The CCD detector Sami Dib, Max-Planck-Institute for Astronomy, Heidelberg Jean Surdej, Institut d’Astrophysique et de Géophysique, Liègemodified byMartin Hennemann, Stefan Hippler and Jutta Stegmaier (2006)1 Introduction2 History of the CCD3 How does a CCD work ?4 Advantages of CCDs5 Observations with a CCD
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.
2 HistoryIn 1969 Willard S. Boyle and George E. Smith, while working at Bell Laboratories, designedthe 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 digitalphotography and video. In January 2006 they have been honored with the Charles StarkDraper Prize which is presented by the National Academy of Engineering.
3 How does a CCD work? (1)Determining the distribution of an astronomical object (star, planet, galaxy, a martianspacecraft (?)) with the help of a CCD is similar to measuring the quantity of infalling rain ona field. As soon as the rain stops, collecting buckets are displaced horizontally on conveyorbelts. Then the water content of the buckets is collected in other buckets on a verticalconveyor belt. The overall content is sent onto a weighting system. 4
3 How does a CCD work? (2) pixel output register (a) (b)electrodes electrons to output amplifierThe way a CCD works is illustrated by means of a simplified CCD made out of 9 pixels, anoutput register and an amplifier. Each pixel is divided into 3 regions (electrodes who create apotential well). (a) For the charge collection process during an exposure the central electrodeof each pixel is maintained at a higher potential (yellow) than the others (green). (b) At theend of the exposure, the electrodes’ potentials are changed and the charges transferred fromone electrode to the other. 5
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.
4 Advantages of CCDs (1)1) Good spatial resolution2) Very high quantum efficiency3) Large spectral window4) Very low noise5) Large variations in the signal strength allowed (high dynamic range)6) High photometric precision7) Very good linearity8) A reliable rigidity
4 Advantages of CCDs (2)Spatial ResolutionMosaic of 4 CCDs containing fourtimes 2040 x 2048 pixels. Thiscomposite detector is about 6 cmlarge and contains a total of 16millions pixels (Kitt Peak NationalObservatory, Arizona).
4 Advantages of CCDs (3)Quantum EfficiencyAbove you see several quantum efficiency curves of different types of CCDs as a function ofthe wavelength. The large domain of wavelengths for the spectral response of CCDsbecomes obvious.
4 Advantages of CCDs (4)SpectralRangeFI: frontilluminatedBN: backilluminated, nocoatingDD: deepdepletion
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.
4 Advantages of CCDs (6)Flat field technique (a) (b) (c)
5 Observations with a CCD (15) CCD image of Arp 188 and the Tadpoles Tidal Tail taken with Hubble’s ACS camera.