CCDs do not work at wavelengths longer than about 
m, as the
photon energies are smaller than the band-gap of silicon. IR detectors
therefore use different and exotic materials, typically either
HgCdTe (Mercury-Cadmium-Telluride) or InSb (Indium Antimonide). These
materials were mainly developed by US defense researchers for use in
missiles, and nearly all the world's IR detector chips are
purchased from an handful of US companies, for very large prices (
A$ 100,000). The technology is evolving very rapidly, and detectors
even a couple of years old are often considered obsolete.
In near-IR detector chips, the incoming photons create a voltage pattern in an array of pixels. Each pixel is bonded onto a circuit network, which enables the control electronics to read the voltage of any individual pixel at any time, without altering it. This is quite different from a CCD chip, where measuring the charge pattern requires reading out the whole chip. What this means in practice is that IR arrays can be read out extremely quickly; typically in less than a second.
As compared to CCDs, IR arrays have reasonable quantum efficiencies
(around 50%), but high read-out noise and dark currents. However,
the latter two hardly matter for imaging applications, as the brightness of
the sky swamps all other sources of noise. Currently HgCdTe chips have
better quantum efficiency, but they only work out to wavelengths of
m. InSb chips are a little less efficient, but work out to
m.
There are currently two near-IR cameras in use in Australia:
128 in size, with 60
m square pixels. The camera is
very flexible, with an amazing number of observing modes. It can be
operated as an imager with pixel scales of between 1.94 arcsec and
0.24 arcsec, and for spectroscopy with resolution 300 or 400. Being
HgCdTe, it only operates out to the K-band. Details are on-line at the
AAO web site, or contact Stuart Lumsden.
256), which can be set to have imaging scales of
or 
arcsec per pixel. A variety of spectral modes are
being implemented, giving resolutions of 500 to 1100. Being more modern
than IRIS, its quantum efficiency is higher, and its sensitivity is
comparable to IRIS, despite the smaller aperture of the 2.3-m. Being
InSb, it works out to wavelengths of m. Details (including
an excellent manual) are on-line at the Mt Stromlo web site, or contact
Peter McGregor.
Experience suggests that for imaging purposes, the superior quantum efficiency of CASPIR cancels the mirror size advantage of IRIS and both have comparable sensitivity. This means that CASPIR (with its larger field of view) is to be preferred for most purposes. The AAO are currently planning to build a much larger and more powerful IR camera, with a 1k chip, in which case the balance of IR power will shift down the mountain once more!
Overseas telescopes with modern HgCdTe detectors, on cold, dry high altitude sites, can do substantially better.