Download The Imaging Chain for Optical Astronomy

The Imaging Chain
for Optical Astronomy
Review/overview
• The imaging chain typically includes the
following elements:
–
–
–
–
–
–
energy source
object
collection
detection
processing
display & analysis
Source/object
• In astronomy, the source of energy (light) is
almost always also the object of the imaging
– exceptions: planets, dust reflecting starlight
• Astronomical sources place specific requirements
on astronomical imaging systems
– requirements are often conflicting:
• excellent angular resolution; wide field of view
• high sensitivity; large dynamic range
• broad wavelength coverage; spectral lines
More luminous objects can be
detected out to larger distances
Lines of constant
apparent brightness
More distant objects are usually
larger in physical size
Lines of constant
angular size
Angular sizes span a wide range
Atmosphere modifies source
• For ground-based optical astronomy, Earth’s
atmosphere plays a large role in determining the
character of the source
– scintillation modifies source angular size
• twinkling of stars = smearing of point sources
– extinction cuts down on light intensity
• atmosphere scatters a small amount of light, especially at short
(bluer) wavelengths
• water vapor blocks out specific wavelengths, esp. in 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
Collection: Telescopes
• 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
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
3.5 meter
WIYN
telescope
mirror, Kitt
Peak, Arizona
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
Optical Reflecting Telescopes
Schematic of
10-meter
Keck
telescope
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)
Keck
telescope
mirror
(note
person)
Summit of Mauna Kea, with Maui in background
Why build big telescopes?
• Larger aperture means more light gathering
power
– sensitivity goes like D2, where D is diameter of
main light collecting element (e.g., primary
mirror)
• Larger aperture means better angular
resolution
– resolution goes like lambda/D, where lambda is
wavelength and D is diameter of mirror
Why build small telescopes?
• Smaller aperture means less chance of
saturation (“overexposure”) on bright
sources
• Smaller aperture generally means larger
field of view
– recall F ratio, F=f/D, where f is focal length of
collecting element and D is diameter of aperture
– for two reflecting telescopes with same F ratio and the
same size detector, the telescope with smaller D
produces images that cover a wider angle
Detection: Cameras for Astronomy
• Camera usually includes:
– filters
• most experiments require specific wavelength range(s)
• broad-band vs. narrow-band
– reimaging optics
• enlarge or reduce image formed by primary collecting element
– detector
• Most common detectors:
– The eye
– Photographic emulsion
• film
• plates
– CCDs
The eye as astronomical detector
• Must reimage the image formed by the primary (or
objective) such that the light rays are parallel as they enter
the eye (i.e. rays appear to come from infinity)
– reimaging is accomplished by the eyepiece
• Point sources (stars) appear brighter to the eye through a
telescope by a factor D2/P2 , where D is telescope diameter
and P is the diameter of the eye’s pupil
– for maximum effect, magnification has to be sufficient for light to
fill pupil
• Extended sources (for example, nebulae) do not appear
brighter through a telescope
– Gain in light gathering power is exactly compensated by
magnification of image, which spreads light out
Photographic techniques:
silver halide
• film
– large amount of work is still done by amateurs using
highly sensitive B&W and color film
• plates
– from the earliest development of AgX techniques until
advent of CCDs in late 70’s, most images were
captured on photographic plates
• panes of glass overlaid with silver halide emulsion
CCDs
• charge coupled devices (CCDs) are now standard light
detection medium for professional and amateur
astronomical imaging systems alike
• numerous advantages over film:
– high quantum efficiency (QE), meaning most photons incident on a
CCD are detected
– linear response, meaning signal builds up in direct proportion to
number of photons collected
– fast processing turnaround (CCD readout speeds ~1 sec)
– regular grid of pixels (as opposed to random distribution of AgX
grains)
– image delivered in computer-ready form
Image processing
• Once images are collected, they need to be
corrected for:
– Atmosphere (to the 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 & analysis
• Often, this step in the imaging chain is where the
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