The CCD detector

1
The CCD detector

• Sami Dib, Max-Planck-Institute for Astronomy,
  Heidelberg
• Jean Surdej, Institut dAstrophysique 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.
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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)

output register
pixel
(a)
(b)
to output amplifier
electrodes
electrons
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.
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3 How does a CCD work? (3)
(b)
(a)
impurity (doping)

• 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.

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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

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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).
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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.
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4 Advantages of CCDs (4)

• Spectral
• Range
• FI front
• illuminated
• BN back
• illuminated, no
• coating
• DD deep
• depletion

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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.
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4 Advantages of CCDs (6)

• Flat field technique

(b)
(c)
(a)
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5 Observations with a CCD (1)
4 exposures of the galaxy M100 with exposure
times of 1, 10, 100 and 1000 seconds (obtained
with a 11 inch Celestron telescope).
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5 Observations with a CCD (2)

• 5.1 Subtraction of the bias

Processed image
Raw image ...
15
5 Observations with a CCD (3)

• 5.2 The darks (1)
• Sn(t) Rn0 2(T - T0) / ?T t (5.2.1)

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5 Observations with a CCD (4)

• 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)

17
5 Observations with a CCD (5)

• 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)

18
5 Observations with a CCD (6)

• 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).
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5 Observations with a CCD (7)

• 5.4 Cosmic rays

The impact of many cosmic rays are visible on
this dark image
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5 Observations with a CCD (8)

• 5.5 Improving the S/N ratio of astronomical
  observations
• N ??N12 N22 N32 ...?
  (6.5.1)
• S So Sn Sc
  (6.5.2)
• N2 No2 Nn2 y2 Nc2
  (6.5.3)
• S/N (So Sn Sc) / ?? No2 Nn2 y2 Nc2?
  (6.5.4)

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5 Observations with a CCD (9)

• 5.5 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)

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5 Observations with a CCD (10)

• 5.5 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)

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5 Observations with a CCD (11)

• 5.5 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)

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5 Observations with a CCD (12)

• 5.6 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)

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5 Observations with a CCD (13)

• 5.6 Determination of the gain (and read out
  noise) of a CCD with
• the photon-transfer method

Linear slope CCD gain in units of e-/ADU
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5 Observations with a CCD (14)

• 5.6 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)

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5 Observations with a CCD (15)
CCD image of Arp 188 and the Tadpole's Tidal Tail
taken with Hubbles ACS camera.