CCDs
AST443, Lecture 5
Stanimir Metchev
Administrative
• Homework 1 due today
2
Outline
• Overview of previous lecture
– electromagnetic radiation
– telescopes
• CCDs
3
Interstellar Extinction Law
extinction is highest at ~100 nm = 0.1 µm
unimportant for >10 µm
4
source: Kitt Peak National Observatory
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Photometric Bands: NearInfrared
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Atmospheric Refraction
n (3200 Å) = 1.0003049
n (5400 Å) = 1.0002929
n (10,000 Å) = 1.0002890
differential atmospheric
refraction D between
3200 Å and 5400 Å
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Focusing
• focal length (fL), focal plane
• object size (α, s) in the focal plane
s = fL tan α ≈ fLα
• plate/pixel scale
P = α/s = 1/fL
– Lick observatory 3m
• fL = 15.2m, P = 14″/mm
• energy per unit detector area
Ep ∝ (d / fL)2
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Fraunhofer Diffraction
Circular Aperture
• Airy disk
• Airy nulls at 1.220, 2.233,
3.238, … λ/d
• angular resolution
$ #r 2 J1 (2m) ' 2
I(" ) = &
)
m
%
(
#r sin "
m=
*
– θmin ~ 1.22 λ/d
• Rayleigh criterion
!
• gives 74% drop in intensity
between peaks
– can do as little as ~80% of that
• 3% drop in intensity between
peaks (Dawes criterion)
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Point Spread Function (PSF)
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Imaging through a Turbulent
Atmosphere: Seeing
• FWHM of seeing disk
– θseeing <1.0″ at a good site
• r0: Fried parameter
– θseeing = 1.2 λ/r0
– r0 ∝ λ6/5 (cos z)3/5
– θseeing ∝ λ–1/5
• t0: coherence time
– t0 = r0 / vwind
– vwind ~ several m/s
– t0 is tens of milli-sec
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Adaptive Optics
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Adaptive Optics
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Outline
• Overview of previous lecture
– electromagnetic radiation
– telescopes
• CCDs
14
Context: Plates, PMTs, CCDs
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Basic Concept
A charge-coupled
device (CCD) converts
photons to electrons
• works thanks to photoelectric effect
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Basic Concept
•
•
electron-hole pair generation
doping:
– n-type (electrons)
– p-type (holes)
– creates additional energy levels
within band gap
– increases conductivity
•
silicon
– band gap: 1.12 eV (11 300Å)
– free-electron energy: 4 eV (3000Å)
– 1 photon -> 1 electron
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Basic Concept
•
•
electron-hole pair generation
doping:
– n-type (electrons)
– p-type (holes)
– creates additional energy levels
within band gap
– increases conductivity
•
silicon
– band gap: 1.12 eV (11 300Å)
– free-electron energy: 4 eV (3000Å)
– 1 photon -> 1 electron
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Basic Concept:
A P-N Photo Diode
•
depleted region
– low conductivity
– can support an E field
•
•
•
net positive charge (higher charge density near top)
additional E-field applied
subsequently generated electrons get trapped in potential well near top
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Charge Trapping
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Charge Transfer
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Buried Channel CCDs
• surface channel
– charge transfer via overlapping
gates
– but trapping can occur at gates due
to impurities
• low CTE (~99%)
• buried channels
– CTE > 99.9995%
– lower potential well
• allow low light illumination
• higher dynamic range and sensitivity
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23
Front- vs. Back-Illumination
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Advanced CCD Technology
•
orthogonal transfer
–
–
–
–
•
30–100 Hz readout
tip/tilt wavefont correction
~30% improvement in “seeing”
large-format CCDs
low-light CCDs
– gain register clocked out with
higher voltage (40–60V vs.
~10V)
– 1–2% probability of generating
2nd electron at each gate
transfer
– total gain enhancement:
~1.01N = 145 for N=500
transfers
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Analog-to-Digital Converters
• X electrons = 1 digital unit (counts)
– X is “gain”: usually 1–10
• CCD saturation depends on
– well capacity
• ~300,000 photolectrons for “deep depletion” CCDs
– number of bits in ADC
• n=16 bits: maximum is 216–1 = 65535 counts
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CCD Quantum Efficiency
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QE Improvements
• UV coatings
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QE Improvements
• UV coatings
• anti-reflection
(AR) coatings
n1
n2
d
n3
– n2 = sqrt(n1 * n3)
– n2*d = λ/4
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Charge Diffusion
•
•
due to substrate impurities
in front-illuminated CCDs, red
photons
– are absorbed near back of CCD
– see shallower potential well
– can move into neighboring
pixels
•
problematic / uneven in thinned
CCDs
– HST ACS
• 0.5 mag loss at short
wavelengths
• alters shape of PSF
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Read Noise
• electrons / pix / read
• sources
– A/D conversion not perfectly repeatable
– spurious electrons from electronics (e.g.,
from amplifier heating)
• aleviated through
• nowadays: <3–10 electrons
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Dark Current
• electrons / pixel / second
• source
– thermal noise at non-zero detector temperature
• higher at room temperature (~10,000)
• at cryogenic temperatures
– LN2, –100 C
– 0.1–20 e–/pix/s
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Dark Current
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Non-Linearity
• differential (digitization noise)
• integral
– examples of non-linearity in SDSS CCDs:
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Large-Area CCD Mosaics:
The Sloan Digital Sky Survey (SDSS)
SDSS 2.5 m telescope at Apache Point, NM
Ritchey-Chretien design
(Cassegrain-like)
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Large-Area CCD Mosaics:
The Sloan Digital Sky Survey (SDSS)
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Large-Area CCD Mosaics:
The Sloan Digital Sky Survey (SDSS)
u
g
r
i
(ansgtroms)
z
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Proxima Cen
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