New Generation Ground-Based Optical/Infrared Telescopes 727
FIGURE 8 Improvement in angular resolution at optical
wavelengths. The development of adaptive optics has permitted
diffraction-limited observations from ground-based observatories
since 1990, largely eliminating the effects of the atmosphere. The
dashed line shows the theoretical diffraction-limited resolution for the
telescope. The solid line shows the seeing limit imposed by the
atmosphere. Improvements were obtained by going to very good
seeing sites. The resolution of the Hubble Space Telescope is shown,
(From P. Bely, 2003.)and motivation for future planetary missions. This is likely to
continue in the coming decades as the push to build ever-
larger telescopes continues.
Several groups in the US are proposing the next leap
in technology to a telescope in the 20–30-m class, and
the engineering studies have started. One proposal is
the Thirty-Meter Telescope, an international consortium
consisting of research groups in the US and Canada
(http://www.tmt.org/). This project proposes to build a tele-
scope similar in concept to the Keck telescopes that will
have over 700 hexagonal segments composing the pri-
mary mirror. As the name implies, the collecting area is
equivalent to a circular mirror 30 m in diameter. The
other project is the Giant Magellan Telescope, which is
supported by a group of public and private institutions in
the US (http://www.gmto.org/). This telescope concept con-
sists of seven 8.4-m mirrors to create a single telescope with
the collecting area equivalent to a 21.4-m circular mirror.
The European Southern Observatory is also considering
an even larger telescope concept (see http://www.eso.org/
projects/owl/). Thus it seems inevitable that a ground-based
telescope larger than 10 m will be built.
3. Advances with Detector Arrays
Initial observations with telescopes were conducted solely
with the human eye (still much recommended for the non-
professional), but the advantages of using photographic
plates to record and archive observations of the sky were
quickly exploited beginning in the 1850s. Photographic
plates were eventually supplemented with electronic de-
vices like the photomultiplier tube, which amplified the
signal from stars by about one million. At infrared wave-
lengths, there were specialized detectors that employed
bolometers, photovoltaic devices, and photoconductive de-
vices. However, photographic plates were a necessity for
recording high-resolution images of large areas of sky and
recording spectra with a wide wavelength range.
Images recorded by photographic plates depend on the
chemical reaction that is induced by a photon of light. Al-
though the efficiency of the photographic plate in convert-
ing a photon to an image is only a few percent, it allows
quantitative measurements to be made on the brightness
of stars and the strength of spectral lines. Most importantly,
the information is archived on the photographic plate for
future use. This was absolutely necessary for the develop-
ment of astrophysics.
The next technological revolution came with the inven-
tion of the charge-coupled device (CCD) in 1973. CCDs
are composed of millions of picture elements, or pixels.
Each pixel is a single detector and is capable of converting
photons to electrons. The accumulated electrons can then
be sent to an amplifier to be “read out” and recorded by
a computer. CCD technology is employed in digital cam-
eras, and just as digital photography is gradually replacing
photography, a similar transformation has taken place in
astronomy.
The impact of the CCD on astronomy was immediately
apparent after its first use. CCDs have two major advantages
over the photographic plate: the capability to directly record
photons with an efficiency of 80–90% and to store data
electronically. The stored data can then be processed with
a computer. Until recently, the main deficiency of the CCD
relative to the photographic plate was the relatively small
amount of sky that could be covered. However, the recent
development of very large CCD mosaics now permits larger
areas of sky to be covered by a CCD than by a photographic