Download The Imaging Chain in Optical Astronomy

Review and Overview
The Imaging Chain
in Optical Astronomy
“Imaging Chain” includes these elements:
1.
2.
3.
4.
5.
6.
7.
8.
energy source
object
collector
detector (or sensor)
processor
display
analysis
storage (if any)
Source and/or Object
Optical Imaging Chain
• 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
1: source
– Examples:
• Galaxies
• Stars
5: processing
– Exceptions:
2: object
6: display
7: analysis
3: collector
4: sensor
Optical Imaging Chain in
Astronomy until 1980 or so
• Planets and the moon
• Dust and gas that reflects or absorbs starlight
Optical Imaging Chain in Modern
Astronomy (post-1980)
5: processing
1: source
2: object
5: processing
1: source
2: object
6: display
7: analysis
3: collector
4: sensor
6: display
7: analysis
3: collector
4: sensor
8: storage
8: storage
(stack of glass)
1
Transition (“Catch-up”) Phase:
Digitize Plates
Optical Imaging Chain in Radio
Astronomy
6: display
7: analysis
1,2
+
3,4
Scanner
radio waves
receiver where
waves are collected
waves
converted into
electro signals
5
8: storage
computer
received as signal
6,7
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
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)
– broad wavelength coverage AND ability to measure
narrow spectral lines
Sensitivity vs. Dynamic Range
• Sensitivity
– ability to measure faint brightness
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)
• Dynamic Range
– ability to image “bright” and “faint” sources in same
system
• Spectral Resolution
– Ability to detect and measure narrow spectral lines
– Limited by:
• “Spectrometer” Resolution (number of lines in diffraction
grating)
2
Optical Collection (Link #3):
Refracting Telescopes
• Lenses collect light
• BIG disadvantages
Optical Collector (Link #3)
– 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
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
Thin and Light (Weight) Mirrors
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
Hale 200" Telescope
Palomar Mountain, CA
• Light weight ⇒Easier to point
– “light-duty” mechanical systems ⇒ cheaper
• Thin Glass ⇒ Less “Thermal Mass”
– Reaches Equilibrium (“cools down” to ambient
temperature) quicker
http://www.cmog.org/page.cfm?page=374
http://www.astro.caltech.edu/observatories/palomar/overview.html
3
200" mirror (5 meters)
for Hale Telescope
•
•
•
•
•
•
•
Keck telescopes, Mauna Kea, HI
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
http://www2.keck.hawaii.edu/geninfo/about.html
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 (< 15 tons)
• Telescope weighs 270 tons
• “Alt-azimuth” mount (left-right, up-down
motion)
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)
– follows sky with two motions + rotation
Prime Focus
Sensor
Newtonian Reflector
f
Mirror diameter must be large to ensure that
obstruction is not significant
Sensor
4
Feature of Cassegrain
Telescope
Cassegrain Telescope
• Long Focal Length in
Short Tube
f
Sensor
Location of
Equivalent Thin Lens
Secondary
Convex Mirror
Coudé or Nasmyth Telescope
Optical Reflecting Telescopes
Schematic
of 10-meter
Keck
telescope
(segmented
mirror)
Sensor
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)
Keck
telescope
mirror
(note
person)
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
Summit of Mauna Kea, with Maui in background
http://seds.lpl.arizona.edu/billa/bigeyes.html
5
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)
–
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#
Determined by “F ratio” or “F#”
F#≡
f
D
f = focal length of collecting element
D = diameter of aperture
Small F#
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
Large F#
F Ratio: F#
• Two reflecting telescopes with different F#
and same detector have different “Fields of
View”:
• Keck 10-m on Mauna Kea: f/1.75
– focal length of primary mirror is:
1.75 × 10m = 17.5 m ≅ 58 m
large ∆θ
small ∆θ
Small F#
Large F#
Astronomical Cameras
Usually Include:
1. Spectral Filters
–
Sensors (Link #4)
–
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
6
Astronomical Sensors
• Most common detectors:
– Human Eye
– Photographic Emulsion
Angular Resolution
• Fundamental Limit due to Diffraction in
“Optical Collector” (Link #3)
∆θ ≅
• film
• plates
λ
D
– Electronic Sensors
• But Also Limited by Resolution of Sensor!
• CCDs
Sensor Resolution
ChargeCharge-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)
• 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!
• 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
∆x
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
Photographic techniques:
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
of AgX techniques until CCDs in late 70’s
7
Eye as Astronomical Detector
Eye with Telescope
• Eye includes its own lens
Without Eyepiece
– 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)
With Eyepiece
Light entering eye
is “collimated”
– reimaging is performed by “eyepiece”
Eye as Astronomical Detector
• Point sources (stars) appear brighter to eye through
telescope
• Large role in ground-based optical astronomy
– scintillation modifies source angular size
2
• Factor is
Atmospheric Effects on Image
D
P2
• twinkling of stars = “smearing” of point sources
– extinction reduces light intensity
– D is telescope diameter
– P is diameter of eye pupil
– Magnification should make light fill the eye pupil (“exit pupil”)
• atmosphere scatters a small amount of light, especially at
short (bluer) wavelengths
• water vapor blocks specific wavelengths, especially nearIR
– scattered light produces interfering “background”
• 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.
Scattering
• “Wavelength Dependent”
– Depends on color of light
– Long wavelengths are scattered “less”
• astronomical images are never limited to light from
source alone; always include “source” + “background
sky”
• “light pollution” worsens sky background
Scattering by Molecules
"Rayleigh Scattering" ∝
1
λ4
• 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)
8
Scattering by Dust
"Mie Scattering" ∝
1
λ
• Dust particles are MUCH larger than
molecules
Link #5: Image Processing
– e.g., from volcanos, dust storms
• Blue light is scattered by dust “somewhat
more” than red light
Link #5: Image Processing
Image Processing
• Once collected, images must be corrected for:
• 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)
–…
– 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”
Image Display and Analysis
Links #6 and #7
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
9
Storage
• Glass plates
Link #8: Storage
– 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
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