Telescope and Sensor Optical Astronomy Imaging Chain ... · • Revolutionized Astronomical Imaging...

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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


Now Empty

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Empty Second Buckets in Column Into First Buckets

Start Conveyor Belt

Measure& Drain

10

Measure& Drain

Measure& Drain

11


Start Conveyor Belt

Measure& Drain

12

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

123

+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

-5V

+5V

0V

-5V

1

2

3


123

+5V

0V

-5V

+5V

0V

-5V

+5V

0V

-5V

1

2

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123

+5V

0V

-5V

+5V

0V

-5V

+5V

0V

-5V

1

2

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123

+5V

0V

-5V

+5V

0V

-5V

+5V

0V

-5V

1

2

3


123

+5V

0V

-5V

+5V

0V

-5V

+5V

0V

-5V

1

2

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123

+5V

0V

-5V

+5V

0V

-5V

+5V

0V

-5V

1

2

3


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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

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Flow of bloomed

charge

Channel “Stops” (Charge Barrier)

Charge Spreads in Column• Up AND Down


ChargeTransferDirection

Bloomed Star Imageswith “Streaks”

M42


• 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

Telescope and Sensor Optical Astronomy Imaging Chain ... · • Revolutionized Astronomical Imaging...

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Transcript of Telescope and Sensor Optical Astronomy Imaging Chain ... · • Revolutionized Astronomical Imaging...