The CCD detector This is an auxiliary document for the lab FP30 done at the 70cm King telescope of the MPIA (2001/02)

2 History CCDs were first developed at the end of the sixties by two researchers of the AT&T Bell Telephone Labs, Williard S. Boyle and George E. Smith, who tried to build a new type of electronic circuits with high memory capacity for computers (semiconductor bubble memory). It appeared very quickly that this new circuit, build on a silicate based semi-conductor substrate, is very sensitive to visible light. This produced a revolution in the technology of light detectors, especially for astronomical applications. Astronomers have been among the first scientists who recognized the extraordinary and promising capabilities and performances of CCDs, which started its life only as a memory device. Already in 1972, scientists at the Jet Propulsion Laboratory (JPL) in Pasadena (CA) have established a development program of CCDs for space astronomy. Three years later, a team from JPL in collaboration with astronomers from the university of Arizona have obtained the first astronomical image with a CCD camera (see the image on the previous page). In January 2006, Boyle and Smith received the Charles Stark Draper Prize which is presented by the National Academy of Engineering for their work on the CCD. Nowadays, CCDs are present in every instrument which uses electronic imaging (e.g., TV, video systems, cameras, etc...), as well as in all modern telescopes. Mainly, CCD detectors are present in visible light cameras for direct imaging, but they also serve as detectors for spectrographs. Additionally, most of the recent and future space missions (e.g., Galileo, Hubble telescope, etc.) are/will be equipped with instruments whose detectors are CCDs.

3 How does a CCD work? The way a CCD works is very easy in principle. A very nice analogy, suggested by Jerome Kristian, is often used (see figure above): One can imagine a network of large buckets distributed in a regular manner on a large surface farm. After a strong downpour, the buckets filled with water are transported on conveyor belts to a weighting station where the quantity of water which poured on the field‘s ground will be measured with high accuracy. Therefore it becomes possible to determine the overall quantity of rain which fell on the field as well as its spatial distribution. A CCD detector works with the same principles: rain drops are replaced by photons, the buckets by pixels, etc. (also see next figure).

3 How does a CCD work? … continued In order to produce an image, a CCD must accomplish four functions: generate photoelectrons (i.e., rain drops) collect electrons (i.e., the buckets) transfer the collected charges (i.e., the conveyor belts) read the charges (i.e., weighting device) The first function is based on the photoelectric effect. The light absorption in the silicate network of the CCD generates these photoelectrons, in proportion to the number of incident photons. The latter are immediately collected in “picture elements” so-called pixels (i.e., the buckets), closest to where the photons fell on the chip. Those pixels are defined by means of an electrode network which covers the surface of a CCD. The electrodes form potential wells to prevent the collected charges from escaping.

3 How does a CCD work? … continued When the collection of the charges is finished, their transfer (i.e., displacement of buckets on the conveyor belt) is realized by changing the potentials of each electrode in a synchronized way. Therefore electrons can move horizontally from pixel to pixel and finally to the counting device (output register) at the end of each horizontal line. The output register consists of a series of electrodes, which lie outside the photosensitive zone of the CCD and perpendicular to it. This output register sends each charge packages to an output amplifier where charges are digitalized and stocked into a computer hard-disk. Afterwards the registered signal can be calibrated, analyzed, etc.. Therefore the distribution of the observed astronomical object can be regained from the form of the numerical image. The CCD as a 3 dimensional detector Photons in the ultraviolet domain (  1200-3500 Å), far ultraviolet (  120-1200 Å) and soft X rays (  1,2-120 Å) are much more energetic than visible photons. When they get absorbed in the silicate network of the CCD, these high energy photons generate much more electrons; the exact number depends on the wavelength of the photon. For example, an X ray photon which has a wavelength of 2,1 Å is capable to generate, in average, 1620 electrons. For wavelengths typically shorter than 100 Å, a single photon is detected each time and its energy is determined by measuring directly the quantity of generated charges.

4 Advantages of CCDs … continued The resolution and the field of an image taken by a CCD depend on the number of pixels the CCD is made of. The first large CCDs produced in 1973 by the Fairchild Semiconductor company were made of 10 000 pixels (100 columns and 100 lines). Today, most common CCDs have 2048  2048 pixels (about 4 millions pixels). But there exist even larger CCDs with 4096  4096 pixels or 4096  8192 pixels (10 k x 10 k will be build soon). For realizing even larger chips (and since larger CCD chips are very expensive), several small chips can be placed together resembling a CCD mosaic (see figure above). E.g., the WFI camera at the 2.2 m telescope in Chile built at MPIA, has 8 chips with 2048  4096 pixels each. The size of a single pixel is in the order of 8  8, 15  15 or 25  25 microns. Therefore the size of CCD chips remain quite small, especially by comparing CCDs to classical photographic plate images. E.g., a CCD with 2048  2048 pixels of 15 microns measures only 3  3 cm². In contrast a photographic plate for a Schmidt telescope can be as big as 30  30 cm², equivalently to a CCD chip with 400 million pixels!

Detector CCD Inventores da fibra óptica e câmera digital ganham Nobel Três pesquisadores que criaram a tecnologia responsável pela fotografia digital e ajudaram a diminuir as distâncias do mundo por meio de redes de fibra óptica dividiram o prêmio Nobel de Física de 2009. O anúncio da categoria foi feito hoje, em Estocolmo, na Suécia. O cientista Charles K. Kao foi agraciado por seu trabalho na inovação em matéria de transmissão de luz através de fibras ópticas. Já Willard S. Boyle e George E. Smith venceram por inventar um circuito semicondutor de imagens conhecido como sensor CCD (Charge Coupled Device). Vamos conhecer um pouco o CCD? Referencia http://www.mpia-hd.mpg.de/AO/INSTRUMENTS/FPRAKT/CCD_online/CCD_onlineSep2006.ppt_files/CCD_onlineSep2006.ppt.ppt Adaptado pro profa Marisa Almeida Cavalcante (marisac@pucsp.br)

In 1969 Willard S. Boyle and George E. Smith, no Laboratório Bell, mostrando a captura de imagem pelo CCD (Charge Coupled Device). Historia

3 Uma analogia Supondo a chuva (intensidade de luz) e os Baldes (pixels). Um sistema de ”esteiras” acopladas, permite obter informações da intensidade de chuva que foi capturada com cada balde.

4 Como funciona Acima um exemplo de um CCD com 9 pixels. Esta matriz é constituida por fotosensores distribuidos em linhas e colunas. (A) Cada pixel na incidencia de luz emite eletron que são coletados nos eletrodos amarelos. B) Estes eletrons darão origem a uma corrente eletrica . O registro se faz atraves de uma varredura em tempo sincronizada. Se o sincronismo for bem esrtuturado será possivel reconstruir a intensidade de luz que cada pixel foi submetido, do mesmo modo que nos baldes da figura anterior. output register

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. (b) (a) impurity (doping) 3 How does a CCD work? (3)

Um imagem de um detector CCD 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).