Transcript CCD-1 - RIT Center for Imaging Science

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
• When Made of Silicon (Si), has same LightSensitive Properties as Light Meters
– Use them to “Measure” Light
• Applied to Imaging as Sensor
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 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”
1. “Digitized”
–
Analog Measurements (“Voltages”) Converted to Integer
Values at Discrete Locations
2. 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
http://www.webelements.com/webelements/elements/text/Si/xtal.html
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
Increasing
energy
Conduction Band of Electron States
“Gap”
- - Valence Band of Electron States
“Gap” = 1.12 electron-volts
(eV)
Comparison of State Structure in
Crystal with Bohr Model
Conduction Band
Orbitals
Valence Band
“Gap”
States “Blur” Together
To Form “Bands”
Discrete Transition
Isolated Atom (as in Gas)
Single Atom in Crystal
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”
• 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
• 1 eV = 1.602  10-12 erg = 1.602  10-12 Joule

8 m 
6.624 10 erg  sec    3 10


hc
sec 

 
E

12 erg 
1.12eV  1.602 10

eV


 1.107 106 m  1107 nm
27
 To Energize Electron in Si Lattice Requires
 < 1.1 m
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….
• 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
Conduction Band
Hole
Electron
Valence Band
Spontaneous Recombination
Conduction Band
Valence Band
Prevent Spontaneous
Recombination by Applying
Voltage to “Sweep” Electrons
+
+
+
+

+
Ammeter
Prevent Spontaneous
Recombination by Applying
Voltage to “Sweep” Electrons




+
+
+
+

+
Ammeter
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”)?
• 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
• Electron Charge Generated by Photons is
“Transferred” from Pixel to “Edge” of Array
• Transferred Charges are “Counted” to
Measure Number of Photons
Rain of
Photons
BUCKETS (PIXELS)
CONVEYOR BELT
(SERIAL REGISTER)
VERTICAL
COLUMNS
of PIXELS
MEASURING
CYLINDER
(OUTPUT
AMPLIFIER)
Rain of
Photons
Shutter
Empty First Buckets in Column
Into Buckets in Conveyor Belt
CONVEYOR BELT
(SERIAL REGISTER)
MEASURING
CYLINDER
(OUTPUT
AMPLIFIER)
CONVEYOR BELT
(SERIAL REGISTER)
MEASURING
CYLINDER
(OUTPUT
AMPLIFIER)
Empty Second Buckets in Column
Into First Buckets
Empty Third Buckets in Column
Into Second Buckets
Start Conveyor Belt
After each bucket has been measured,
the measuring cylinder is emptied,
ready for the next bucket load.
Measure
& Drain
Measure
& Drain
Empty First Buckets in Column
Into Buckets in Conveyor Belt
Now Empty
Empty Second Buckets in Column
Into First Buckets
Start Conveyor Belt
Measure
& Drain
Measure
& Drain
Measure
& Drain
Empty First Buckets in Column
Into Buckets in Conveyor Belt
Start Conveyor Belt
Measure
& Drain
Measure
& Drain
Measure
& Drain
Ready for New Exposure
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
“Leaky” Bucket Loses Water (Charge)
for this Pixel
AND following Pixel
 Less Charge Measured
for This Column
Structure of Astronomical CCDs
Image Area
Package
Serial register
(Conveyor Belt)
• Image Area of
Connection pins
CCD Located at
Focal Plane of
Gold bond wires
Telescope
Bond pads
• Image Builds Up
During Exposure
• Image Transferred,
Silicon chip
pixel-by-pixel to
Output amplifier
Output Amplifier
CCD Manufacture
Don Groom LBNL
Fabricated CCD
Kodak KAF1401
1317  1035 pixels (1,363,095 pixels)
Charges (“Buckets” are Moved
by Changing Voltage Pattern
Apply Voltages
Here
1
2
3
Charge Transfer
1
2
3
Charge Transfer - 1
+5V
2
0V
-5V
+5V
1
2
3
1
0V
-5V
+5V
3
0V
-5V
Time-slice shown in diagram
Charge Transfer - 2
+5V
2
0V
-5V
+5V
1
2
3
1
0V
-5V
+5V
3
0V
-5V
Charge Transfer - 3
+5V
2
0V
-5V
+5V
1
2
3
1
0V
-5V
+5V
3
0V
-5V
Charge Transfer - 4
+5V
2
0V
-5V
+5V
1
2
3
1
0V
-5V
+5V
3
0V
-5V
Charge Transfer - 5
+5V
2
0V
-5V
+5V
1
2
3
1
0V
-5V
+5V
3
0V
-5V
Charge Transfer - 6
+5V
2
0V
-5V
+5V
1
2
3
1
0V
-5V
+5V
3
0V
-5V
Charge Transfer - 7
+5V
2
0V
-5V
+5V
1
2
3
1
0V
-5V
+5V
3
0V
-5V
CCD “Blooming” - 1
Charge Capacity of CCD pixel is Finite
(Up to 300,000 Electrons)
After Pixel Fills, Charge Leaks into adjacent pixels.
Spillage
pixel
boundary
Photons
pixel
boundary
Overflowing
charge packet
Photons
Spillage
CCD “Blooming” - 2
Channel “Stops” (Charge Barrier)
Charge
Transfer
Direction
Charge Spreads in Column
• Up AND Down
Flow of
bloomed
charge
CCD “Blooming” - 3
M42
• Long Exposure for
Faint Nebulosity
 Star Images are
Overexposed
Bloomed Star Images
with “Streaks”
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 Defects
1. Bright Columns
Bright
Column
–
2. Hot Spots
–
Cluster of
Hot Spots
Electron “Traps”
–
Pixels with Larger Dark
Current
Caused by Fabrication
Problems
3. Cosmic Rays ()
Cosmic rays
–
–
–
Unavoidable
Ionization of e- in Si
Can Damage CCD if
High Energy (HST)
CCD Image Defects
M51
Negative Image
Dark Column
Hot Spots, Bright Columns
Bright First Row
• incorrect operation of
signal processing electronics
CCD Image Processing
•
•
“Raw” CCD Image Must Be Processed to
Correct for Image Errors
CCD Image is Combination of 4 Images:
1.
2.
3.
4.
“Raw” Image of Scene
“Bias” Image
“Dark Field” Image with Shutter Closed
“Flat Field” Image of Uniformly Lit Scene
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 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
• 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 File
r  x, y  d  x, y
r  x, y 
Dark Frame
d  x, y
Flat Field Image
“Raw”  “Dark”
“Flat”  “Bias”
f  x, y  b  x, y
b  x, y 
r  x, y   d  x, y 
f  x, y   b  x, y 
f  x, y
Bias Image
“Raw”  “Dark”
“Flat”  “Bias”
Output
Image
Correction of Raw Image
w/ Flat Image, w/o Dark Image
Raw File
r  x, y   b  x, y 
r  x, y 
“Raw”  “Bias”
Bias Image
b  x, y 
f  x, y  b  x, y
Assumes Small Dark Current
(Cooled Camera)
“Raw”  “Bias”
“Flat”  “Bias”
r  x, y   b  x, y 
f  x, y   b  x, y 
Flat Field Image
f  x, y
“Flat”  “Bias”
Output
Image