Basic Principles of CCD Imaging in Astronomy Based on Slides by Simon Tulloch available from

“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

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 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 =  erg =  Joule  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”

photon Hole Electron Conduction Band Valence Band Generation of CCD Carriers

photon 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) VERTICAL COLUMNS 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 Measured for 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 Package Image Area Serial register (Conveyor Belt) Output amplifier

CCD Manufacture Don Groom LBNL

Fabricated CCD Kodak KAF  1035 pixels (1,363,095 pixels)

Charges (“Buckets” are Moved by Changing Voltage Pattern Apply Voltages Here

1 2 3 Charge Transfer

V 0V -5V +5V 0V -5V +5V 0V -5V Time-slice shown in diagram Charge Transfer - 1

V 0V -5V +5V 0V -5V +5V 0V -5V Charge Transfer - 2

V 0V -5V +5V 0V -5V +5V 0V -5V Charge Transfer - 3

V 0V -5V +5V 0V -5V +5V 0V -5V Charge Transfer - 4

V 0V -5V +5V 0V -5V +5V 0V -5V Charge Transfer - 5

V 0V -5V +5V 0V -5V +5V 0V -5V Charge Transfer - 6

V 0V -5V +5V 0V -5V +5V 0V -5V Charge Transfer - 7

pixel boundary Photons Charge Capacity of CCD pixel is Finite (Up to 300,000 Electrons) After Pixel Fills, Charge Leaks into adjacent pixels. Photons Overflowing charge packet Spillage pixel boundary CCD “Blooming” - 1

Flow of bloomed charge Channel “Stops” (Charge Barrier) Charge Spreads in Column Up AND Down CCD “Blooming” - 2 Charge Transfer Direction

Bloomed Star Images with “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 of Hot Spots Bright Column

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 Scene 2.“Bias” Image 3.“Dark Field” Image with Shutter Closed 4.“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 Flat Field Image Bias Image Output Image Dark Frame Raw File “Flat”  “Bias” “Raw”  “Dark” “Flat”  “Bias”

Correction of Raw Image w/ Flat Image, w/o Dark Image Flat Field Image Bias Image Output Image Raw File “Flat”  “Bias” “Raw”  “Bias” “Flat”  “Bias” “Raw”  “Bias” Assumes Small Dark Current (Cooled Camera)