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ISNS 3371 - Phenomena of Nature
The Eye
The eye consists of pupil that allows light into the eye - it controls the amount
of light allowed in through the lens - acts like a simple glass lens which
focuses the light on the retina - which consists of light sensitive cells that
send signals to the brain via the optic nerve. An eye with perfect vision has its
focus on the retina when the muscles controlling the shape of the lens are
completely relaxed - when viewing an object far away - essentially at infinity.
ISNS 3371 - Phenomena of Nature
When viewing an object not at infinity, the eye muscles contract and
change the shape of the lens so that the focal plane is at the retina (in an
eye with perfect vision). The image is inverted as with a single lens - the
brain interprets the image and rights it.
ISNS 3371 - Phenomena of Nature
Types of Optical Telescopes
ISNS 3371 - Phenomena of Nature
Magnification Using Two Lenses - Refracting Telescope
f1 = 0.5 m
f2 = 0.1 m
f1 = 0.5 m
f2 = 0.3 m
Refracting telescope - consists of two lenses - the objective and the
eyepiece (ocular). Incident light rays (from the left) are refracted by the
objective and the eyepiece and reach the eye of the person looking through
the telescope (to the right of the eyepiece). If the focal length of the
objective (f1) is bigger than the focal length of the eyepiece (f1), the
refracting astronomical telescope produces an enlarged, inverted image:
magnification = f1 /f2
ISNS 3371 - Phenomena of Nature
Refracting Telescope
Uses lens to focus light from
distant object - the eyepiece
contains a small lens that
brings the collected light to a
focus and magnifies it for an
observer looking through it.
ISNS 3371 - Phenomena of Nature
The largest refracting telescope in the world is the at the University of
Chicago’s Yerkes Observatory - it is 40 inches in diameter and 63 feet long.
ISNS 3371 - Phenomena of Nature
Reflecting Telescope
The primary mirror focuses light at the prime focus. A camera or another
mirror that reflects the light into an eyepiece is placed at the prime focus.
ISNS 3371 - Phenomena of Nature
Types of Reflecting Telescopes
Each design incorporates a small mirror just in front of the prime focus to
reflect the light to a convenient location for viewing.
ISNS 3371 - Phenomena of Nature
The Keck Telescopes
Largest in the world - on Mauna Kea in Hawaii. 36 hexagonal mirrors
function as single 10-meter mirror.
ISNS 3371 - Phenomena of Nature
The Hubble Space Telescope
The Hubble Space Telescope is 43.5 ft long and weighs 24,500 lbs. Its
primary mirror is 2.4 m (7 ft 10.5 in) in diameter.
ISNS 3371 - Phenomena of Nature
Refracting vs Reflecting Telescopes
Reflecting telescopes are primary astronomical tools used for research:
1. Lens of refracting telescope very heavy - must be placed at end of
telescope - difficult to stabilize and prevent from deforming
2. Light losses from passing through thick glass of refracting lens must be very high quality and perfectly shaped on both sides
3. Refracting lenses subject to chromatic aberration
ISNS 3371 - Phenomena of Nature
Lens and Mirror Aberrations
SPHERICAL (lens and mirror)
Light passing through different parts of a lens or reflected from
different parts of a mirror comes to focus at different distances from
the lens.
Result: fuzzy image
CHROMATIC (lens only)
Objective lens acts like a prism.
Light of different wavelengths (colors) comes to focus at different
distances from the lens.
Result: fuzzy image
ISNS 3371 - Phenomena of Nature
Chromatic Aberration in Lenses
Focal point
for blue light
Simple lenses suffer from
the fact that different colors
of light have slightly
different focal lengths. This
defect is corrected by
adding a second lens
The problem
Focal point
for red light
Focal point
for all light
The solution
ISNS 3371 - Phenomena of Nature
Spherical Aberration in Lenses
Simple lenses suffer
from the fact that light
rays entering different
parts of the lens have
slightly difference focal
lengths. As with
chromatic aberration,
this defect is corrected
with the addition of a
second lens.
The problem
One focal point
for all light rays
The solution
ISNS 3371 - Phenomena of Nature
Spherical Aberration in Mirrors
The Problem
Simple concave mirrors suffer
from the fact that light rays
reflected from different locations
on the mirror have slightly
different locations on the mirror
have slightly different focal
lengths. This defect is corrected
by making sure the concave
surface of the mirror is parabolic
The Solution
All light rays converge
at a single point
ISNS 3371 - Phenomena of Nature
The image from an reflecting telescope is inverted.
Focus Inversion Animation
The focus is adjusted by changing the secondary mirror position.
Mirror Position and Focus Animation
ISNS 3371 - Phenomena of Nature
Uses of Telescopes
1.
Imaging
–
use a camera to take pictures (images)
–
Photometry  measure total amount of light from an object
2.
Spectroscopy
–
use a spectrograph to separate the light into its different
–
wavelengths (colors)
3.
Timing
–
measure how the amount of light changes with time
(sometimes in a fraction of a second)
ISNS 3371 - Phenomena of Nature
Imaging
• In astronomy, filters are
usually placed in front of
a camera to allow only
certain colors to be
imaged
• Single color images are
superimposed to form
true color images.
ISNS 3371 - Phenomena of Nature
Nonvisible Light
•
•
•
Most light is invisible to the human eye - gamma rays, x-rays, ultraviolet,
infrared, radio waves.
Special detectors/receivers can record such light - each type of light can
provide information not available from other types.
Digital images are reconstructed using false-color coding so that we can see
this light.
Chandra X-ray image of the Center of the Milky Way Galaxy
ISNS 3371 - Phenomena of Nature
The Crab Nebula
Visible
Radio Waves
Infrared
X-rays
ISNS 3371 - Phenomena of Nature
Atmospheric Effects
Earth’s atmosphere causes problems for astronomers on the ground:
• Bad weather makes it impossible to observe the night sky.
• Man-made light is reflected by the atmosphere, thus making the night
sky brighter.
– this is called light pollution
• The atmosphere absorbs light - dependent on wavelength
• Air turbulence in the atmosphere distorts light.
– That is why the stars appear to “twinkle”.
– Angular resolution is degraded.
ISNS 3371 - Phenomena of Nature
Atmospheric Absorption of Light
• Earth’s atmosphere absorbs most types of light.
– good thing it does, or we would be dead!
• Only visible, radio, and certain IR and UV light make it through to the
ground.
To observe the other wavelengths, we must put our telescopes in space!
ISNS 3371 - Phenomena of Nature
Space Astronomy
ISNS 3371 - Phenomena of Nature
Space Based Telescopes
Chandra X-ray Obs.
FUSE (Far UV)
Compton Gamma Ray Obs.
Hubble Space Telescope
Spitzer Space Telescope (IR)
ISNS 3371 - Phenomena of Nature
Radio Telescopes
• The wavelengths of radio waves are long.
• So the dishes which reflect them must be very large to achieve any
reasonable angular resolution.
305-meter radio telescope at Arecibo, Puerto Rico
ISNS 3371 - Phenomena of Nature
Spectrum of a Low Density Cloud Animation
ISNS 3371 - Phenomena of Nature
ISNS 3371 - Phenomena of Nature
Kirchhoff’s Laws of Radiation
First Law. A luminous solid, liquid or gas, such as a light bulb filament,
emits light of all wavelengths thus producing a continuous spectrum
of thermal radiation.
Second Law. If thermal radiation passes through a thin gas that is
cooler than the thermal emitter, dark absorption lines are superimposed
on the continuous spectrum. The gas absorbs certain wavelengths.
This is called an absorption spectrum or dark line spectrum.
Third Law. Viewed against a cold, dark background, the same gas
produces an emission line spectrum. It emits light of discrete
wavelengths. This is called an emission spectrum or bright line
spectrum.
.
ISNS 3371 - Phenomena of Nature
So what astronomical body has this spectrum?
ISNS 3371 - Phenomena of Nature
The Doppler Effect
ISNS 3371 - Phenomena of Nature
Doppler Effect for Light Animation
ISNS 3371 - Phenomena of Nature
The Doppler Effect
1. Light emitted from an object moving towards you will have its
wavelength shortened.
BLUESHIFT
2. Light emitted from an object moving away from you will have its
wavelength lengthened.
REDSHIFT
3. Light emitted from an object moving perpendicular to your line-ofsight will not change its wavelength.
ISNS 3371 - Phenomena of Nature
Doppler Shift of Emission Lines Animation
ISNS 3371 - Phenomena of Nature
The amount of spectral shift tells us the velocity of the object:
 = v

c
ISNS 3371 - Phenomena of Nature
The Doppler shift only tells us part of the object’s full motion - the radial
part or the part directed toward or away from us.