio (mirror di-
ameter to mirror thickness ratio) of 46. This is a very thin
mirror compared with the 5.1-m Hale telescope, which
has a weight of 14.5 tons and an aspect ratio of 9.
(2) Altitude-azimuth (alt-az) mounts reduce the size
of the required telescope enclosure. An 8-m alt-az tele-
scope can fit into the same size enclosure as a 4-m equa-
torial telescope. An alt-az telescope requires computer-
controlled pointing and tracking on two axes (whereas
the traditional mount requires tracking on only a single
axis). The Hale telescope is the largest equatorial tele-
scope ever built. All larger and more recent telescopes
use alt-az mountings. Figure 5 illustrates the basic types
of telescope mounts, and Figure 6 shows examples of the
equatorial and alt-az mounts.
(3) Advances in mirror casting and computer-
controlled mirror polishing allow the production of larger
primary mirrors with shorter focal lengths. A shorter fo-
cal length allows the telescope structure to be smaller,
thus lowering the weight and cost of the telescope. It also
greatly reduces the cost of the dome enclosure. The state-
of-the-art in short focal length primary mirrors are those
with a focal length to diameter ratio (f/no) of 1.14 installed
in the Large Binocular Telescope. This can be compared
to the Hale telescope primary mirror that has an f/no of
3.3. The smaller telescope structure with reduced mass re-
quires less time to reach thermal equilibrium, and its lower
mass makes it easier to move. This is extremely important
in achieving the best image quality and to efficiently repo-
sition in the telescope.
(4) Advances in reducing dome seeing led to signifi-
cant improvement in image quality. Dome seeing is caused
by temperature differences within the dome, especially
differences between the mirror and the surrounding air.
To reduce dome seeing, it is necessary to flush the dome
with outside air at night, refrigerate it during the daytime,
and cool the primary mirror to about 0.5âŚC below the am-
bient air temperature. Dome seeing is so important that
large telescope projects use wind tunnel experiments to
determine what type of dome design to employ. Careful
attention to dome design is critical in eliminating dome
seeing and achieving the very best seeing at the obser-
vatory site. Figure 6b shows an innovative approach to
providing dome flushing by providing slits in the dome.
(5) Advances in telescope construction have led to
novel methods of reducing the cost of building extremely
large telescopes. For example, the 10-m Keck telescopes
have segmented mirrors to make up the primary mirror
(Fig. 6c). Although this technique had been used to build
radio telescopes, the difficulty of making the segments and