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Basic Principles of CCD Imaging in Astronomy

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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_Primer.htm. What is a CCD?. “CCD” = “Charge-Coupled Device” Invented in 1970s, originally for: Memory Devices Arithmetic Processing of Data

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slide1

Basic Principles of CCD Imaging in Astronomy

Based on Slides by Simon Tulloch

available from

http://www.ing.iac.es/~smt/CCD_Primer/CCD_Primer.htm

what is a ccd
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
ccds in astronomy
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
what is a ccd1
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
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

Si Crystal Structure

http://www.webelements.com/webelements/elements/text/Si/xtal.html

electron states in si crystal

-

-

-

-

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

comparison of state structure in crystal with bohr model
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)

action of light on electron states
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
action of charge carriers
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
maximum to jump si band gap
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

 < 1.1 m

energy and wavelength
Energy and Wavelength
  • Incident Wavelength  > 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

after electron is excited into conduction band
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”
generation of ccd carriers

Hole

Electron

Generation of CCD Carriers

photon

photon

Conduction Band

Valence Band

spontaneous recombination
Spontaneous Recombination

photon

photon

Conduction Band

Valence Band

prevent spontaneous recombination by applying voltage to sweep electrons

+

+

+

+

Prevent Spontaneous Recombination by Applying Voltage to “Sweep” Electrons

+



Ammeter

prevent spontaneous recombination by applying voltage to sweep electrons1
Prevent Spontaneous Recombination by Applying Voltage to “Sweep” Electrons

+

+

+

+

+



Ammeter

thermal noise
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)
how do we count charge carriers photoelectrons
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
bucket brigade ccd analogy
“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
slide20

Rain of

Photons

VERTICAL

COLUMNS

of PIXELS

BUCKETS (PIXELS)

MEASURING

CYLINDER

(OUTPUT

AMPLIFIER)

CONVEYOR BELT

(SERIAL REGISTER)

slide21

Rain of

Photons

Shutter

slide22

Empty First Buckets in Column

Into Buckets in Conveyor Belt

MEASURING

CYLINDER

(OUTPUT

AMPLIFIER)

CONVEYOR BELT

(SERIAL REGISTER)

slide23

MEASURING

CYLINDER

(OUTPUT

AMPLIFIER)

CONVEYOR BELT

(SERIAL REGISTER)

slide29

After each bucket has been measured,

the measuring cylinder is emptied,

ready for the next bucket load.

Measure

& Drain

slide31

Measure

& Drain

slide34

Empty First Buckets in Column

Into Buckets in Conveyor Belt

Now Empty

slide40

Measure

& Drain

slide43

Measure

& Drain

slide46

Measure

& Drain

slide47

Empty First Buckets in Column

Into Buckets in Conveyor Belt

slide51

Measure

& Drain

slide53

Measure

& Drain

slide55

Measure

& Drain

features of ccd readout
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
slide58

“Leaky” Bucket Loses Water (Charge)

for this Pixel

AND following Pixel

 Less Charge Measured

for This Column

structure of astronomical ccds
Structure of Astronomical CCDs

Image Area

Package

Connection pins

Gold bond wires

Bond pads

Silicon chip

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

Output amplifier

Serial register

(Conveyor Belt)

ccd manufacture
CCD Manufacture

Don Groom LBNL

fabricated ccd
Fabricated CCD

Kodak KAF1401

1317  1035 pixels (1,363,095 pixels)

slide64

1

2

1

2

3

3

Charge Transfer - 1

+5V

0V

-5V

+5V

0V

-5V

+5V

0V

-5V

Time-slice shown in diagram

slide65

1

2

1

2

3

3

Charge Transfer - 2

+5V

0V

-5V

+5V

0V

-5V

+5V

0V

-5V

slide66

1

2

1

2

3

3

Charge Transfer - 3

+5V

0V

-5V

+5V

0V

-5V

+5V

0V

-5V

slide67

1

2

1

2

3

3

Charge Transfer - 4

+5V

0V

-5V

+5V

0V

-5V

+5V

0V

-5V

slide68

1

2

1

2

3

3

Charge Transfer - 5

+5V

0V

-5V

+5V

0V

-5V

+5V

0V

-5V

slide69

1

2

1

2

3

3

Charge Transfer - 6

+5V

0V

-5V

+5V

0V

-5V

+5V

0V

-5V

slide70

1

2

1

2

3

3

Charge Transfer - 7

+5V

0V

-5V

+5V

0V

-5V

+5V

0V

-5V

ccd blooming 1
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

slide72

CCD “Blooming” - 2

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

Charge

Transfer

Direction

Flow of

bloomed

charge

slide73

CCD “Blooming” - 3

M42

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

Bloomed Star Images

with “Streaks”

ccd image defects
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

ccd image defects1
CCD Image Defects

Bright

Column

  • 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)

Cluster of

Hot Spots

Cosmic rays

slide76

CCD Image Defects

M51

Negative Image

Dark Column

Hot Spots, Bright Columns

  • Bright First Row
  • incorrect operation of
  • signal processing electronics
ccd image processing
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
bias frame
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%
dark field image
“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

flat field image
“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
correction of raw image with bias dark flat images

“Raw”  “Dark”

“Flat”  “Bias”

Correction of Raw Imagewith Bias, Dark, Flat Images

Raw File

Dark Frame

“Raw”  “Dark”

Flat Field Image

Output

Image

Bias Image

“Flat”  “Bias”

correction of raw image w flat image w o dark image

“Raw”  “Bias”

“Flat”  “Bias”

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”

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