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Optics and Telescopes

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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? – PowerPoint PPT presentation

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Title: Optics and Telescopes


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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
    f-ratio, 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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  • Theres always a trade-off in optics. The
    problem of chromatic aberration is worst with
    fast or low f-ratio lenses. These are the
    lenses wed like to use most!
  • The problem is fixed by making compound lenses
    out of 2 or more different kinds of glass.
  • Mirror-based telescopes dont 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 1500s 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 TelescopesAdvantages
  • 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 TelescopesDisadvantages
  • 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 scopesthe 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 doesnt 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 doesnt
    appear to rotate over time.

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  • What is the function of a telescope? Its 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 smaller, but very bright image?

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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 telescopes LGP to that of a
    typical eye, use the formula
  • LGP 4D2
  • where D is the telescopes 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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  • Theres a practical limit to resolution for a
    ground-based telescopethe 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 telescopes
    objective mirror without making any other
    changes, how would the telescopes 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) Semi-conductor layer (acts as
an electron filter) Collector layer (holds the
electrons until counted)
This stack of 3 layers is one pixel.
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Why use CCDs 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 available in seconds.
  • Pictures can be digitally added together.
  • Initial cost is similar to film but operating
    costs are much lower.
  • Pictures must be developed (hours to days)
  • Digital techniques are possible, but more
    difficult.
  • Operating costs higher.

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A typical, high-res image produced by a CCD.
Credit solarsystem.nasa.gov/multimedia/gallery/PI
A02888.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 BW, 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 doesnt absorb radio waves
  • Poorest resolution of any type of light (doesnt
    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 youre 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
    whats inside the dust clouds!
  • Since stars form where theres 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
IRCredit Akira Fujii / NASA
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The Spitzer Space Telescope, part of the Great
Telescope SeriesCredit 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
(artists conception)
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A gamma ray burst beginning.Credit NASA
(artists conception)
The GLAST ( Gamma-ray Large Area
Space Telescope, renamed FERMI )Credit General
Dynamics for NASA (artists conception)
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