Optical Astronomy Imaging Chain: Telescopes & CCDs · 2005. 3. 30. · • Convert photons to ......
Transcript of Optical Astronomy Imaging Chain: Telescopes & CCDs · 2005. 3. 30. · • Convert photons to ......
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Optical Astronomy Optical Astronomy Imaging Chain: Imaging Chain:
Telescopes & Telescopes & CCDsCCDs
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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
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ChargeCharge--Coupled Device = CCDCoupled Device = CCD
• Individual “Picture elements” (= “Pixels”)
• Convert photons to electrons
• Pixel Size ⇒ “Resolution” in image
• Area of Pixels ⇒ “coverage”
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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
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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
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C(“center of curvature”)
Spherical MirrorSpherical Mirror
Concave mirror on topConvex mirror on bottom
Same “radius of curvature” R
R
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• 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
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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
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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
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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= = =
⋅
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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−×≅
⋅=
⋅=
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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!!
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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 =×=
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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
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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
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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
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• “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
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• 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
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Film/Plates Still Useful!!Film/Plates Still Useful!!
• Large field of view• Cheap!
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• 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?
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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
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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
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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
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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
−
−
−
⎛ ⎞× − ⋅ ×⎜ ⎟⎝ ⎠= =
⎛ ⎞× ×⎜ ⎟⎝ ⎠
= × =
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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
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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”
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photonph
oton
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)
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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
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““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
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BUCKETS (PIXELS)
VERTICALCOLUMNS of PIXELS
CONVEYOR BELT(SERIAL REGISTER)
MEASURING CYLINDER(OUTPUT
AMPLIFIER)
Rain of Photons
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Shutter
Rain of Photons
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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)
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Empty Second Buckets in Column Into First Buckets
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Empty Third Buckets in Column Into Second Buckets
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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.
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Measure& Drain
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Empty First Buckets in Column Into Buckets in Conveyor Belt
Now Empty
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Empty Second Buckets in Column Into First Buckets
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Start Conveyor Belt
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Measure& Drain
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Measure& Drain
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Measure& Drain
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Empty First Buckets in Column Into Buckets in Conveyor Belt
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Start Conveyor Belt
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Measure& Drain
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Measure& Drain
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Measure& Drain
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Ready for New Exposure
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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
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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
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CCD ManufactureCCD Manufacture
Don Groom LBNL
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Fabricated CCDFabricated CCD
Kodak KAF1401 1317 × 1035 pixels (1,363,095 pixels)
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Charges (Charges (““BucketsBuckets”” are Moved are Moved by Changing Voltage Patternby Changing Voltage Pattern
123
Apply VoltagesHere
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Charge TransferCharge Transfer
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+5V
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+5V
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Time-slice shown in diagram
1
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Charge Transfer Charge Transfer -- 11
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Charge Transfer Charge Transfer -- 22
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Charge Transfer Charge Transfer -- 33
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Charge Transfer Charge Transfer -- 44
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Charge Transfer Charge Transfer -- 55
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Charge Transfer Charge Transfer -- 66
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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
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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
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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
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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
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CCDsCCDs: artifacts and defects : artifacts and defects -- 44• Charge loss
– during pixel charge transfer & readout
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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
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Flow of bloomed
charge
Channel “Stops” (Charge Barrier)
Charge Spreads in Column• Up AND Down
CCD CCD ““BloomingBlooming”” -- 22
ChargeTransferDirection
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Bloomed Star Imageswith “Streaks”
M42
CCD CCD ““BloomingBlooming”” -- 33
• Long Exposure forFaint Nebulosity
⇒ Star Images areOverexposed
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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
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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
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M51
Dark Column
Hot Spots, Bright Columns
Bright First Row • incorrect operation of
signal processing electronics
CCD Image DefectsCCD Image Defects
Negative Image
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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%
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““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
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““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
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[ ] [ ], ,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”
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[ ] [ ], ,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)
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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