The Imaging Chain The Imaging Chain in Optical Astronomy in - - PDF document

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The Imaging Chain The Imaging Chain in Optical Astronomy in Optical Astronomy

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Review and Overview Review and Overview

“Imaging Chain” includes these elements:

• 1. energy source
• 2. object
• 3. collector
• 4. detector (or sensor)
• 5. processor
• 6. display
• 7. analysis
• 8. storage (if any)

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Optical Imaging Chain Optical Imaging Chain

1: source 2: object 3: collector 4: sensor 5: processing 6: display 7: analysis

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Source and/or Object Source and/or Object

• In astronomy, the source of energy (1) and the
• bject (2) are almost always one and the same!
• i.e., The object emits the light

– Examples:

• Galaxies
• Stars

– Exceptions:

• Planets and the moon
• Dust and gas that reflects or absorbs starlight

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Optical Imaging Chain in Optical Imaging Chain in Astronomy until 1980 or so Astronomy until 1980 or so

1: source 2: object 3: collector 4: sensor 5: processing 6: display 7: analysis 8: storage

(stack of glass)

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Optical Imaging Chain in Modern Optical Imaging Chain in Modern Astronomy (post Astronomy (post-

• 1980)

1980)

1: source 2: object 3: collector 4: sensor 5: processing 6: display 7: analysis 8: storage

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Transition ( Transition (“ “Catch Catch-

• up

up” ”) Phase: ) Phase: Digitize Plates Digitize Plates

6: display 7: analysis 8: storage

+

Scanner

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Optical Imaging Chain in Radio Optical Imaging Chain in Radio Astronomy Astronomy

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radio waves receiver where waves are collected waves converted into electro signals computer received as signal

3,4 5 6,7

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Specific Requirements for Specific Requirements for Astronomical Imaging Systems Astronomical Imaging Systems

• Requirements always conflict

– Always want more than you can have ⇒must “trade off” desirable attributes − Deciding the relative merits is a difficult task

• “general-purpose” instruments (cameras) may not be

sufficient

• Want simultaneously to have:

– excellent angular resolution AND wide field of view – high sensitivity AND wide dynamic range

• Dynamic range is the ability to image “bright” and “faint”

sources

– broad wavelength coverage AND ability to measure narrow spectral lines

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Angular Resolution Angular Resolution

• vs. Field of View
• vs. Field of View
• Angular Resolution: ability to distinguish sources

that are separated by small angles

– Limited by:

• Optical Diffraction
• Sensor Resolution
• Field of View: angular size of the image field

– Limited by:

• Optics
• Sensor Size (area)

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Sensitivity vs. Dynamic Range Sensitivity vs. Dynamic Range

• Sensitivity

– ability to measure faint brightness

• Dynamic Range

– ability to image “bright” and “faint” sources in same system

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Wavelength Coverage Wavelength Coverage

• vs. Spectral Resolution
• vs. Spectral Resolution
• Wavelength Coverage

– Ability to image over a wide range of wavelengths – Limited by:

• Spectral Transmission of Optics (Glass cuts off UV, far IR)
• Spectral Resolution

– Ability to detect and measure narrow spectral lines – Limited by:

• “Spectrometer” Resolution (number of lines in diffraction

grating)

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Optical Collector (Link #3) Optical Collector (Link #3)

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Optical Collection (Link #3): Optical Collection (Link #3): Refracting Telescopes Refracting Telescopes

• Lenses collect light
• BIG disadvantages

– Chromatic Aberrations (due to dispersion of glass) – Lenses are HEAVY and supported only on periphery

• Limits the Lens Diameter
• Largest is 40" at Yerkes Observatory, Wisconsin

http://astro.uchicago.edu/vtour/40inch/kyle3.jpg

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Optical Collection (Link #3): Optical Collection (Link #3): Reflecting Telescopes Reflecting Telescopes

• Mirrors collect light
• Chromatic Aberrations eliminated
• Fabrication techniques continue to improve
• Mirrors may be supported from behind

⇒ Mirrors may be made much larger than refractive lenses

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Optical Reflecting Telescopes Optical Reflecting Telescopes

• Concave

parabolic primary mirror to collect light from source

– modern mirrors for large telescopes are thin, lightweight & deformable, to

• ptimize image

quality

3.5 meter WIYN telescope mirror, Kitt Peak, Arizona

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Thin and Light (Weight) Mirrors Thin and Light (Weight) Mirrors

• Light weight ⇒Easier to point

– “light-duty” mechanical systems ⇒ cheaper

• Thin Glass ⇒ Less “Thermal Mass”

– Reaches Equilibrium (“cools down” to ambient temperature) quicker

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Hale 200 Hale 200" " Telescope Telescope Palomar Mountain, CA Palomar Mountain, CA

http://www.astro.caltech.edu/observatories/palomar/overview.html http://www.cmog.org/page.cfm?page=374

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200 200" mirror (5 meters) " mirror (5 meters) for Hale Telescope for Hale Telescope

• Monolithic Mirror (single piece)
• Several feet thick
• 10 months to cool
• 7.5 years to grind
• Mirror weighs 20 tons
• Telescope weighs 400 tons
• “Equatorial” Mount

– follows sky with one motion

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Keck Keck telescopes

telescopes, Mauna Kea, HI

, Mauna Kea, HI

http://www2.keck.hawaii.edu/geninfo/about.html

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400 400" mirror (10 meters) " mirror (10 meters) for Keck Telescope for Keck Telescope

• 36 segments
• 3" thick
• Each segment weighs 400 kg (880 pounds)

– Total weight of mirror is 14,400 kg (< 15 tons)

• Telescope weighs 270 tons
• “Alt-azimuth” mount (left-right, up-down

motion)

– follows sky with two motions + rotation

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Basic Designs of Optical Basic Designs of Optical Reflecting Telescopes Reflecting Telescopes

1. Prime focus: light focused by primary mirror alone 2. Newtonian: use flat, diagonal secondary mirror to deflect light out side of tube 3. Cassegrain: use convex secondary mirror to reflect light back through hole in primary 4. Nasmyth (or Coudé) focus (coudé ⇒ French for “bend” or “elbow”): uses a tertiary mirror to redirect light to external instruments (e.g., a spectrograph)

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Prime Focus Prime Focus

f Sensor Mirror diameter must be large to ensure that

• bstruction is not significant

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Newtonian Reflector Newtonian Reflector

Sensor

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Cassegrain Telescope Cassegrain Telescope

Sensor

Secondary Convex Mirror

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Feature of Cassegrain Feature of Cassegrain Telescope Telescope

• Long Focal Length in

Short Tube

Location of Equivalent Thin Lens

f

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Coud Coudé é or

• r Nasmyth

Nasmyth Telescope Telescope

Sensor

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Optical Reflecting Telescopes Optical Reflecting Telescopes

Schematic

• f 10-meter

Keck telescope (segmented mirror)

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Large Optical Telescopes Large Optical Telescopes

Telescopes with largest diameters (in use or under construction:

– 10-meter Keck (Mauna Kea, Hawaii) – 8-meter Subaru (Mauna Kea) – 8-meter Gemini (twin telescopes: Mauna Kea & Cerro Pachon, Chile) – 6.5-meter Mt. Hopkins (Arizona) – 5-meter Mt. Palomar (California) – 4-meter NOAO (Kitt Peak, AZ & Cerro Tololo, Chile)

http://seds.lpl.arizona.edu/billa/bigeyes.html

Summit of Mauna Kea, with Maui in background Keck telescope mirror (note person)

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Why Build Large Telescopes? Why Build Large Telescopes?

• 1. Larger Aperture ⇒ Gathers MORE Light

– Light-Gathering Power ∝ Area – Area of Circular Aperture = πD2 / 4 ∝ D2

• D = diameter of primary collecting element
• 2. Larger aperture ⇒ better angular

resolution

– recall that:

∆θ λ ≅ D

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Why Build Small Telescopes? Why Build Small Telescopes?

• 1. Smaller aperture ⇒ collects less light
• ⇒ less chance of saturation

(“overexposure”) on bright sources

• 2. Smaller aperture ⇒ larger field of view

(generally)

– Determined by “F ratio” or “F#”

f = focal length of collecting element D = diameter of aperture

F f D #≡

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F Ratio: F# F Ratio: F#

• F# describes the ability of the optic to

“deflect” or “focus” light

– Smaller F# ⇒ optic “deflects” light more than system with larger F#

Small F# Large F#

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F# of Large Telescopes F# of Large Telescopes

• Hale 200" on Palomar: f/3.3

– focal length of primary mirror is: 3.3 × 200" = 660" = 55' ≅ 16.8 m – Dome must be large enough to enclose

• Keck 10-m on Mauna Kea: f/1.75

– focal length of primary mirror is: 1.75 × 10m = 17.5 m ≅ 58 m

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F Ratio: F# F Ratio: F#

• Two reflecting telescopes with different F#

and same detector have different “Fields of View”:

Small F# Large F# large ∆θ small ∆θ

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Sensors (Link #4) Sensors (Link #4)

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Astronomical Cameras Astronomical Cameras Usually Include: Usually Include:

1. Spectral Filters

– most experiments require specific wavelength range(s) – broad-band or narrow-band

2. “Reimaging” Optics

– enlarge or reduce image formed by primary collecting element

3. Light-Sensitive Detector: Sensor

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Astronomical Sensors Astronomical Sensors

• Most common detectors:

– Human Eye – Photographic Emulsion

• film
• plates

– Electronic Sensors

• CCDs

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Angular Resolution Angular Resolution

• Fundamental Limit due to Diffraction in

“Optical Collector” (Link #3)

• But Also Limited by Resolution of Sensor!

∆θ λ ≅ D

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Charge Charge-

• Coupled Devices (

Coupled Devices (CCDs CCDs) )

• Standard light detection medium for BOTH professional and

amateur astronomical imaging systems

– Significant decrease in price

• numerous advantages over film:

– high quantum efficiency (QE)

• meaning most of the photons incident on CCD are “counted”

– linear response

• measured signal is proportional to number of photons collected

– fast processing turnaround (CCD readout speeds ~1 sec)

• NO development of emulsion!

– regular grid of sensor elements (pixels)

• as opposed to random distribution of AgX grains

– image delivered in computer-ready form

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Sensor Resolution Sensor Resolution

• Obvious for Electronic Sensors (e.g.,

CCDs)

∆x

• Elements have finite size
• Light is summed over area
• f sensor element (“integrated”)
• Light from two stars that falls on

same element is added together

• stars cannot be distinguished

in image!

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Same Effect in Photographic Same Effect in Photographic Emulsions Emulsions

• More difficult to quantify
• Light-sensitive “grains” of silver

halide in the emulsion

• Placed “randomly” in emulsion
• “Random” sizes
• “large” grains are more sensitive
• (respond to few photons)
• “small” grains produce better

resolution

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Photographic techniques: Photographic techniques: silver halide silver halide

• Film

– Emulsion on “flexible” substrate – Still used by amateurs using sensitive film

• B&W and color
• Special treatment to increase sensitivity
• Photographic Plates

– Emulsion on glass plates – Most common detector from earliest development

• f AgX techniques until CCDs in late 70’s

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Eye as Astronomical Detector Eye as Astronomical Detector

• Eye includes its own lens

– focuses light on retina ( “sensor”)

• When used with a telescope, must add yet

another lens

– redirect rays from primary optic – make them parallel (“collimated”)

• rays appear to come from “infinity” (infinite distance

away)

– reimaging is performed by “eyepiece”

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Eye with Telescope Eye with Telescope

Without Eyepiece With Eyepiece Light entering eye is “collimated”

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Eye as Astronomical Detector Eye as Astronomical Detector

• Point sources (stars) appear brighter to eye through

telescope

• Factor is

– D is telescope diameter – P is diameter of eye pupil – Magnification should make light fill the eye pupil (“exit pupil”)

• Extended sources (for example, nebulae) do not

appear brighter through a telescope

– Gain in light gathering power exactly compensated by image magnification, spreads light out over larger angle.

2 2

D P

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Atmospheric Effects on Image Atmospheric Effects on Image

• Large role in ground-based optical astronomy

– scintillation modifies source angular size

• twinkling of stars = “smearing” of point sources

– extinction reduces light intensity

• atmosphere scatters a small amount of light, especially at

short (bluer) wavelengths

• water vapor blocks specific wavelengths, especially near-

IR

– scattered light produces interfering “background”

• astronomical images are never limited to light from

source alone; always include “source” + “background sky”

• “light pollution” worsens sky background

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Scattering Scattering

• “Wavelength Dependent”

– Depends on color of light – Long wavelengths are scattered “less”

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Scattering by Molecules Scattering by Molecules

• Molecules are SMALL
• “Blue” light is scattered MUCH more than

red light

– Reason for BOTH

• blue sky (blue light scattered from sun in all

directions)

• red sunset (blue light is scattered out of the sun’s

direct rays)

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1 "Rayleigh Scattering" λ ∝

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Scattering by Dust Scattering by Dust

• Dust particles are MUCH larger than

molecules

– e.g., from volcanos, dust storms

• Blue light is scattered by dust “somewhat

more” than red light

1 "Mie Scattering" λ ∝

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Link #5: Image Processing Link #5: Image Processing

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Link #5: Image Processing Link #5: Image Processing

• Formerly: performed in darkroom

– e.g., David Malin’s “Unsharp Masking”

• Subtract a blurred copy from a “sharp” positive
• (or, add a blurred negative to a “sharp” positive)
• Now performed in computers, e.g.,

– contrast enhancement – “sharpening” – “normalization” (background division) – …

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Image Processing Image Processing

• Once collected, images must be corrected for:

– Atmosphere (to extent possible)

• e.g., sequence of images obtained at a variety of telescope

elevations usually can be corrected for atmospheric extinction

– CCD defects and artifacts

• dark current

– CCD pixel reports a signal even when not exposed to light

• bad pixels

– some pixels will be dead, hot, or even “flickering”

• variations in pixel-to-pixel sensitivity

– every pixel has its own QE – can be characterized by “flat field”

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Links #6 and #7 Links #6 and #7 Image Display and Analysis Image Display and Analysis

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Image Display and Analysis Image Display and Analysis

• This step often is where astronomy really begins.
• Type and extent of display and analysis depends on

purpose of imaging experiment

• Common examples:

– evaluating whether an object has been detected or not – determining total CCD signal (counts) for an object, such as a star – determining relative intensities of an object from images at two different wavelengths – determining relative sizes of an extended object from images at two different wavelengths

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Link #8: Storage Link #8: Storage

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Storage Storage

• Glass plates

– Lots of climate-controlled storage space – expensive – available to one user at a time – now being “digitized” (scanned), as in the archive you use with DS9

• Digital Images

– Lots of disk space – cheaper all the time – available to many users