Transcript Optics and Telescopes PowerPoint

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Optics and Telescopes
Credit: www.sherwoods-photo.com
Credit: www.telescopeguides.net
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This evening we will investigate:
• how lenses and mirrors can be used to focus
light and form an image.
• the 3 basic telescope designs and the
advantages and disadvantages of each.
• some numbers that characterize a telescope: fratio, light gathering power, resolution,
magnification
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This evening we will investigate:
• recording the images produced by a telescope.
• telescopes that use the other wavelengths of
light.
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• Optics – The science of reflecting and/or
refracting (bending) light so as to produce
an image of an object. The image is
usually recorded so that it can be studied
more extensively.
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• Regarding Mirrors
– The Law of Reflection: When a ray of light
strikes a shiny or “specular” surface, the ray
reflects away at the same angle at which it
struck the surface. The angle of incidence
equals the angle of reflection, as measured
from a ‘normal’ to the surface.
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i = r
a “shiny” or reflective surface
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• If the reflecting
surface is curved
correctly, the light
can be focused to
a point, called the
focal point. An
image forms near
the focal point.
Credit: www.antonine-education.co.uk
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• Regarding Lenses
– The Law of Refraction: When light moves
from a less dense medium (empty space or
air) to a denser medium (glass), the light
slows down and bends INTO the denser
medium.
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speed of light in air =
3 x 108 m/s
speed of light in glass =
2 x 108 m/s
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• Glass can be formed into a convex lens
which will also focus light. An image forms
near the focal point. The focal length is
the distance from the centerline of the lens
to the focal point.
focal length
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• The f-ratio is a way to compare or rate
convex (converging) lenses.
– The f-ratio is the focal length of the lens
divided by the lens’ diameter.
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Thicker lenses tend to focus
closer to the lens and give
brighter images. These are
“fast” lenses.
Do these lenses have low or
high f-ratios?
But these lenses have other
problems.
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Thinner lenses focus farther from the lens,
give less-bright images, and are described
as “slow” lenses.
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• When taking photographs of space
objects, using a “fast” lens with a low
f-ratio means less time is needed for the
photograph. This results in less blurring
due to vibration of the telescope and the
motion of the stars.
Credit: Gemini Observatory/AURA
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• Chromatic Aberration – a problem with
lenses
– The edges of lenses act like prisms. They
split “white” light into all the colors of the
rainbow.
– Problem: the different colors focus at different
focal points. This means that if you focus the
blue color of an object, the red is fuzzy, and
vice versa.
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Chromatic Aberration
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• There’s always a trade-off in optics. The
problem of chromatic aberration is worst
with “fast” or low f-ratio lenses. These are
the lenses we’d like to use most!
• The problem is fixed by making compound
lenses out of 2 or more different kinds of
glass.
• Mirror-based telescopes don’t have this
problem – a definite advantage!
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• 3 Types of Telescopes
• Refractors (gathers light with a lens)
• Reflectors (gathers light with a mirror)
• Mixed (uses a combination of lenses and
mirrors)
– Schmidt-Cassegrain Telescopes
– Maksutov-Cassegrain Telescopes
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• Refracting Telescopes
– The “original” type, invented in the 1500’s and
first used by Galileo to explore space.
– Sharpest, brightest images.
– Lenses are heavy and expensive!
– Prone to chromatic aberration.
– Give an inverted (upside-down) image.
– Can only be made up to about 40 inches in
diameter.
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Credit: library.thinkquest.org
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• Reflecting Telescopes…Advantages
– Mirrors are much cheaper to make than
lenses, and are very light-weight, easy to
carry.
– Mirrors can be VERY large. Multiple mirrors
can be combined to simulate a single gigantic
mirror.
– No chromatic aberration.
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• Reflecting Telescopes…Disadvantages
• Not quite as sharp or bright an image as
the same size refractor.
• Large scopes get currents of different
temperature air inside their tubes. This
can make images blurry.
• Mirrors will oxidize (corrode) over time.
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• Combination ‘scopes…the Cassegrains
– Very short tube length, because the light gets
“folded” back on itself twice. This makes the
scope easy to handle & transport.
– Moderately expensive.
– Best choice for amateur astrophotography,
because the tube doesn’t vibrate or shake
very much.
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The corrector plate is a type of lens. A secondary mirror is glued to
its inner surface.
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• The telescope mount is as important as the
optics! There are two types…
• Altitude-Azimuth. Like aiming a tank.
Point it in the compass direction (azimuth)
you want, then point it up to the angle
(altitude) you want.
– Easy to use, but image rotates over time.
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• Equatorial. Part of the mount is aimed at
the north celestial pole. The mount then
swivels east-west to follow an object
through the sky.
– Disadvantage: a real bear to use!
– Advantage: the picture in the telescope
doesn’t appear to rotate over time.
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• What is the function of a telescope? It’s
not just to make the image bigger!
– Gathering light
– Resolving details
– Magnifying the image
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• A Telescope is a Light Funnel
• Gathering light from dim objects is the
MOST important function of a telescope.
• Which would you rather see, a large
but very dim image or a
image
?
smaller, but very bright
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• Light-gathering power (LGP)
– How much light can the human eye gather? A
“typical” human eye has a pupil that is about 0.5 cm in
diameter when fully dilated at night.
– Area of the pupil =  r2 =  (0.25 cm)2 = about 0.2
cm2.
– The main purpose of the telescope is to take light
from a much larger area and “funnel” it into your pupil.
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• How much light can a telescope gather?
• A 10 inch diameter scope (25 cm
diameter) gathers (12.5cm)2 = 490 cm2.
• This is 490 cm2 / 0.2cm2 = almost 2500
times more light than the naked eye.
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• To compare a telescope’s LGP to that of a
“typical” eye, use the formula
LGP = 4D2
where D is the telescope’s lens/mirror
diameter in centimeters. (2.54 cm/inch)
• What is the LGP of a 6 inch telescope?
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• Seeing Small Details – Resolution
– Resolution is defined as the minimum angle
between 2 objects, that will allow you to see
them as 2 separate objects and not one big
blob.
– Units are arcseconds (1/3600th of a degree)
– The smaller the theoretical resolution number
is, the smaller the details you can see.
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• Theoretical Resolution ()=
(2.1x105)(wavelength in m)
(diameter of objective mirror or lens)
• The diameter is in meters, not inches!
• What is the resolution of a 10 inch scope for blue
light (450 nm or 4.5 x 10-7 meters)?
• Calculate the resolution again for red light (7.0 x
10-7 meters)
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• Resolution – not the same for all light!
– What color of visible light would have the
poorest resolution? The best?
– What “color” of all the types of light would
have the poorest resolution? How is this
limitation overcome?
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• There’s a practical limit to resolution for a
ground-based telescope…the
Atmosphere!
• Air currents in the atmosphere will make the
image blurry. Think twinkling stars!
• The best time for viewing is in the hours before
dawn, since the air currents are least.
• Are there any other accommodations that could
be made?
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• Magnification – the least important
function of a telescope
• M = focal length of the objective lens or mirror
focal length of eyepiece lens
• What is the magnification factor (power) of a
telescope with a 1000 mm focal length, using an
eyepiece with a 2.5 cm focal length?
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• My 10 inch (25 cm) Schmidt-Cassegrain
telescope has a 250 cm focal length. If I
use an eyepiece with a 1.25 cm focal
length, what is the magnification?
• If I want to increase the magnification,
should I use a 2.5 cm focal length
eyepiece, or a 0.75 cm focal length
eyepiece?
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• A bit of review:
• If you doubled the size of a telescope’s
objective mirror without making any other
changes, how would the telescope’s
properties change?
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• Why do astronomers no longer use film in
their cameras?
• Film has been replaced by CCD chips
(Charge-Coupled Device).
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Credit: rst.gsfc.nasa.gov/Intro/ccd.jpg
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The surface of a CCD
chip is divided up into
rows of rectangular
light-sensitive pixels
(picture elements).
Films have irregularly
shaped and distributed
grains of light-sensitive
chemicals.
The pixels are usually
much more sensitive
than the chemical grains.
Advantage???
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Film Emulsions
Credit: www.imx.nl/photosite/technical/Filmbasics/grainshapes.jpg
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Individual
pixel
Light-sensitive layer
(gives off electrons
when struck by light)
This
stack of
3 layers
is one
pixel.
Semi-conductor layer
(acts as an electron
filter)
Collector layer (holds
the electrons until
counted)
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Why use CCD’s instead of film?
CCD Detector
• 70% efficient
• Shorter exposures
• Resolution can be
higher (8 Mpixels
or higher)
Film
• 5% to 10% efficient
• 7 to 14 times
longer exposures
• Resolution is
limited by grain
size
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• Pictures are
• Pictures must be
available in
developed (hours
seconds.
to days)
• Pictures can be
• Digital techniques
digitally added
are possible, but
together.
more difficult.
• Initial cost is similar • Operating costs
to film but
higher.
operating costs are
much lower.
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A typical,
high-res
image
produced
by a CCD.
Credit: solarsystem.nasa.gov/multimedia/gallery/PIA02888.jpg
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• All astrophotographs are black & white.
• Photographs can be taken in color, but you
lose resolution.
• 4 pixels must be “binned” or clustered for
color photographs (1 B&W, 1 red, 1 green,
1 blue) This makes the overall pixel size 4
times bigger = lower resolution.
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1 big pixel if the photo is
taken in color
4 smaller pixels if the
photo is taken in B/W.
Better resolution.
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• So how can we see all those beautiful
“color” photographs?
NGC 2393 – The “Eskimo” Nebula
Credit: Andrew Fruchter (STScI) et al., WFPC2, HST, NASA
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• We take 4 pictures in succession then
combine them into a single image:
– one photo through a red filter.
– one photo through a green filter.
– one photo through a blue filter.
– one photo in B/W (often called a Luminance
filter) for overall brightness levels.
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M57 Ring Nebula taken through red, green, and blue filters.
Notice the different details which come out.
Credit: Chris Brown, University of Manitoba
The composite
color photo.
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• Telescopes which “see” at other wavelengths
than visible light.
• Not all objects are visible at optical wavelengths
(400 – 700 nm).
• Many hot objects are only visible at shorter
wavelengths (UV, X-rays, -rays)
• Many cool objects are only visible at longer
wavelengths (IR, microwaves, radio waves)
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• Radio Telescopes
– Detect cool gases: H+ H H2
– Can detect molecules out in space:
•
•
•
•
•
oxygen O2,
carbon dioxide CO2
hydrogen cyanide HCN
formaldehyde H2CO
Ethanol CH3COOH
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• Advantages & Problems
• Operate night or day
• Atmosphere doesn’t absorb radio waves
• Poorest resolution of any type of light
(doesn’t see details well)
• Solution is to make antennas (dishes)
VERY large
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The Arecibo Radio Telescope, Puerto Rico.
Credit: National Radio Astronomy Observatory
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The Green Bank Telescope
(GBT) in Green Bank, W.Va.
The largest steerable dish in
the world. As tall as the
Statue of Liberty, the dish
would hold the building
you’re in.
Credit: National Radio Astronomy Observatory
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The Very Large Array (VLA), Socorro, N.M.
Credit: National Radio Astronomy Observatory
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• Infrared Telescopes
– Very similar to visible wavelength telescopes,
except for the detector, called a bolometer.
– IR scopes detect heat from warm gas or warm
objects. “Warm” means not hot enough to
glow in visible light.
– These scopes must be kept very cold or the
heat that the ‘scope itself radiates will swamp
out what is being observed.
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• What kinds of objects do IR telescopes
observe?
• IR telescopes “see” molecules & dust. In
some cases, they can look through cooler
dust to see what’s inside the dust clouds!
• Since stars form where there’s lots of dust,
these ‘scopes are used for for looking
inside dusty nebulas where new stars
form.
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Star-forming regions around Orion, in visible and IR
Credit: Akira Fujii / NASA
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The Spitzer Space Telescope,
part of the “Great Telescope” Series
Credit: NASA/JPL-Caltech
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The Sombrero Galaxy
(in Leo) in IR and
in visible light.
Credit: JPL / NASA (top)
Credit: NASA/ESA and
The Hubble Heritage Team
STScI/AURA)
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• Ultraviolet Telescopes
– Look for hot, young stars.
– These stars help us better define star-forming
regions, which contributes to a better
understanding of the evolution of our galaxy.
– They also look for hot, distant galaxies, as
they looked in the early universe.
– What famous ‘scope is also a UV telescope?
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GALEX Telescope (Galaxy Evolution Explorer)
Credit: JPL / NASA
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Galaxy NGC 300 in Sculptor Constellation,
7 million light years away Credit: NASA/JPL-Caltech/Las Campanas
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• X-ray and Gamma Ray Telescopes
• “See” very hot objects:
– Black Holes
– Pulsars & Neutron Stars
– Supernovas
• VERY good resolution – great ability
to observe fine details
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Core of the Elliptical Galaxy NGC 4261 (accretion
disk of a black hole.) Credit: NASA/ESA and The Hubble
Heritage Team STScI/AURA)
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Cassiopeia A - the remnant of a supernova which
exploded about 300 years ago.
Credit: X-ray: NASA/CXC/SAO; Optical: NASA/STScI; Infrared: NASA/JPL-Caltech
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The Chandra X-ray Telescope, part of the ‘Great Telescopes’
series. Credit: chandra.nasa.gov (artist’s conception)
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A gamma ray burst beginning.
Credit: NASA (artist’s conception)
The GLAST ( Gamma-ray Large Area Space
Telescope, renamed FERMI )
Credit: General Dynamics for NASA (artist’s conception)