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Telescopes: Portals of Discovery
How do light and matter interact?
• Emission
• Absorption
• Transmission
– Transparent objects transmit light
– Opaque objects block (absorb) light
• Reflection or Scattering
Reflection and Scattering
Mirror reflects
light in a particular
direction
Movie screen scatters light
in all directions
Reflection of light with mirrors
The flat surface enables an incoming light
beam to perfectly bounce
Angle of Incidence = Angle of Reflection
Reflection and Scattering with
Matter
Interactions between light and matter determine the
appearance of everything around us
Thought Question
Why is a rose red?
a)
b)
c)
d)
The rose absorbs red light.
The rose transmits red light.
The rose emits red light.
The rose reflects red light.
Thought Question
Why is a rose red?
a)
b)
c)
d)
The rose absorbs red light.
The rose transmits red light.
The rose emits red light.
The rose reflects red light.
Refraction
(occurs during transmission)
• The universal speed
of light is measured
in a vacuum.
• The speed of light is
slower in different
substances.
Refraction
• E.g. Light travels
slower through
glass or water than
through air.
• Refraction is the
bending of light
when it passes
from one substance
into another
Focusing Light
• The speed of light through the liquid in your
eye is slower than the speed of light in air.
• Refraction can cause parallel light rays to
converge to a focus
Image Formation
• The focal plane is where light from different
directions comes into focus
• The image behind a single (convex) lens is actually
upside-down!
What have we learned?
• How do light and matter ineract?
– 4 forms of interaction, including reflection
and transmission
– Refraction is when light slows and bends
when travelling through a substance
• How does your eye form an image?
– It uses refraction to bend parallel light rays
so that they form an image.
– Cameras focus light like your eye and record
the image with a detector (CCDs in digital
cameras).
Telescopes: Giant Eyes
• Our goals for learning
• What are the two most important properties
of a telescope?
• What are the two basic designs of
telescopes?
• What do astronomers do with telescopes?
What are the two most important
properties of a telescope?
1. Light-collecting area: Telescopes with a
larger collecting area can gather a greater
amount of light in a shorter time.
2. Angular resolution: Telescopes that are
larger are capable of taking images with
greater detail.
Light Collecting Area
• A telescope’s diameter tells us its lightcollecting area: Area = π(diameter/2)2
• i.e The larger the aperture size, the better the
telescope
• The largest telescopes currently in use have a
aperture diameter of about 10 meters
Bigger is better
Thought Question
How does the collecting area of a 10-meter
telescope compare with that of a 2-meter
telescope?
a) It’s 5 times greater.
b) It’s 10 times greater.
c) It’s 25 times greater.
Angular Resolution
• Distant objects look very
close together; i.e. a small
'angular separation'
• Angular resolution is the
minimum gap between
objects that the telescope
can distinguish.
• Eventually the car is so far
away that you see the
headlights as one light.
-you have reached the limit of
your eye to resolve the
angular separation.
Angular Resolution
• Ultimate limit to
resolution comes
from interference of
light waves within a
telescope.
• Larger telescopes
are capable of
greater resolution
because there’s less
interference for the
amount of light
What are the basic designs of
telescopes?
• Refracting telescope: Focuses light with
lenses
• Reflecting telescope: Focuses light with
mirrors
• Catadioptric telescope: Focuses light with
both lenses and mirrors
Refracting Telescope
• Refracting
telescopes
need to be
very long,
with large,
heavy lenses
Basic Refractor Telescope 1608
Galileo’s Telescopes 1609
Chromatic Aberration
• Light focuses imperfectly in a lens because the
light must transmit though glass
• All lenses suffer from chromatic aberration
(slight blurring caused by the short wavelength light
focusing before the long wavelengths)
Reflecting Telescope
• Reflecting telescopes do not suffer chromatic aberration
• Reflecting telescopes can have much greater diameters.
• Most modern telescopes are reflectors
Mirrors in Reflecting Telescopes
The largest mirrors in the world are
in the Keck telescope on Hawaii
Twin Keck telescopes
on Mauna Kea
Segmented 10-meter mirror
of a Keck telescope
Designs for Reflecting Telescopes
Newtonian Telescope 1672
One of the most common
amateur telescopes for
deep sky observing.
Early
Newtonian
used by
Herschel
Modern
version with
a Dobsonian
mount
Cassegrain Reflector Telescope, 1672
More efficient, but requires a more complex secondary mirror.
(hyperbolic rather than flat)
Some Cassegrain Correctors -Catadioptrics
Schmidt-Cassegrain Telescope 1930
One of most
common amateur
telescopes
Maksutov-Cassegrain Telescope 1941
Catadioptric Telescopes
Corrector lens
Schmidt-Cassegrain
telescope on an
equatorial mount
What do astronomers do with
telescopes?
• Imaging: Taking pictures of the sky
• Spectroscopy: Breaking light into spectra
• Timing: Measuring how light output varies
with time
Imaging
• Early astronomers had no choice, they observed
by eye and recorded with descriptions and
drawings.
– Visible light only.
• Camera - permanent record, non-subjective,
reproducible world-wide in publications.
– Can be built to detect any wavelength.
Imaging
• Astronomical
detectors
generally
record only
one color of
light at a time
• Several images
must be
combined to
make full-color
pictures
Imaging
• Astronomical
detectors can
record forms of
light our eyes
can’t see
• Color is
sometimes used
to represent
different
energies of
nonvisible light
Imaging
• True color image
– The colors are the same as
if you were viewing with
your own eyes
• False color image
– The colors have been
arbitrarily assigned to
assist image interpretation
The famous image of the
Eagle nebula is false color!
(Nebula in true color)
Spectroscopy
Light from
only one star
enters
• A spectrograph
separates the
different
wavelengths of
Diffraction
light before they
grating breaks
light into
hit the detector
spectrum
Detector
records
spectrum
Spectroscopy
• Graphing
relative
brightness of
light at each
wavelength
shows the
details in a
spectrum
Timing
• A light curve represents a series of brightness
measurements made over a period of time
What have we learned?
• What are the two most important properties of a
telescope?
– Collecting area determines how much light a
telescope can gather
– Angular resolution is the minimum angular
separation a telescope can distinguish
• What are the basic designs of telescopes?
– Refracting telescopes focus light with lenses
– Reflecting telescopes focus light with mirrors
– Catadioptric telescopes focus light with both.
– The vast majority of professional telescopes are
reflectors
What have we learned?
• What do astronomers do with telescopes?
– Imaging
– Spectroscopy
– Timing