Charge Coupled Devices CCDs

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Charge Coupled Devices (CCDs)

• Analogy to rain falling in buckets
• Conveyor belt moves buckets off to left and out
• actually the buckets dont move just the
  water like

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Shifting the water out
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The array
The CCD array a a grid of light sensitive picture
elements (pixels) that record the image that
falls on it. Each pixel as seen here accumulates
a charge signal proportional to the incident
light. As the exposure proceeds these numbers
increase
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Image accumulation

• As illustrated here the signal increases linearly
  with time.
• The limit is when the full well capacity is
  reached for a pixel.

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Linearity

• Signal is linear to exposure over a certain
  range.
• Beyond that

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Saturation and blooming
the pixel saturates and you get blooming
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CCD electronics semiconductors
We use semiconductors which are based on
tri-valent elements like boron or pentavalent
elements like phosphorus, doped in small
concentrations into a matrix of silicon (or
germanium). Chalk board
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CCDs basic pixel construction
This shows the basic layer structure of a single
picture element (pixel) of a CCD.
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Completing the pixel circuit
Providing a bias voltage causes a depletion
region to forma lack of holes in the p-type
layer. Note this leaves some charge there even
with no lightthe removal is by subtracting
bias images produced by reading the chip with
no exposure.
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Now, illuminate our pixel

• Electron-hole separation charging like a
  capacitor, q ? I
• Illuminated from above frontside
  illuminated chip
• Illuminated from below backside illuminated
  (may be thinned)

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A simple array
Shown a simple linear array (one row or column
of a 2-D CCD. Light pattern now mapped as
charges at each pixel.
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Reading out pixels by voltage changes
Voltages define the charge separation and can be
used to clock it out by coupling the charges to
adjacent pixels. (Chalk board) Hence, Charge
Coupled Device
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The charge-transfer cycle
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Putting it all together
Three voltages/pixel used to shift the signal out
of each row, in this 3-cycle system. Clock rates
in 100s of kilohertz
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CCD readout full-frame
In this full-frame transfer chip the rows are
shifted up one at a time, read out the serial
register and amplified and A/D converted. (details
chalk talk) Note a mechanical shutter must
block the light during the readout.
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CCD readout frame transfer
In the frame-transfer method and entire
(light-masked) region of the chip receives the
transferred image, which is read out while the
next exposure is under way. Good for video. The
STV cameras use this.
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Some Notes

• Charge Transfer Efficiency the CTE must be
  better than 0.99999 from pixel-to-pixel. For
  example, 0.99992048 0.81 (19 loss)
• A bad pixel can kill or change all charge
  transferred through it.
• CCDs graded on of defects and where on the chip
  and type of defects
• Cosmic rays leave a small streakneed to remove
  from images of darks, flats, biases with median
  averaging
• Too bright an object can leave a trace image that
  will gradually go away over tens of minutes (or,
  recycle it thermallywarm it up)
• Lets look at properties of detectors