basic principles of ccd imaging in astronomy
DESCRIPTION
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 - PowerPoint PPT PresentationTRANSCRIPT
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
• “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
What is a CCD?
• 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
CCDs in Astronomy
• 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
What is a CCD?
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
Incr
easi
ng e
nerg
y
Valence Band of Electron States
Conduction Band of Electron States
“Gap” = 1.12 electron-volts(eV) - - -
-“Gap”
Comparison of State Structure in Crystal with Bohr Model
Orbitals
Discrete Transition
Isolated Atom (as in Gas)
Conduction Band
Valence Band
Single Atom in Crystal
“Gap”
States “Blur” Together To Form “Bands”
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
To Energize Electron in Si Lattice Requires < 1.1 m
27 8
12
6
6.624 10 sec 3 10sec
1.12 1.602 10
1.107 10 1107
merghc
ergE eVeV
m nm
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”
photon
phot
on
Hole
Electron
Conduction Band
Valence Band
Generation of CCD Carriers
photon
phot
on
Conduction Band
Valence Band
Spontaneous Recombination
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
BUCKETS (PIXELS)
VERTICALCOLUMNS of PIXELS
CONVEYOR BELT
(SERIAL REGISTER)
MEASURING CYLINDER(OUTPUT
AMPLIFIER)
Rain of Photons
Shutter
Rain of Photons
CONVEYOR BELT
(SERIAL REGISTER)
MEASURING CYLINDER(OUTPUT AMPLIFIER)
Empty First Buckets in Column Into Buckets in Conveyor Belt
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
Measure& Drain
After each bucket has been measured,the measuring cylinder is emptied,
ready for the next bucket load.
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
for this Pixel
“Leaky” Bucket Loses Water (Charge)
AND following Pixel
Less Charge Measuredfor This Column
Structure of Astronomical CCDs• 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
PackageImage Area
Serial register(Conveyor Belt)
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
123
Apply VoltagesHere
123
Charge Transfer
123
+5V
0V
-5V
+5V
0V
-5V
+5V
0V
-5V
Time-slice shown in diagram
1
2
3
Charge Transfer - 1
123
+5V
0V
-5V
+5V
0V
-5V
+5V
0V
-5V
1
2
3
Charge Transfer - 2
123
+5V
0V
-5V
+5V
0V
-5V
+5V
0V
-5V
1
2
3
Charge Transfer - 3
123
+5V
0V
-5V
+5V
0V
-5V
+5V
0V
-5V
1
2
3
Charge Transfer - 4
123
+5V
0V
-5V
+5V
0V
-5V
+5V
0V
-5V
1
2
3
Charge Transfer - 5
123
+5V
0V
-5V
+5V
0V
-5V
+5V
0V
-5V
1
2
3
Charge Transfer - 6
123
+5V
0V
-5V
+5V
0V
-5V
+5V
0V
-5V
1
2
3
Charge Transfer - 7
pixe
l bo
unda
ry
Phot
ons
Charge Capacity of CCD pixel is Finite (Up to 300,000 Electrons)
After Pixel Fills, Charge Leaks into adjacent pixels.
Phot
ons
Overflowingcharge packet
Spillage Spillagepi
xel
boun
dary
CCD “Blooming” - 1
Flow of bloomed
charge
Channel “Stops” (Charge Barrier)
Charge Spreads in Column• Up AND Down
CCD “Blooming” - 2
ChargeTransferDirection
Bloomed Star Imageswith “Streaks”
M42
CCD “Blooming” - 3
• Long Exposure for Faint Nebulosity
Star Images are Overexposed
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
1. Bright Columns– Electron “Traps”
2. Hot Spots– Pixels with Larger Dark Current– Caused by Fabrication Problems
3. Cosmic Rays ()– Unavoidable– Ionization of e- in Si– Can Damage CCD if High
Energy (HST)
CCD Image Defects
Cosmic rays
Cluster ofHot Spots
BrightColumn
M51
Dark Column
Hot Spots, Bright Columns
Bright First Row • incorrect operation of signal processing electronics
CCD Image Defects
Negative Image
CCD Image Processing
• “Raw” CCD Image Must Be Processed to Correct for Image Errors
• CCD Image is Combination of 4 Images:1. “Raw” Image of Scene2. “Bias” Image3. “Dark Field” Image with Shutter Closed4. “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
, ,r x y d x y
Correction of Raw Imagewith Bias, Dark, Flat Images
Flat Field Image
Bias Image
OutputImage
Dark Frame
Raw File ,r x y
,d x y
,f x y
,b x y
, ,f x y b x y
“Flat” “Bias”
“Raw” “Dark”
, ,, ,
r x y d x yf x y b x y
“Raw” “Dark”“Flat” “Bias”
, ,r x y b x y
Correction of Raw Imagew/ Flat Image, w/o Dark Image
Flat Field Image
Bias Image
OutputImage
Raw File
,r x y
,f x y
,b x y , ,f x y b x y
“Flat” “Bias”
, ,, ,
r x y b x yf x y b x y
“Raw” “Bias”“Flat” “Bias”
“Raw” “Bias”
Assumes Small Dark Current(Cooled Camera)