telescope and sensor optical astronomy imaging chain ... · • revolutionized astronomical imaging...
TRANSCRIPT
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Optical Astronomy Optical Astronomy Imaging Chain: Imaging Chain:
Telescopes & Telescopes & CCDsCCDs
Telescope and SensorTelescope and Sensor• Telescope:
– Collects and focuses light to make the image– Generally a “reflecting” telescope
• X-ray, Ultraviolet, Optical (visible), IR, Radio• No variation in image with wavelength (“color”)
• Sensor:– Measures the light at each position– Generally a “charge-coupled device” (CCD)
• Converts light (“photons”) to electrons
ChargeCharge--Coupled Device = CCDCoupled Device = CCD
• Individual “Picture elements” (= “Pixels”)
• Convert photons to electrons
• Pixel Size ⇒ “Resolution” in image
• Area of Pixels ⇒ “coverage”
Reflector telescopes: Reflector telescopes: basic principlesbasic principles
• For Reflection, we know that: angle of incidence = angle of reflection
(angle in = angle out)• angles measured from “normal”
(perpendicular to surface)θin θout
Reflector telescopes: Reflector telescopes: basic principlesbasic principles
• Easy to make concave mirrors with a “spherical” profileGrind mirror on second piece of glass – the “tool”
water& “grit”
Force
top piece becomes concave spherebottom piece becomes convex sphere C
(“center of curvature”)
Spherical MirrorSpherical Mirror
Concave mirror on topConvex mirror on bottom
Same “radius of curvature” R
R
2
• Reflected rays from source at ∞ at different “heights” do not “focus” (cross optic axis) at same distance from mirror
• This is called “Spherical Aberration!”– This is what plagued the Hubble Space Telescope
Concave Concave ““SphericalSpherical”” Mirror Mirror Works Poorly for Imaging StarsWorks Poorly for Imaging Stars
Correct Mirror Surface for Correct Mirror Surface for Object at Object at ∞∞
• Paraboloid!– somewhat “shallower” curve than sphere
• z = kx2 for paraboloid– parallel incident rays brought to common
focus paraboloidsphere
z
x
Basic Designs of Optical Basic Designs of Optical Reflecting TelescopesReflecting Telescopes
• “Prime focus”– light is brought to focus by primary mirror only!
• “Newtonian”– flat, diagonal secondary mirror deflects light out of tube
• “Cassegrain”– convex secondary mirror reflects light through hole in primary
• “Nasmyth” (or coudé) focus– tertiary mirror to redirect light to external instruments– “coudé” = “elbow” in French
F# (FF# (F--ratio) and ratio) and ““Plate ScalePlate Scale””•
– D = diameter– f = focal length– must consider focal length of combination of
primary & any secondary mirrors• Determines “plate scale”
– angle increment of image per unit length at focal plane (e.g., arcsec per mm)
– estimated from (our old friend): small-angle relation
# fFD
=
Sf
θ =
1 1plate scale #S f F D
θ= = =
⋅
Example of Plate ScaleExample of Plate Scale
• 10"-diameter f/16 telescope
1plate scale #S F D
θ= =
⋅
mmmmarcseconds105.2
254161
DF#1 scale plate 4−×≅
⋅=
⋅=
Sensors with Sensors with ““PixelsPixels””
(different from (different from ““emulsionsemulsions””))
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Basic Concepts of CCD SensorsBasic Concepts of CCD Sensors• “Pixelated” ⇒ discrete picture elements
(“pixels”)• Converts Photons to Electrons by
absorption and conversion of energy• Sensitive over wide range of wavelengths
(“colors”)• Pixels are “read out” in sequence
– cannot be randomly accessed!!
CCDsCCDs: : ““pixel scalepixel scale””
• Example: – assume plate scale of image = 50 arcsec per mm– CCD pixel size (linear dimension)
= 25 microns = 25 µm = 0.025 mm = 25,000 nm
⇒ pixel scale = 1.25 arcsec per pixel
pixelarcseconds25.1
pixelmm025.0
mmarcseconds 50 scale pixel =×=
CCDsCCDs: : ““field of viewfield of view””• Example:
– CCD with 1,000,000 pixels (1 Mpixel) in 1000×1000 array
– Each pixel is 25 µm × 25 µm– Pixel size is 1.25 arcsec
⇒ field of view is:1000 pixels × 1.25 arcsec per pixel = 1250 arcsec
≅ 21 arcmin– could image most of Moon’s surface on this CCD
through this telescope
CCDsCCDs: field of view: field of view• Want to match CCD pixel scale to image
“blur” due to diffraction• Recall main sources of image blur
– angular resolution of telescope due to “diffraction limit”
– random variations in atmosphere ⇒ time-varying movement
• Ideal pixel scale: 2 CCD pixels ≥ width of optical “blur”⇒ Image field of view then limited by size of
CCD (number of pixels) F CCD bi f ll i l i
Basic Principles of CCD Imaging in Astronomy
Based on Slides by Simon Tulloch: available fromhttp://www.ing.iac.es/~smt/CCD_Primer/CCD_Primer.htm
• “CCD” = “Charge-Coupled Device”• Invented in 1970s, originally for:
– Memory devices – Arithmetic data processing (computer
chips)• Usually made of Silicon (“Si”)
⇒Has Same Light-Sensitive Properties as Silicon Light Meters
What is a CCD?What is a CCD?
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Spectral Response (sensitivity) Spectral Response (sensitivity) of Typical CCDof Typical CCD
• Response is large in visible region, falls off for ultraviolet (UV) and infrared (IR)
300 400 500 600 700 800 900 1000Incident Wavelength [nm]
RelativeResponse
Visible Light IRUV
• Light-Sensitive Properties applied to Imaging• Revolutionized Astronomical Imaging
– Improved Light-Gathering Power of Telescopes by nearly 100× (5 magnitudes!!)
• 2005 Amateur w/ 15-cm (6") Telescope + CCD can get similar performance as 1960 Professional with 1-m (40") Telescope + Photography
• Now Considered to be “Standard” Sensor in Astronomical Imaging– Special Arrangements with Observatory Necessary
to use Photographic Plates or Film
CCDsCCDs in Astronomyin Astronomy
Film/Plates Still Useful!!Film/Plates Still Useful!!
• Large field of view• Cheap!
• Crystal Form of Matter (typically Si)• Converts “Light” to “Electronic Charge”
– Pattern of Charge = “Image”1. “Digitized”
– Analog Measurements (“Voltages”) Converted to Integer Values at Pixels
2. “Digitized” Measurements Stored as Computer File
What is a CCD?What is a CCD?
SiSi Crystal StructureCrystal Structure
• Regular Pattern of Siatoms– Fixed Separations
• Atomic Structure Pattern “Perturbs”Electron Orbitals– Changes Layout of
Available Electron States
http://www.webelements.com/webelements/elements/text/Si/xtal.html
Electron States in Electron States in SiSi CrystalCrystal• 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
ngen
ergy
Valence Band of Electron States
Conduction Band of Electron States
“Gap” ≈ 1.26 electron-volts(eV) - - -
-“Gap”
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Action of Light on Electron StatesAction of Light on Electron States
• Incoming Photon with Energy ≥ 1.26 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 Action of ““Charge CarriersCharge Carriers””• Carriers are “Free” to Move in the
Corresponding Band– Electron e- moves in Conduction Band– Hole h+ moves in Valence Band
• Charge Carriers may be “Counted”Electronically – Measure the Number of Absorbed Electrons
≈ Number of Absorbed Photons
Wavelength Wavelength λλ corresponding to corresponding to E = 1.26 electron VoltsE = 1.26 electron Volts
• 1 eV = 1.602 × 10-12 erg = 1.602 × 10-12 Joule
⇒ To Energize Electron in Si Lattice Requiresλ < 984 nm ≅ 1 µm
( )27 8
12
7
6.624 10 sec 3 10sec
1.26 1.602 10
9.84 10 984
merghc
ergE eVeV
m nm
λ
−
−
−
⎛ ⎞× − ⋅ ×⎜ ⎟⎝ ⎠= =
⎛ ⎞× ×⎜ ⎟⎝ ⎠
= × =
Energy and WavelengthEnergy and Wavelength• If Incident Wavelength λ > 1 µm, Photon
CANNOT be Absorbed!– Insufficient Energy to “Kick” Electron to
Conduction Band
⇒ Silicon is “Transparent” to long λ⇒ CCDs constructed from Silicon are Not
Sensitive to Long Wavelengths
After Electron is Excited into After Electron is Excited into Conduction BandConduction Band……..
• Electron and Hole Usually “Recombine” Quickly– Charge Carriers are “Lost”
• Prevent by Applying External Electric Field to “Separate”Electrons from Holes
• Field Attracts “Sweeps” Electrons and Holes in Opposite Directions:– Field “Sweeps” Electrons and Holes Apart
⇒They don’t recombine• Maintains Population of Charge Carriers
– Allows Carriers to be “Counted”
photon
phot
on
Hole
Electron
Conduction Band
Valence Band
Generation of CCD CarriersGeneration of CCD Carriers
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Thermal Thermal ““NoiseNoise””• BUT: Other Forms of Energy has Same Effect
as Light• 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 How Do We ““CountCount”” the Charge the Charge Carriers (Carriers (““PhotoelectronsPhotoelectrons””)?)?
• Must “Move” Charges to an “Amplifier”• Astronomical CCDs: Amplifier Located at “Edge”
of Light-Sensitive Region of CCD– Most of CCD Area “Sensitive” to Light– Charge Transfer is “Slow”
• Video and Amateur Camera CCDs: Must Transfer Charge QUICKLY– Less Area Available to Collect Light
““Bucket BrigadeBucket Brigade”” CCD AnalogyCCD 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(OUTPUTAMPLIFIER)
Empty First Buckets in Column Into Buckets in Conveyor Belt
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CONVEYOR BELT
(SERIAL REGISTER)
MEASURING CYLINDER(OUTPUTAMPLIFIER)
Empty Second Buckets in Column Into First Buckets
Empty Third Buckets in Column Into Second Buckets
Start Conveyor Belt
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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
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Empty Second Buckets in Column Into First Buckets
Start Conveyor Belt
Measure& Drain
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Measure& Drain
Measure& Drain
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Empty First Buckets in Column Into Buckets in Conveyor Belt
Start Conveyor Belt
Measure& Drain
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Measure& Drain
Measure& Drain
Ready for New Exposure
Features of CCD ReadoutFeatures 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
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Structure of Astronomical Structure of Astronomical CCDsCCDs
• 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 ManufactureCCD Manufacture
Don Groom LBNL
Fabricated CCDFabricated CCD
Kodak KAF1401 1317 × 1035 pixels (1,363,095 pixels)
Charges (Charges (““BucketsBuckets”” are Moved are Moved by Changing Voltage Patternby Changing Voltage Pattern
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Apply VoltagesHere
123
Charge TransferCharge Transfer
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+5V
0V
-5V
+5V
0V
-5V
+5V
0V
-5V
Time-slice shown in diagram
1
2
3
Charge Transfer Charge Transfer -- 11
14
123
+5V
0V
-5V
+5V
0V
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+5V
0V
-5V
1
2
3
Charge Transfer Charge Transfer -- 22
123
+5V
0V
-5V
+5V
0V
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+5V
0V
-5V
1
2
3
Charge Transfer Charge Transfer -- 33
123
+5V
0V
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+5V
0V
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+5V
0V
-5V
1
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Charge Transfer Charge Transfer -- 44
123
+5V
0V
-5V
+5V
0V
-5V
+5V
0V
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1
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Charge Transfer Charge Transfer -- 55
123
+5V
0V
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+5V
0V
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+5V
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1
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Charge Transfer Charge Transfer -- 66
123
+5V
0V
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+5V
0V
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+5V
0V
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1
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3
Charge Transfer Charge Transfer -- 77
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CCDsCCDs: noise sources: noise sources• “dark current”
– can be “removed” by subtracting image obtained without exposing CCD
• leave CCD covered: dark frame
• “read noise”– detector electronics subject to uncertainty in
reading out the number of electrons in each pixel• “photon counting”
– Poisson statistics: if N photons are measured, the uncertainty in my photon count (the “noise”) is √N
CCDsCCDs: artifacts and defects : artifacts and defects -- 11• “bad” pixels
– “dead,” “hot,” “flickering,” …
• methods for correcting:– replace bad pixel with average value of the pixel’s
neighbors– “dither” the telescope
• take series of images• move telescope slightly between exposures• ensures that image falls on good pixels at least some of
the time
CCDsCCDs: artifacts and defects : artifacts and defects -- 22• pixel-to-pixel variation in “efficiency”
– “Quantum Efficiency” = “QE”– Some pixels are more sensitive than others
• Method for Correction:– Construct a “flat field”
• Image of a uniformly illuminated scene• Flat-field image measures efficiency of each
pixel – Divide each image by flat field
CCDsCCDs: artifacts and defects : artifacts and defects -- 33• Pixel “Saturation”
– a pixel can hold a limited amount of electric charge
• limited “well depth”– once pixel is “saturated”, it stops detecting
and counting new photons • analogous to “overexposure” on photographic
emulsion
• charge loss occurs during pixel charge transfer & readout
CCDsCCDs: artifacts and defects : artifacts and defects -- 44• Charge loss
– during pixel charge transfer & readout
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 Spillage
pixe
l bo
unda
ry
CCD CCD ““BloomingBlooming”” -- 11
16
Flow of bloomed
charge
Channel “Stops” (Charge Barrier)
Charge Spreads in Column• Up AND Down
CCD CCD ““BloomingBlooming”” -- 22
ChargeTransferDirection
Bloomed Star Imageswith “Streaks”
M42
CCD CCD ““BloomingBlooming”” -- 33
• Long Exposure forFaint Nebulosity
⇒ Star Images areOverexposed
CCD Image DefectsCCD 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
Problems3. Cosmic Rays (γ)
– Unavoidable– Ionization of e- in Si– Can Damage CCD if
High Energy (HST)
CCD Image DefectsCCD 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 DefectsCCD Image Defects
Negative Image
CCD Image ProcessingCCD 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
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Bias FrameBias 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 FieldDark Field”” ImageImage• 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 FramesDark Frame
““Flat FieldFlat Field”” ImageImage• 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 ImageCorrection of Raw Imagewith Bias, Dark, Flat Imageswith 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 ImageCorrection of Raw Imagew/ Flat Image, w/o Dark 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)
FiltersFilters• Because CCDs have broad spectral
response, need to use filters to determine e.g. star colors in visible
• broad-band: filter width is about 10% of filter’s central wavelength– example: V filter at 550 nm will allow light from
500 to 600 nm to pass through– astronomers use BVRI: blue, ‘visible’, red, IR
• narrow-band: filter width is <1%– example: “H-alpha” covers 650 to 660 nm