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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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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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

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Apply VoltagesHere

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Charge TransferCharge Transfer

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123

+5V

0V

-5V

+5V

0V

-5V

+5V

0V

-5V

Time-slice shown in diagram

1

2

3

Charge Transfer Charge Transfer -- 11

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123

+5V

0V

-5V

+5V

0V

-5V

+5V

0V

-5V

1

2

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0V

-5V

+5V

0V

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+5V

0V

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1

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+5V

0V

-5V

+5V

0V

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+5V

0V

-5V

1

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+5V

0V

-5V

+5V

0V

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+5V

0V

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1

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+5V

0V

-5V

+5V

0V

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+5V

0V

-5V

1

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123

+5V

0V

-5V

+5V

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-5V

+5V

0V

-5V

1

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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


ChargeTransferDirection

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Bloomed Star Imageswith “Streaks”

M42


• 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

Optical Astronomy Imaging Chain: Telescopes & CCDs · 2005. 3. 30. · • Convert photons to ......

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Transcript of Optical Astronomy Imaging Chain: Telescopes & CCDs · 2005. 3. 30. · • Convert photons to ......