Basic Principles of CCD Imaging in Astronomy

1. 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

2. 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

3. 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

4. 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

5. 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

6. - - - - 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

7. 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)

8. 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

9. 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

10. 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

11. 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

12. 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”

13. Hole Electron Generation of CCD Carriers photon photon Conduction Band Valence Band

14. Spontaneous Recombination photon photon Conduction Band Valence Band

15. + + + + Prevent Spontaneous Recombination by Applying Voltage to “Sweep” Electrons +  Ammeter

16. Prevent Spontaneous Recombination by Applying Voltage to “Sweep” Electrons +  +  +  +  +  Ammeter

17. 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)

18. 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

19. “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

20. Rain of Photons VERTICAL COLUMNS of PIXELS BUCKETS (PIXELS) MEASURING CYLINDER (OUTPUT AMPLIFIER) CONVEYOR BELT (SERIAL REGISTER)

21. Rain of Photons Shutter

22. Empty First Buckets in Column Into Buckets in Conveyor Belt MEASURING CYLINDER (OUTPUT AMPLIFIER) CONVEYOR BELT (SERIAL REGISTER)

23. MEASURING CYLINDER (OUTPUT AMPLIFIER) CONVEYOR BELT (SERIAL REGISTER)

24. Empty Second Buckets in Column Into First Buckets

26. Empty Third Buckets in Column Into Second Buckets

27. Start Conveyor Belt

29. After each bucket has been measured, the measuring cylinder is emptied, ready for the next bucket load. Measure & Drain

31. Measure & Drain

34. Empty First Buckets in Column Into Buckets in Conveyor Belt Now Empty

36. Empty Second Buckets in Column Into First Buckets

38. Start Conveyor Belt

40. Measure & Drain

43. Measure & Drain

46. Measure & Drain

47. Empty First Buckets in Column Into Buckets in Conveyor Belt

49. Start Conveyor Belt