The%20Imaging%20Chain%20in%20Optical%20Astronomy

1
The Imaging Chain in Optical Astronomy
2
Review and Overview

• Imaging Chain includes these elements
• energy source
• object
• collector
• detector (or sensor)
• processor
• display
• analysis
• storage (if any)

3
Optical Imaging Chain
1 source
5 processing
6 display 7 analysis
3 collector 4 sensor
2 object
4
Source and/or Object

• In astronomy, the source of energy (1) and the
  object (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

5
Optical Imaging Chain in Astronomy
5 processing
1 source 2 object
6 display 7 analysis
3 collector 4 sensor
8 storage
6
Imaging Chain in Radio Astronomy
1,2
7
Specific Requirements for 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
  light intensities at specific wavelengths

8
Angular Resolution 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)

9
Sensitivity vs. Dynamic Range

• Sensitivity
• ability to measure faint brightnesses
• Dynamic Range
• ability to image bright and faint sources in
  same system

10
Wavelength Coverage 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 light intensities
  at specific wavelengths
• Limited by
• Spectrometer Resolution (for ex., number of
  lines in a diffraction grating)

11
Optical Collector (Link 3)
12
Optical Collection (Link 3) 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
13
Optical Collection (Link 3) 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

14
Optical Reflecting Telescopes

• Concave parabolic primary mirror to collect light
  from source
• modern mirrors for large telescopes are thin,
  lightweight deformable, to optimize image
  quality

3.5 meter WIYN telescope mirror, Kitt Peak,
Arizona
15
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

16
Hale 200" TelescopePalomar Mountain, CA
http//www.cmog.org/page.cfm?page374
http//www.astro.caltech.edu/observatories/palomar
/overview.html
17
200" mirror (5 meters)for Hale Telescope

• Monolith (one 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

18
Keck telescopes, Mauna Kea, HI
http//www2.keck.hawaii.edu/geninfo/about.html
19
400" mirror (10 meters)for Keck Telescope

• 36 segments
• 3" thick
• Each segment weighs 400 kg (880 pounds)
• Total weight of mirror is 14,400 kg (lt 15 tons)
• Telescope weighs 270 tons
• Alt-azimuth mount (left-right, up-down motion)
• follows sky with two motions rotation

20
Basic Designs of Optical 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)

21
Prime Focus
f
Sensor
Mirror diameter must be large to ensure
that obstruction is not significant
22
Newtonian Reflector
Sensor
23
Cassegrain Telescope
Sensor
Secondary Convex Mirror
24
Feature of Cassegrain Telescope

• Long Focal Length in Short Tube

f
Location of Equivalent Thin Lens
25
Coudé or Nasmyth Telescope
Sensor
26
Optical Reflecting Telescopes
Schematic of 10-meter Keck telescope (segmented
mirror)
27
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

Keck telescope mirror (note person)
Summit of Mauna Kea, with Maui in background
28
Why Build Large Telescopes?

• Larger Aperture ? Gathers MORE Light
• Light-Gathering Power ? Area
• Area of Circular Aperture ?D2 / 4 ? D2
• D diameter of primary collecting element
• Larger aperture ? better angular resolution
• recall that

29
Why Build Small Telescopes?

• Smaller aperture ? collects less light
• ? less chance of saturation (overexposure) on
  bright sources
• Smaller aperture ? larger field of view
  (generally)
• Determined by F ratio or F
• f focal length of collecting element
• D diameter of aperture

30
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
31
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 feet

32
F Ratio F

• Two reflecting telescopes with different F and
  same detector have different Fields of View

large ??
small ??
Small F
Large F
33
Sensors (Link 4)
34
Astronomical CamerasUsually Include

• Spectral Filters
• most experiments require specific wavelength
  range(s)
• broad-band or narrow-band
• Reimaging Optics
• enlarge or reduce image formed by primary
  collecting element
• Light-Sensitive Detector Sensor

35
Astronomical Sensors (visual wavelengths)

• Most common detectors
• Human Eye
• Photographic Emulsion
• film
• plates
• Electronic Sensors
• CCDs

36
Angular Resolution

• Fundamental Limit due to Diffraction in Optical
  Collector (Link 3)
• But Also Limited by Resolution of Sensor!

37
Charge-Coupled Devices (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

38
CCD Basics

• Light-sensitive electronic element based on
  crystalline silicon
• crystal lattice of atoms at regular spacings
• acts as though electrons have two states
• bound to atom
• free to roam through lattice

39
CCD Basics

• Incident photon adds energy to electron to kick
  it up into the free states
• energy of photon must be sufficiently large for
  electron to reach the free states
• to be absorbed by CCDs silicon, the photon
  wavelength ? must be less than maximum ?max ?
  1100 nm (near infrared)

40
CCD Basics

• Silicon structure is divided into pixels
• e- transferred and counted one pixel at a time

http//www.byte.com/art/9510/img/505099d2.htm
41
Sensor Resolution

• Obvious for Electronic Sensors (e.g., CCDs)

• Elements have finite size
• Light is summed over area
• of sensor element (integrated)
• Light from two stars that falls on
• same element is added together
• stars cannot be distinguished
• in image!

?x
42
Same Effect in Photographic 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

43
Photographic techniquessilver halide

• Film
• Emulsion on flexible substrate
• Still used by amateurs using sensitive film
• BW and color
• Special treatment to increase sensitivity
• Photographic Plates
• Emulsion on glass plates
• Most common detector from earliest development of
  AgX techniques until CCDs in late 70s

44
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

45
Eye with Telescope
With Eyepiece Light entering eye is collimated
46
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.

47
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

48
Scattering

• Wavelength Dependent
• Depends on color of light
• Long wavelengths are scattered less

49
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
  suns direct rays)

50
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

51
Link 5 Image Processing
52
Link 5 Image Processing

• Formerly performed in darkroom
• e.g., David Malins 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)

53
Example of Unsharp Masking
http//www.hawastsoc.org/messier/fslide53.html
http//www.seds.org/messier/m/m042.html
Unprocessed
After Unsharp Masking
n.b., Increased visibility of fine structure in
bright and dark regions of cloud after unsharp
masking
54
Blurring vs. Sharpening

• Blurring
• Local Averaging of Pixels in Scene
• Averages out fine detail in image more than
  large-scale structure
• Sharpening
• Inverse of Blurring ? Local Differencing of
  Pixels

55
Image Processing to Correct for

• Atmosphere (to extent possible)
• e.g., images obtained of object at different
  heights in sky exhibit different atmospheric
  extinction
• images usually can be corrected to compare
  brightnesses
• CCD defects and artifacts
• dark current
• Pixel gives output response even when not exposed
  to light
• Bad pixels
• Due to manufacturing flaws
• Dead, Hot, Flickering (time-variable
  response)
• Variations in pixel-to-pixel sensitivity
• every pixel has its own Quantum Efficiency (QE)
• Characterized by measuring response to uniform
  flat field and subsequently divided out

56
Links 6 and 7Image Display and Analysis
57
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

58
Link 8 Storage
59
Storage

• Glass plates
• Requires MUCH climate-controlled storage space
• Expensive to store and retrieve
• 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

60
Objects with Large Luminosities can be Detected
at Large Distances
Lines of constant apparent brightness
61
Distant Objects are Usually Physically Larger
62
Angular Sizes Span Wide Range