1. The CCD detector
Sami Dib, Max-Planck-Institute for Astronomy, Heidelberg
Jean Surdej, Institut d’Astrophysique et de Géophysique, Liège
modified by
Martin Hennemann, Stefan Hippler and Jutta Stegmaier (2006)
1 Introduction
2 History of the CCD
3 How does a CCD work ?
4 Advantages of CCDs
5 Observations with a CCD
2. 1 Introduction
It seems that this near-infrared
(8900 Å) picture of Uranus was the
first celestial object to be
photographed by a CCD in 1975 by
astronomers at the JPL and
University of Arizona. This image
has been obtained by the 61 inch
telescopes located at Santa
Catalina mountains near Tucson
(Arizona).
The dark region in the image
correspond to an absorption
region with some Methane bands
close to the southern pole of
Uranus.
3. 2 History
In 1969 Willard S. Boyle and George E. Smith, while working at Bell Laboratories, designed
the first Charge Coupled Device (CCD), a working version was produced just a year later.
The CCD has become the bedrock of the digital imaging revolution including digital
photography and video. In January 2006 they have been honored with the Charles Stark
Draper Prize which is presented by the National Academy of Engineering.
4. 3 How does a CCD work? (1)
Determining the distribution of an astronomical object (star, planet, galaxy, a martian
spacecraft (?)) with the help of a CCD is similar to measuring the quantity of infalling rain on
a field. As soon as the rain stops, collecting buckets are displaced horizontally on conveyor
belts. Then the water content of the buckets is collected in other buckets on a vertical
conveyor belt. The overall content is sent onto a weighting system.
4
5. 3 How does a CCD work? (2)
pixel output
register
(a) (b)
electrodes electrons to output amplifier
The way a CCD works is illustrated by means of a simplified CCD made out of 9 pixels, an
output register and an amplifier. Each pixel is divided into 3 regions (electrodes who create a
potential well). (a) For the charge collection process during an exposure the central electrode
of each pixel is maintained at a higher potential (yellow) than the others (green). (b) At the
end of the exposure, the electrodes’ potentials are changed and the charges transferred from
one electrode to the other.
5
6. 3 How does a CCD work? (3)
(a) (b)
impurity (doping)
(a) By changing the potential of the electrodes in a synchronized way, electrons are
transferred from pixel to pixel. Charges on the right are guided to the output register
(b) The horizontal transfer of charges is then stopped and each charge package at the output
register is transferred vertically to an output amplifier and then read one by one. The
cycle starts again until all the charges have been read. The reading time amounts to
about one minute for a large CCD.
7. 4 Advantages of CCDs (1)
1) Good spatial resolution
2) Very high quantum efficiency
3) Large spectral window
4) Very low noise
5) Large variations in the signal strength allowed (high dynamic
range)
6) High photometric precision
7) Very good linearity
8) A reliable rigidity
8. 4 Advantages of CCDs (2)
Spatial Resolution
Mosaic of 4 CCDs containing four
times 2040 x 2048 pixels. This
composite detector is about 6 cm
large and contains a total of 16
millions pixels (Kitt Peak National
Observatory, Arizona).
9. 4 Advantages of CCDs (3)
Quantum Efficiency
Above you see several quantum efficiency curves of different types of CCDs as a function of
the wavelength. The large domain of wavelengths for the spectral response of CCDs
becomes obvious.
10. 4 Advantages of CCDs (4)
Spectral
Range
FI: front
illuminated
BN: back
illuminated, no
coating
DD: deep
depletion
11. 4 Advantages of CCDs (5)
Linearity and Dynamic Range
Dynamic range = ratio between brightest and faintest detectable signal
CCDs are extremely linear detectors, i.e.,
the received signal increases linearly with
the exposure time (see figure on the left).
Therefore CCDs enable the simultaneous
detection of both very faint and very
bright objects. In contrast photographic
plates have a very limited linear regime:
there is a minimum exposure time for an
image of an object to form. Further on
during the exposure, the image gets
saturated quickly (S-shape gamma
curve). The dynamic range of CCDs is
about 100 times larger compared to films.
