CCD-1

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Basic Principles of CCD Imaging in Astronomy
Based on Slides by Simon Tulloch available
from http//www.ing.iac.es/smt/CCD_Primer/CCD_Pri
mer.htm
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What is a CCD?

• CCD Charge-Coupled Device
• Invented in 1970s, originally for
• Memory Devices
• Arithmetic Processing of Data
• When Made of Silicon (Si), has same
  Light-Sensitive Properties as Light Meters
• Use them to Measure Light
• Applied to Imaging as Sensor

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CCDs in Astronomy

• Revolutionized Astronomical Imaging
• More Sensitive than Photographic Emulsions
• Factor of 100? ? Measure Light only 0.01 as
  Bright
• Improved Light-Gathering Power of Telescopes by
  nearly 100?
• Amateur w/ 15-cm (6") Telescope CCD can get
  similar performance as 1960s Professional with
  1-m (40") Telescope Photography
• Now Considered to be Standard Sensor in
  Astronomical Imaging
• Special Arrangements with Observatory Now
  Necessary to use Photographic Plates or Film

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What is a CCD?

• Made from Crystalline Material
• Typically Silicon (Si)
• CCD Converts Light to Electronic Charge
• Spatial Pattern of Light Produces a Spatial
  Pattern of Charge Image
• Digitized
• Analog Measurements (Voltages) Converted to
  Integer Values at Discrete Locations
• Stored as Computer File

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Si Crystal Structure

• Regular Pattern of Si atoms
• Fixed Separations Between Atoms
• Atomic Structure Pattern Perturbs Electron
  Orbitals
• Changes Layout of Available Electron States from
  Model of Bohr Atom

http//www.webelements.com/webelements/elements/te
xt/Si/xtal.html
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Electron States in Si Crystal

• Available States in Crystal Arranged in Discrete
  Bands of Energies
• Lower Band ? Valence Band
• More electrons
• Upper Band ? Conduction Band
• Fewer electrons
• No States Exist in Gap Between Bands

Conduction Band of Electron States
Increasing energy
Gap 1.12 electron-volts (eV)
Gap
Valence Band of Electron States
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Comparison of State Structure in Crystal with
Bohr Model
Conduction Band
Orbitals
Valence Band
Gap
States Blur Together To Form Bands
Discrete Transition
Single Atom in Crystal
Isolated Atom (as in Gas)
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Action of Light on Electron States

• Incoming Photon w/ Energy ? 1.12 eV Excites
  Electrons From Valence Band to Conduction
  Band
• Electron in Conduction Band Moves in the Crystal
  Lattice
• Excited Electron e- leaves Hole (Lack of
  Electron h) in Valence Band
• Hole Carrier of Positive Charge

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Action of Charge Carriers

• Carriers are Free to Move in the Band
• Electron e- in Conduction Band
• Hole h in Valence Band
• Charge Carriers may be Counted
• Measurement of Number of Absorbed Photons

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Maximum ? to Jump Si Band Gap

• 1 eV 1.602 ? 10-12 erg 1.602 ? 10-12 Joule
• ? To Energize Electron in Si Lattice Requires
• ? lt 1.1 ?m

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Energy and Wavelength

• Incident Wavelength ? gt 1.1 ?m ? Photon CANNOT be
  Absorbed!
• Insufficient Energy to Kick Electron to
  Conduction Band
• ? Silicon is Transparent to long ?
• ? CCDs constructed from Silicon are Not Sensitive
  to Long Wavelengths

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After Electron is Excited into Conduction Band.

• Electron and Hole Usually Recombine Quickly
• Charge Carriers are Lost
• Apply External Electric Field to Separate
  Electrons from Holes
• Sweeps Electrons Away from Holes
• Maintains Population of Free Electrons
• Allows Electrons to be Counted

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Generation of CCD Carriers
photon
photon
Conduction Band
Valence Band
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Spontaneous Recombination
photon
photon
Conduction Band
Valence Band
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Prevent Spontaneous Recombination by Applying
Voltage to Sweep Electrons

??
Ammeter
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Prevent Spontaneous Recombination by Applying
Voltage to Sweep Electrons

?

?

?

?

??
Ammeter
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Thermal Noise

• Big BUT Other Kinds of Energy Have Identical
  Effect
• Thermally Generated Electrons are
  Indistinguishable from Photon-Generated Electrons
  
• Heat Energy can Kick e- into Conduction Band
• Thermal Electrons appear as Noise in Images
• Dark Current
• Keep CCDs COLD to Reduce Number of Thermally
  Generated Carriers (Dark Current)

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How Do We Count Charge Carriers
(Photoelectrons)?

• Must Move Charges to an Amplifier
• Astronomical CCDs Amplifier Located at Edge of
  Light-Sensitive Region of CCD
• Charge Transfer is Slow
• Most of CCD Area Sensitive to Light
• Video and Amateur Camera CCDs Must Transfer
  Charge QUICKLY
• Less Area Available to Collect Light

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Bucket Brigade CCD Analogy

• Electron Charge Generated by Photons is
  Transferred from Pixel to Edge of Array
• Transferred Charges are Counted to Measure
  Number of Photons

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Rain of Photons
VERTICAL COLUMNS of PIXELS
BUCKETS (PIXELS)
MEASURING CYLINDER (OUTPUT AMPLIFIER)
CONVEYOR BELT (SERIAL REGISTER)
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Rain of Photons
Shutter
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Empty First Buckets in Column Into Buckets in
Conveyor Belt
MEASURING CYLINDER (OUTPUT AMPLIFIER)
CONVEYOR BELT (SERIAL REGISTER)
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MEASURING CYLINDER (OUTPUT AMPLIFIER)
CONVEYOR BELT (SERIAL REGISTER)
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Empty Second Buckets in Column Into First Buckets
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Empty Third Buckets in Column Into Second Buckets
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Start Conveyor Belt
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After each bucket has been measured, the
measuring cylinder is emptied, ready for the
next bucket load.
Measure Drain
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Measure Drain
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Empty First Buckets in Column Into Buckets in
Conveyor Belt
Now Empty
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Empty Second Buckets in Column Into First Buckets
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Start Conveyor Belt
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Measure Drain
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Measure Drain
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Measure Drain
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Empty First Buckets in Column Into Buckets in
Conveyor Belt
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Start Conveyor Belt
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Measure Drain
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Measure Drain
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Measure Drain
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Ready for New Exposure
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Features of CCD Readout

• Pixels are Counted in Sequence
• Number of Electrons in One Pixel Measured at One
  Time
• Takes a While to Read Entire Array
• Condition of an Individual Pixel Affects
  Measurements of ALL Following Pixels
• A Leaky Bucket Affects Other Measurements in
  Same Column

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Leaky Bucket Loses Water (Charge)
for this Pixel
AND following Pixel
? Less Charge Measured for This Column
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Structure of Astronomical CCDs
Image Area
Package

• Image Area of CCD Located at Focal Plane of
  Telescope
• Image Builds Up During Exposure
• Image Transferred, pixel-by-pixel to Output
  Amplifier

Connection pins Gold bond wires
Bond pads Silicon chip
Output amplifier
Serial register (Conveyor Belt)
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CCD Manufacture
Don Groom LBNL
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Fabricated CCD
Kodak KAF1401
1317 ? 1035 pixels (1,363,095 pixels)
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Charges (Buckets are Moved by Changing Voltage
Pattern
Apply Voltages Here
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Charge Transfer
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Charge Transfer - 1
5V 0V -5V
5V 0V -5V
5V 0V -5V
Time-slice shown in diagram
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Charge Transfer - 2
5V 0V -5V
5V 0V -5V
5V 0V -5V
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Charge Transfer - 3
5V 0V -5V
5V 0V -5V
5V 0V -5V
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Charge Transfer - 4
5V 0V -5V
5V 0V -5V
5V 0V -5V
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Charge Transfer - 5
5V 0V -5V
5V 0V -5V
5V 0V -5V
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Charge Transfer - 6
5V 0V -5V
5V 0V -5V
5V 0V -5V
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Charge Transfer - 7
5V 0V -5V
5V 0V -5V
5V 0V -5V
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CCD Blooming - 1
Charge Capacity of CCD pixel is Finite (Up to
300,000 Electrons) After Pixel Fills, Charge
Leaks into adjacent pixels.
Spillage
Spillage
pixel boundary
pixel boundary
Overflowing charge packet
Photons
Photons
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CCD Blooming - 2

• Channel Stops (Charge Barrier)
• Charge Spreads in Column
• Up AND Down

Charge Transfer Direction
Flow of bloomed charge
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CCD Blooming - 3
M42

• Long Exposure for
• Faint Nebulosity
• ? Star Images are
• Overexposed

Bloomed Star Images with Streaks
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CCD Image Defects

• Dark Columns
• Charge Traps Block Charge Transfer
• Charge Bucket with a VERY LARGE Leak
• Not Much of a Problem in Astronomy
• 7 Bad Columns out of 2048
• ? Little Loss of Data

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CCD Image Defects

• Bright Columns
• Electron Traps
• Hot Spots
• Pixels with Larger Dark Current
• Caused by Fabrication Problems
• Cosmic Rays (?)
• Unavoidable
• Ionization of e- in Si
• Can Damage CCD if High Energy (HST)

Bright Column
Cluster of Hot Spots
Cosmic rays
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CCD Image Defects
M51
Negative Image
Dark Column
Hot Spots, Bright Columns

• Bright First Row
• incorrect operation of
• signal processing electronics

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CCD Image Processing

• Raw CCD Image Must Be Processed to Correct for
  Image Errors
• CCD Image is Combination of 4 Images
• Raw Image of Scene
• Bias Image
• Dark Field Image with Shutter Closed
• Flat Field Image of Uniformly Lit Scene

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Bias Frame

• Exposure of Zero Duration with Shutter Closed
• Zero Point or Baseline Signal from CCD
• Resulting Structure in Image from Image Defects
  and/or Electronic Noise
• Record ? 5 Bias Frames Before Observing
• Calculate Average to Reduce Camera Readout Noise
  by 1/?5 ? 45

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Dark Field Image

• Dark Current Minimized by Cooling
• Effect of Dark Current is Compensated Using
  Exposures of Same Duration Taken with Shutter
  Closed.
• Dark Frames are Subtracted from Raw Frames

Dark Frame
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Flat Field Image

• Sensitivity to Light Varies from Pixel to Pixel
• Fabrication Problems
• Dust Spots
• Lens Vignetting
• Image of Uniform (Flat) Field
• Twilight Sky at High Magnification
• Inside of Closed Dome

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Correction of Raw Imagewith Bias, Dark, Flat
Images
Raw File
Dark Frame
Raw ? Dark
Flat Field Image
Output Image
Bias Image
Flat ? Bias
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Correction of Raw Imagew/ Flat Image, w/o Dark
Image
Assumes Small Dark Current (Cooled Camera)
Raw File
Raw ? Bias
Bias Image
Output Image
Flat Field Image
Flat ? Bias