728 Encyclopedia of the Solar System
FIGURE 9 Large CCD mosaic installed in MegaCam, a prime
focus camera at the Canada-France-Hawaii. This mosaic consists
of 40 CCDs, each with 9.5 million pixels. In total the camera has
380 million pixels, the largest mosaic CCD currently in use. This
camera is capable of generating 100 billion bytes (100 gigabytes)
per night. Larger mosaic cameras are being planned. Each
telescope of the Pan-STARRS survey telescope will have a
1.4-Gigapixel camera and the Large Synoptic Survey Telescope
will have a single 3.2-Gigapixel camera. (Courtesy of CFHT)
plate. The rapid development of computing power and disk
storage has made it practical to use large CCD mosaics.
While astronomers have worked hard to develop CCD
technology that is optimized for astronomy, they are fortu-
nate that the consumer market has driven the development
of the necessary computing power and storage. Figure 9
shows an example of a state-of-the-art large format CCD.
There has been a similar revolution in the development
of infrared arrays. The first infrared arrays for astronomy
were used in the early 1980s. While initially very modest in
size (32×32 pixels), infrared arrays now typically contain
a million pixels. There are several significant differences
between CCDs and infrared arrays. One is that a CCD
has a single readout amplifier, while an infrared array has
one readout amplifier per pixel. The electrons in a CCD
are transferred to a single readout amplifier (hence the ori-
gin of the term “charge transfer”). Only a single readout
amplifier is needed since the readout electronics and the
detector material are made out of the same semiconductor
material. In an infrared array, the detector material and the
readout amplifier have to be made out of different mate-
rials, so each pixel must have a separate amplifier. A sec-
ond difference is that the infrared arrays must be cooled
to much lower temperatures. CCDs can operate effectively
at about−30 to− 40 ◦C. Infrared arrays must be cooled
to liquid nitrogen (− 196 ◦C) or liquid helium (− 269 ◦C)
temperatures.
We show in Figure 10 an example of Saturn imaged at
a wavelength of 18 micrometers. At these wavelengths, we
are observing the thermal emission (heat) from the planet.
Thus temperatures can be measured in the atmosphere of
Saturn and for the dust particles in the rings.
The development of large-format CCDs and infrared
arrays has enabled astronomers to undertake large-scale
digital sky surveys at visible and infrared wavelengths, just
as the use of large photographic plates enabled the first
deep sky surveys over 50 years ago.4. Advances in Adaptive Optics
Adaptive optics (AO) is a technique that removes the at-
mospheric disturbance and allows a telescope to achieveFIGURE 10 Image of Saturn and
its rings obtained in 2004 with the
10-m Keck I telescope at a
wavelength of 17.6 micrometers.
This is a false color image, where
higher signal levels are shown
lighter. At these wavelengths we
are seeing the heat radiated by the
atmosphere and rings of Saturn.
The South pole has an elevated
temperature (–182 C) compared to
its surrounding. This is likely due
to the fact that the South pole has
been illuminated by the sun for the
past 15 years. (Courtesy of G.
Orton, JPL).