Download Refracting vs Reflecting Telescopes

PHYS-3380 Astronomy
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
3.
Lens must be very high quality and perfectly shaped on both sides
4. Refracting lenses subject to chromatic aberration
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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
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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
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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
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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 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
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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.
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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
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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.
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Cassegrain reflector
- most common form
of astronomical
telescope
- allows room for
large instruments
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Schmidt-Cassegrain focus most
common in small telescopes
- uses catadioptrics - combines
optical advantages of both lenses
and mirrors
- light enters through a thin aspheric
Schmidt correcting lens
- focal length increased by the
magnification of the correcting
lens
- lens carefully matched to the
primary concave mirror to
correct for spherical aberration
- too slightly curved to
introduce serious chromatic
aberration
- shorter physical length
- lighter and more compact
- easy to use
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The Keck Telescopes
On Mauna Kea in Hawaii. 36 hexagonal mirrors function as single 10meter mirror.
- segmented mirrors
- more economical - segments can be made separately
- weighs less
- cools rapidly
- less distortion from uneven expansion and
contraction
- optical shape maintained by computer-driven thrusters
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Two Fundamental Properties of a Telescope
Resolution
smallest angle which can be seen
θ = 1.22 λ / D
The angular resolution of a reflecting telescope is dependent on
the diameter of the mirror (D) and the wavelength of the light
being viewed (λ)
Light-Collecting Area
think of the telescope as a “photon bucket”
The amount of light that can be collected is dependent on the
mirror area A = π (D/2)2
These properties are much more important than magnification which is produced
by placing another lens - the eyepiece - at the mirror focus. Astronomers do not
look through telescopes with their eyes - a light gathering detector (for instance a
camera) records the image which can later on be magnified to any desired size.
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•
•
Angular Resolution
The ability to separate two
objects.
The angle between two
objects decreases as your
distance to them increases.
The smallest angle at which
you can distinguish two
objects is your angular
resolution.
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Angular Resolution of Car Lights Animation
The maximum angular resolution attainable by the human eye is about
one arcminute - in other words two stars will appear distinct if they are
separated by more than one arcminute - remember that Tycho Brahe
produced the best naked eye star charts ever - had resolution of one
arcminute.
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Angular Resolution
Resolving power: Wave nature of light =>
The telescope aperture produces
fringe rings that set a limit to the
resolution of the telescope.
Resolving power = minimum angular
distance amin between two objects that
can be separated.
amin = 1.22 (λ/D)
For optical wavelengths, this gives
amin = 11.6 arcsec / D[cm]
amin
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So: the angular resolution/resolving power of a reflecting telescope is
dependent on the diameter of its mirror
Mirror Angular Resolution Animation
and the wavelength of the light
Wavelength Effect on Resolution
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Light Gathering Ability: Size Does Matter
1. Light-gathering
power: Depends on
the surface area A of
the primary lens /
mirror, proportional
to diameter squared:
D
A = π(D/2)2
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So: light collecting ability of a reflecting telescope is dependent on the
area of the mirror
Light Collecting Area Animation
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Magnifying Power
Magnifying Power = ability of the telescope to make the image
appear bigger.
The magnification depends on the ratio of focal lengths of
the primary mirror/lens (Fo) and the eyepiece (Fe):
M = Fo/Fe
A larger magnification does not improve the resolving
power of the telescope!
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Interferometry
Recall: Resolving power of a telescope depends on diameter D:
amin = 1.22 λ/D.
This holds true even if
not the entire surface is
filled out.
• Combine the signals from
several smaller telescopes
to simulate one big mirror
→ Interferometry
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Uses of Telescopes
1.
Imaging
–
use a camera to take pictures (images)
2. Photometry
measure total amount of light from an object
3. Spectroscopy
–
use a spectrograph to separate the light into its different
wavelengths
4. Timing
–
measure how the amount of light changes with time
(sometimes in a fraction of a second)
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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.
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Spectroscopy
•
The spectrograph reflects light of
a grating: a finely ruled, smooth
surface.
•
Light interferes with itself and
disperses into colors.
•
This spectrum is recorded by a
digital detector called a CCD.
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Nonvisible Light
•
•
Special detectors/receivers record light invisible to the human eye - gamma
rays, x-rays, ultraviolet, infrared, radio waves.
- 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
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Visible
The Crab Nebula
Radio Waves
Infrared
X-rays
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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.
– 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.
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Light Pollution
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Radio Astronomy
Recall: Radio waves of λ ~ 1 cm – 1 m also penetrate the
Earth’s atmosphere and can be observed from the ground.
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Radio Telescopes
Large dish focuses the
energy of radio waves
onto a small receiver
(antenna)
Amplified signals are
stored in computers and
converted into images,
spectra, etc.
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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
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Radio Interferometry
The Very
Large Array
(VLA): 27
dishes are
combined to
simulate a
large dish of
36 km in
diameter.
Even larger arrays consist of dishes spread out over the entire U.S.
(VLBA = Very Long Baseline Array) or even the whole Earth (VLBI
= Very Long Baseline Interferometry)!
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Most sensitive VLBI array in the world - European VLBI Network (EVN).
• brings together the largest European radiotelescopes for typically week-long
sessions
Very Long Baseline Array (VLBA)
• uses ten dedicated, 25-meter telescopes spanning 5351 miles across the
United States
• the largest VLBI array that operates all year round as both an astronomical
and geodesy instrument.
Global VLBI
• Combination of the EVN and VLBA
Space Very Long Baseline Interferometry (SVLBI)
•dedicated VLBI placed in Earth orbit to provide greatly extended baselines.
•HALCA, an 8 meter radio telescope - launched in February 1997 - made
observations until October 2003,
•small size of the dish - only very strong radio sources could be
observed with
•Spektr-R (or RadioAstron) - launched in July 2011.
When Global VLBI combined with one or more space-based VLBI antennas gives
resolution of microarcseconds.
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Science of Radio Astronomy
Radio astronomy reveals several features, not visible at other
wavelengths:
- neutral hydrogen clouds (which don’t emit any visible light),
containing ~ 90 % of all the atoms in the Universe.
- molecules (often located in dense clouds, where visible light is
completely absorbed).
- Radio waves penetrate gas and dust clouds, so we can observe
regions from which visible light is heavily absorbed.
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Atmospheric Distortion
The turbulence (ever-changing motion) of the atmosphere causes
distortion - twinkling of starlight. Bends light in constantly shifting
patterns. Like looking down the road on a hot day and seeing distant
cars rippling and distorting. Why best viewing is when it is cold and
calm.
Atmospheric Distortion Animation
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Adaptive Optics (AO)
It is possible to “de-twinkle” a star.
The wavefronts of a star’s light rays are deformed by the atmosphere.
By monitoring the distortions of the light from a nearby bright star (or a laser):
– a computer can deform the secondary mirror in the opposite way.
– the wavefronts, when reflected, are restored to their original state.
•
•
•
AO mirror off
AO mirror on
Angular resolution
improves.
These two stars are
separated by 0.38ʺ
Without AO, we see
only one star.
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The Sun
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The Sun’s Energy Source
The first scientific theories involved chemical reactions or gravitational
collapse.
- chemical burning ruled out…it can not account for the Sun’s
luminosity
- conversion of gravitational potential energy into heat as the Sun
contracts would only keep the Sun shining for 25 million years
- late 19th-century geological research indicated the Earth was older
than that
Development of nuclear physics led to the correct answer
- the Sun generates energy via nuclear fusion reactions
- Hydrogen is converted into Helium in the Sun’s core
- the mass lost in this conversion is transformed into energy
- the amount of energy is given by Einstein’s equation: E = mc2
- given the Sun’s mass, this will provide enough energy for the Sun to
shine for 10 billion years
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Striking a Balance
The Sun began as a cloud of gas undergoing gravitational collapse.
- the same heating process, once proposed to power the Sun, did cause
the core of the Sun to get hot and dense enough to start nuclear fusion
reactions
Once begun, the fusion reactions generated energy which provided an
outward pressure.
This pressure perfectly balances the
inward force of gravity.
- deep inside the Sun, the pressure is
strongest where gravity is strongest
- near the surface, the pressure is
weakest where gravity is weakest
This balance is called gravitational
equilibrium.
- it causes the Sun’s size to remain
stable
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One second of output from the Sun (luminosity) would provide power
for the human race for the next 500,000 years
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Layers of the Sun
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Core
T = 1.5 x 107 K; depth = 0 – 0.25 R
Density - up to 150,000 kg/m³ (154 times the density of water on
Earth)
Pressure 200 billion times that on the surface of Earth
This is where the Sun’s energy is generated.