The CCD detector

Sami Dib, Max-Planck-Institute for Astronomy, HeidelbergJean Surdej, Institut d’Astrophysique et de Géophysique, Liège

modified by Martin Hennemann, Stefan Hippler and Jutta Stegmaier (2006)

1 Introduction2 History of the CCD 3 How does a CCD work ?4 Advantages of CCDs 5 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 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.

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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.

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3 How does a CCD work? (2)

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.

to output amplifier

outputregister

pixel

electrodes electrons

(a) (b)

(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.

(b)(a)

impurity (doping)

3 How does a CCD work? (3)

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 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).

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.

4 Advantages of CCDs (4)Spectral Range

FI: frontilluminated

BN: backilluminated, nocoating

DD: deep depletion

4 Advantages of CCDs (5)Linearity and Dynamic Range

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.

Dynamic range = ratio between brightest and faintest detectable signal

4 Advantages of CCDs (6)Flat field technique

(a) (b) (c)

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4 exposures of the galaxy M100 with exposure times of 1, 10, 100 and 1000 seconds (obtained with a 11 inch Celestron telescope).

5 Observations with a CCD (1)

5 Observations with a CCD (2)5.1 Subtraction of the bias5.1 Subtraction of the bias

Raw image ...

Processed image

5 Observations with a CCD (3)5.2 The darks (1)5.2 The darks (1)

Sn(t) = Rn0 2(T - T0) / T t (5.2.1)

5 Observations with a CCD (4)5.2 The darks (2)5.2 The darks (2)

ST = n · S and NT2

= (n · N2) (5.2.2)

ST / NT = (S / N) n (5.2.3)

S = Sa - ST and N = (Na2

+ NT2) (5.2.4)

S / N = (Sa - ST) / (Na2

+ NT2) (5.2.5)

5 Observations with a CCD (5)5.3 The flat field technique (1)5.3 The flat field technique (1)

S = So / Sf (5.3.1)

(S/N) = 1 / [(No/So)2 + (Nf/Sf)2] (5.3.2)

5 Observations with a CCD (6)5.3 The flat field technique (2)5.3 The flat field technique (2)

Raw image (left) from which we subtract the Bias image (middle) and the dark image (right).

We then divide the obtained result by the flat field image (left) and obtain the final image (right).

5 Observations with a CCD (7)5.4 Cosmic rays 5.4 Cosmic rays

The impact of many cosmic rays are visibleon this dark image

5 Observations with a CCD (8)5.5 Improving the S/N ratio of astronomical observations5.5 Improving the S/N ratio of astronomical observations

N = N12 + N2

2 + N32 + ... (6.5.1)

S = So + Sn + Sc (6.5.2)

N2 = No2 + Nn

2 + y2 + Nc2 (6.5.3)

S/N = (So + Sn + Sc) / No2 + Nn

2 + y2 + Nc2 (6.5.4)

5 Observations with a CCD (9)5.5 Improving the S/N ratio of astronomical observations5.5 Improving the S/N ratio of astronomical observations

S/N = (So + Sn + Sc) / So + Sn + Sc + y2 (5.5.5)

S/N = Co / 1 + Cc / Co + DC y2 / Co (5.5.6)

S/N = Co (5.5.7)

5 Observations with a CCD (10)5.5 Improving the S/N ratio of astronomical observations5.5 Improving the S/N ratio of astronomical observations

S1 = Si = n · Si, N1 = (Si) = (n · Si), S1/N1 = (n · Si)

(5.5.8)

S2 = n · Si, N2 = S2, S2/N2 = (n · Si) (5.5.9)

5 Observations with a CCD (11)5.5 Improving the S/N ratio of astronomical observations5.5 Improving the S/N ratio of astronomical observations

S1 = Si = n · Si, N1 = ((Si + y2)) (n · y2)

S1/N1 = (n · Si)(Si /y) (5.5.10)

S2 = n · Si, N2 = S2, S2/N2 = (n · Si) (5.5.11)

S1/N1 = S2/N2 (Si / y) S2/N2 (5.5.12)

5 Observations with a CCD (12)5.6 Determination of the gain and the read out noise of a CCD5.6 Determination of the gain and the read out noise of a CCD

g nmax / 216 (5.6.1)

N2 = So + Sn + Sc + y2 (5.6.2)

N2ADU = SADU / g + BDL2 (5.6.3)

5 Observations with a CCD (13)5.6 Determination of the gain (and read out noise) of a CCD with5.6 Determination of the gain (and read out noise) of a CCD with

the photon-transfer methodthe photon-transfer method

Linear slope = CCD gain in units of e-/ADU

5 Observations with a CCD (14)5.6 Determination of the gain and read out noise of a CCD5.6 Determination of the gain and read out noise of a CCD

(f1 / f2) / f1/f2 2 = 1 / (f1/f1)2 + (f2/f2)2 1 / 2(f/f)2 (5.6.8)

f2 = (f2 / 2) (f1/f2)2 (5.6.9)

5 Observations with a CCD (15)

CCD image of Arp 188 and the Tadpole's Tidal Tail taken with Hubble’s ACS camera.