Download Document

This Set of Slides
• This set of slides deals with telescopes.
• Units covered: 26, 27, 28, 29, and 30.
Telescopes
• Telescopes have been used
for hundreds of years to
collect light from the sky and
focus it into an eyepiece. An
astronomer would then look
through the eyepiece at
planets, nebulae, etc.
• The human eye is not very
sensitive to dim light, and
was replaced in astronomy
by the film camera.
• Film is sensitive to only
around 10% of the impinging
light, and is often today
replaced by a…
The Charge-Coupled Device (CCD)
• The CCD, similar to
those found in
commercial digital
cameras and cell phones,
can collect around 75%
of the visible light that is
focused on it.
• It has revolutionized
astronomy – images can
be recorded and
• The science of developing new
downloaded to a
methods for sensing, focusing and
computer anywhere in
imaging light in astronomy is called
the world for analysis.
instrumentation.
Outside the visible spectrum
• Many objects of astronomical interest
are visible only in frequencies other
than the visible.
• Much can be learned from studying a
star, planet or nebula in different
frequencies.
• Radio telescopes can be used from the
ground to image pulsars and other
bodies.
• Observations in other wavelengths
require instrumentation to be lifted
above the Earth’s atmosphere.
• X-ray, Gamma ray and infrared
wavelength telescopes are currently in
orbit.
• Displaying these require false color.
Modern Telescopes
• Modern telescopes are
designed to collect as much
light as possible.
• Collected visible light is of
nanometer wavelength; the
telescopes must be extremely
precise to keep the images
clear and sharp.
Radio Telescopes
• Radio telescopes, like
the one in Arecibo,
Puerto Rico, collect
radio waves from
astronomical objects.
• Radio waves have
long wavelengths. The
telescopes must be
large or arranged in
arrays of smaller
telescopes.
Radio Telescope Project at OSU
• OSU physics department
has a radio telescope
project.
• Utilizing old satellite
dish antennae (for TV)
and homebuilt
electronics, radio signals
have been detected from
space.
• See Dr. Bill Hetherington or
Jim Ketter for more info.
www.physics.oregonstate.edu/~hetheriw/astro/index.html
Size Matters!
• Aperture size is very
important when collecting
light.
• A large collecting area
allows astronomers to
image dimmer and more
distant objects.
• For a telescope with an
aperture diameter D:
Collecting Area 

4
 D2
Telescopes continued…
• How do telescopes focus light? There are two main
ways this is done.
• Refracting telescopes (or refractors) use refraction to
focus light through lenses to form images.
• Reflecting telescopes focus light by reflecting it off
curved mirrors (for visible) or surfaces (for radio.)
Refraction
• The speed of light changes
depending on what substance,
or medium, it moves through.
• The speed of light in vacuum is
around 300,000 km/s. Its speed
through glass or water, for
example, is slightly slower.
• If a beam of light enters a new
medium at an angle, light on the side
of the beam that enters first slows
down.
• This slowing of one part of the beam
causes the light to change direction.
• This bending of the path of light is
called refraction.
Lenses
• A lens is a specially
shaped piece of glass that
bends light rays passing
through it so that they
focus a particular
distance away (the focal
length) at a particular
location (the focal plane).
• A sensor - such as the
human retina, camera
film, or a CCD - placed
in the focal plane can
image the light.
Refracting Telescopes
Large refractors are difficult to build
and have other limitations.
– Glass is heavy, and glass lenses must
be supported only by their rims, a
difficult engineering problem.
– Glass sags under its own weight,
distorting the lens and image.
– Refractors suffer because of
dispersion, a blurring effect due to
changes in the focal plane of the lens
for different wavelengths of light.
Dispersion
• The amount that light is diffracted depends on its frequency.
• A prism or raindrop spreads white light out into colors.
• This dispersion of light is a problem in refracting telescopes (and
cameras too), as the focal plane will be at a slightly different location
for each wavelength (color) of light.
• This leads to chromatic aberration, a blurring effect.
Reflecting Telescopes
• Mirrors can be supported from
behind, and so can be much
larger than refractors.
• Larger sizes mean that more
light can be collected and
focused, allowing astronomers to
image dimmer or more distant
objects.
• Most modern telescopes are
reflectors.
Different styles of reflectors
X-Ray reflectors
• X-rays only reflect at glancing angles, otherwise they are
absorbed or pass through the mirror.
• X-Ray mirrors are designed to gently reflect the
incoming photons, focusing them at the end of a long
tube-shaped array of mirrors.
Very Large Mirrors
• Reflectors can be made even larger
if multiple mirrors are used as the
primary mirror.
• The Keck Telescope uses 36 large
mirrors to create a single huge
primary reflecting surface.
• The positions of the mirrors are
precisely measured by lasers, and
can be individually adjusted to
keep them perfectly aligned.
Diffraction and Resolution
•
•
•
•
Some stars that appear to be a
single object to the unaided eye
are, when viewed through a
telescope, discovered to be two
separate stars.
The telescope is able to separate
the two stars, while the human
eye is not.
The telescope has better
resolution than the human eye.
The telescope’s resolution is
better because it has a larger
aperture, and there is less
diffraction as light passes
through that aperture.
• Diffraction is a “spreading” effect due to
the finite size of an aperture.
• Light waves approach the aperture as flat
plane waves, similar to the straight water
waves seen above.
• As the waves pass through the aperture,
the waves become curved as seen here.
Diffraction Effects
•
•
•
•
Diffracted light waves spread and
interfere with each other.
This results in a diffraction pattern, a
blurring of the image as it passes
through the telescope.
Larger apertures have less diffraction,
and therefore have higher resolution
than smaller apertures.
For observing light of wavelength nm,
the smallest separation angle arcsec a
telescope can resolve is related to the
telescope aperture Dcm by:
 arcsec
0.02  nm

Dcm
Interferometers
• To counter diffraction effects (and
obtain higher resolution),
astronomers use interferometers.
• Signals from these arrays of
widely-separated telescopes are
added together to create images
with very high resolution.
• In fact, the resolution is
equivalent to that of a single
telescope with an aperture as
large as the separation between
telescopes in the array!
Single versus Array
• Image from a single radio
telescope.
– What looks like a single star…
• Image from an array.
– …is actually two stars!
Atmospheric Absorption
• The Earth’s atmosphere absorbs • Visible, radio and some infrared
wavelengths are not absorbed readily
most of the radiation incident on
by the atmosphere.
it from space.
– Optical and radio telescopes work well
• This is a good thing for life –
from the ground.
high energy photons would
• Gamma Rays, X-rays, UV and IR
sterilize the planet!
photons are absorbed.
• This is not a good thing for
– Observatories for these wavelengths
astronomy, however!
must be in orbit above the Earth’s
atmosphere!
Ground- and Space-based Observatories