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Astronomical Imaging
Telescopes and Detectors
Imaging Science Fundamentals
Chester F. Carlson Center for Imaging Science
Astronomical Imaging
• GOAL: image large objects at VERY large
distances (typically measured in light
years, ly)
– Nearest star: alpha Centauri, 4.3 ly
– Nearest galaxy: Andromeda, 3 million ly
– “Edge” of universe: 15 billion ly
• REQUIREMENTS:
– High angular resolution (where possible)
– High telescope/detector sensitivity
Imaging Science Fundamentals
Chester F. Carlson Center for Imaging Science
Angular Resolution
• Angular resolution = ability to distinguish detail
• Easy yardstick for grasping resolution: the Moon
– Moon’s disk: 1/2 degree across (same for Sun)
– 1 degree = 60 arc minutes; 1 arc minute = 60 arc
seconds
– unaided eye can distinguish shapes/shading on
Moon’s surface (resolution: ~1 arc minute)
– w/ small telescope can distinguish large craters
(resolution: a few arc seconds)
– w/ large telescope can see craters 1/2 mile (~1 arc
second) across
Imaging Science Fundamentals
Chester F. Carlson Center for Imaging Science
Angular Resolution
• Factors determining angular resolution:
– Diameter of main light collecting surface (mirror or
lens) of telescope
• determines diffraction limit of telescopic imaging system
– Quality of telescope collecting surface
• smoother surface = better resolution
– Atmospheric effects
• turbulence smears image
• essentially same effect as stars’ “twinkling”
Imaging Science Fundamentals
Chester F. Carlson Center for Imaging Science
Sensitivity
• Sensitivity = ability to detect faint sources of
electromagnetic radiation
• Telescope sensitivity: proportional to its light
collecting area (area of mirror or lens surface)
• Detector sensitivity: measured by its quantum
efficiency (fraction of input photons that
generate signal in detector)
• Also, need the ability to expose the detector
(integrate) for very long periods of time
Imaging Science Fundamentals
Chester F. Carlson Center for Imaging Science
Telescopes: Basic Flavors
• Refractor telescopes
– exclusively use lenses to collect light
– have big disadvantages: aberrations & sheer
weight of lenses
• Reflector telescopes
– use mirrors to collect light
– relatively free of aberrations
– mirror fabrication techniques steadily improving
Imaging Science Fundamentals
Chester F. Carlson Center for Imaging Science
Optical Reflecting Telescopes
• Use parabolic,
concave primary
mirror to collect
light from source
– modern mirrors
for large
telescopes are
lightweight &
deformable, to
optimize image
quality
Imaging Science Fundamentals
3.5 meter
WIYN
telescope
mirror, Kitt
Peak, Arizona
Chester F. Carlson Center for Imaging Science
Optical Reflecting Telescopes
• Basic optical designs:
– Prime focus: light is brought to focus by primary
mirror, without further deflection
– Newtonian: use flat, diagonal secondary mirror to
deflect light out side of tube
– Cassegrain: use convex secondary mirror to reflect
light back through hole in primary
– Nasmyth focus: use tertiary mirror to redirect light
to external instruments
Imaging Science Fundamentals
Chester F. Carlson Center for Imaging Science
Optical Reflecting Telescopes
Schematic of
10-meter
Keck
telescope
Imaging Science Fundamentals
Chester F. Carlson Center for Imaging Science
Big Optical Telescopes
• Largest telescopes in use
or under construction:
– 10 meter Keck (Mauna
Kea, Hawaii)
– 8 meter Subaru (Mauna
Kea)
– 8 meter Gemini (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)
Imaging Science Fundamentals
Keck
telescope
mirror
(note
person)
Summit of Mauna Kea, with Maui in background
Chester F. Carlson Center for Imaging Science
Radio Telescopes
• Usually
Cassegrain in
design
– primary “mirror”
is replaced by
parabolic
reflector “dish”
– secondary is
called
subreflector
Imaging Science Fundamentals
12 meter radio telescope, Kitt Peak, Arizona
Chester F. Carlson Center for Imaging Science
Radio Telescopes
• Since wavelength of interest is longer, must
increase telescope aperture to achieve good
angular resolution
– alternative is to use an array of radio telescopes
Very
Large
Array,
New
Mexico
Imaging Science Fundamentals
Chester F. Carlson Center for Imaging Science
X-ray Telescopes
• Use grazing
incidence optics to
defeat tendency for
X-rays to be
absorbed by mirrors
Chandra
X-ray
telescope
mirror
design
– Tiny wavelength, so
exceedingly difficult
to produce “smooth”
mirrors for tight
focus
– Chandra is first X-ray
telescope to achieve
<1 arcsecond
resolution
Imaging Science Fundamentals
Chester F. Carlson Center for Imaging Science
Detectors
• Optical: CCDs rule
– film replaced by CCDs by early 80’s
– detector formats (sizes) continually growing
• 1024x1024: industry standard
• 4096x4096, CCD arrays: no longer uncommon
• IR: CIDs (near-IR), bolometers (far-IR)
– CIDs: similar to CCDs but each pixel addressed
independently
– bolometers: directly measure heat input
Imaging Science Fundamentals
Chester F. Carlson Center for Imaging Science
Detectors
• Radio: receivers
– original (50’s-60’s) technology similar to that of home
stereo use
– now emphasize extremely high sensitivity and
extremes in radio frequency range
• X-ray: proportional counters, CCDs
– prop. counters efficiently convert X-ray
energies to voltages
– CCDs provide better X-ray position & energy
determination
Imaging Science Fundamentals
Chester F. Carlson Center for Imaging Science
Observatory Sites
• The best telescope/detector is useless at
a bad site!
• Factors for consideration of appropriate
site:
– atmospheric transparency at wavelength of
interest
– atmospheric turbulence
– sky brightness
– accessibility
Imaging Science Fundamentals
Chester F. Carlson Center for Imaging Science
Observatory Sites
• Optical work:
– need dark, cloud-free site
– helps to remove atmosphere from system (e.g., Hubble)!
• IR work:
– need cold site
– dry site very important at certain wavelengths
• radio work:
– need dry site (shorter wavelengths)
– need interference-free site (longer “)
• X-ray work:
– need to be above atmosphere
Imaging Science Fundamentals
Chester F. Carlson Center for Imaging Science
Optical/IR Telescopes
• Dark, high, & dry: most big optical/IR telescopes
are placed on mountaintops in deserts
Kitt Peak, Arizona
Gemini South, Chile
Imaging Science Fundamentals
Mauna Kea, Hawaii
Chester F. Carlson Center for Imaging Science
IR Telescopes
• For optimum IR work, need high, dry, cold site
– South Pole works well, but accessibility an issue
Center for
Astronomical
Research in
Antarctica
Imaging Science Fundamentals
Chester F. Carlson Center for Imaging Science
IR Telescopes
• Helps to go into space, or at least above the
bulk of the atmosphere
SIRTF: NASA’s Space Infrared Telescope Facility
SOFIA: NASA’s Stratospheric Observatory for IR Astronomy
Imaging Science Fundamentals
Chester F. Carlson Center for Imaging Science
X-ray Telescopes
• Must go above
atmosphere to
detect celestial
objects! (X-rays
are absorbed by
Earth’s
atmosphere)
Chandra is in high
Earth orbit
Imaging Science Fundamentals
Chester F. Carlson Center for Imaging Science