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

merg

hcergE 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

Ph

oton

s

Charge Capacity of CCD pixel is Finite (Up to 300,000 Electrons)

After Pixel Fills, Charge Leaks into adjacent pixels.

Ph

oton

s

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 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

, ,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 y

f 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 y

f x y b x y

“Raw” “Bias”

“Flat” “Bias”

“Raw” “Bias”

Assumes Small Dark Current(Cooled Camera)