Transcript Chapter 4

Chapter 5
Telescopes modified for Geology
360/Physics 380
Copyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Tools of the Trade: Telescopes
• Stars and other celestial objects are too far away to
test directly
– Astronomers passively collect radiation emitted from
distant objects
– Extremely faint objects make collection of radiation
difficult
• Specialized Instruments Required
– Need to measure brightness, spectra, and positions with
high precision
– Astronomers use mirrored telescopes and observatories
• Modern Astronomers are rarely at the eyepiece,
more often they are at a computer terminal!
The Powers of a Telescope
• Collecting Power
– Bigger telescope, more
light collected!
• Focusing Power
– Use mirrors or lenses to
bend the path of light
rays to create images
• Resolving Power
– Picking out the details
in an image
Anatomy of the Human Eye
Leads to the
occipital
cortex at the
posterior
(back) of the
brain
The Macula
The macula lutea is the small, yellowish
central portion of the retina. It is the area
providing the clearest, most distinct
vision. When one looks directly at something,
the light from that object forms an image on
one’s macula. A healthy macula ordinarily is
capable of achieving at least 20/201 (“normal”)
vision or visual acuity, even if this is with a
correction in glasses or contact lenses.
1 Vision sharper than this sharp may be due to there being more cones
per square millimeter of the macula than in the average eye, enabling that
eye to distinguish much greater detail than normal.
fovea centralis
The center of the macula is called the fovea
centralis, an area where all of the photoreceptors
are cones; there are no rods in the fovea. The
fovea is the point of sharpest, most acute visual
acuity. The very center of the fovea is the “foveola.”
Because the fovea has no rods, small dim objects in
the dark cannot be seen if one looks directly at
them. For instance, to detect faint stars in the sky,
one must look just to one side of them so that their
light falls on a retinal area, containing numerous
rods, outside of the macular zone. Rods detect dim
light, as well as movement.
The astronomical telescope
The Refractor
Light Gathering Power
• Light collected
proportional to
“collector” area
– Pupil for the eye
– Mirror or lens for a
telescope
• Telescope “funnels”
light to our eyes for a
brighter image
• Small changes in
“collector” radius give
large change in number
of photons caught
• Telescopes described by lens
or mirror diameter (inches)
Focusing Power
• Refraction
– Light moving at an
angle from one
material to another
will bend due to a
process called
refraction
– Refraction occurs
because the speed of
light is different in
different materials
Refraction
Refraction
• Refraction is also
responsible for
seeing
– Twinkling of stars
– AKA
Scintillation
• Temperature and
density
differences in
pockets of air shift
the image of the
star
Atmospheric Refraction
Distortion of Sun near the
Horizon
6
How your perception may be fooled.
Both circles in the sky and
the bottom circle look
smaller than the circle on
the horizon.
Indeed
all the
circles
are the
same
size!
From Explorations An Introduction to Astronomy 3rd ed, Thomas Arny p 123
Refracting Telescopes
• A lens employs
refraction to bend
light
• Telescopes that
employ lenses to
collect and focus
light are called
refractors
Disadvantages to Refractors
• Lenses have many disadvantages in
large telescopes!
– Large lenses are extremely expensive to
fabricate
– A large lens will sag in the center since it
can only be supported on the edges
– Dispersion causes images to have colored
fringes
– Many lens materials absorb shortwavelength light
Reflecting Telescopes
• Reflectors
– Used almost exclusively
by astronomers today
– Twin Keck telescopes,
located on the 14,000
foot volcanic peak
Mauna Kea in Hawaii,
have 10-meter collector
mirrors!
– Light is focused in front
of the mirror
Reflecting Telescopes
• A secondary mirror may
be used to deflect the
light to the side or
through a hole in the
primary mirror
• Multi-mirror instruments
and extremely thin
mirrors are two modern
approaches to dealing
with large pieces of glass
in a telescope system
Styles of Refractors
11
Basic Type of Telescopes
Basic Diagram of Schmidt-Cassegrain Technology
9
A classical Newtonian reflecting telescope.
(Image by Duncan Kopernicki.)
Small reflectors are often in a Newtonian configuration (shown above).
They have a paraboloid primary mirror which brings the light of any object
in the field of the telescope to a focus near the top end of the tube, called
the prime focus. A flat mirror is placed at 45 to the axis of the tube and
reflects the light out to an eyepiece at the secondary focus.
Resolving Power
• A telescope’s ability to
discern detail is referred to as
its resolving power
• Resolving power is limited by
the wave nature of light
through a phenomenon called
diffraction
• Waves are diffracted as they
pass through narrow
openings
• A diffracted point source of
light appears as a point
surrounded by rings of light
Resolving Power and Aperture
• Two points of light separated by an angle a (in
arcsec) can be seen at a wavelength l (in nm) only
if the telescope diameter D (in cm) satisfies:
D > 0.02 l/a
What causes diffraction in telescopes?
From Hyperphysics web site
http://hyperphysics.phy-astr.gsu.edu/hbase/HFrame.html
Text Problem in Resolving Power
Problem at the End of Chapter 4, Resolving Power
Magnifying Power (not discussed in detail in text)
This is only for your information—not in exams
“The ability of a telescope to enlarge images is the best-known feature
of a telescope. Though it is so well-known, the magnifying power is
the least important power of a telescope because it enlarges any
distortions due to the telescope and atmosphere. A small, fuzzy faint
blob becomes only a big, fuzzy blob. Also, the light becomes more
spread out under higher magnification so the image appears fainter!
The magnifying power = (focal length of objective) / (focal length of
eyepiece); both focal lengths must be in the same length units. A rough
rule for the maximum magnification to use on your telescope is 20 × D
to 24 × D, where the objective diameter D is measured in centimeters.
So an observer with a 15-centimeter telescope should not use
magnification higher than about 24 × 15 = 360-power. “
from http://www.astronomynotes.com/ Nick Strobel’s
Astronomy Notes
Increasing Resolving Power:
Interferometers
– For a given
wavelength, resolution
is increased for a larger
telescope diameter
– An interferometer
accomplishes this by
simultaneously
combining
observations from two
or more widely-spaced
telescopes
Interferometers
• The resolution is
determined by the
individual telescope
separations and not the
individual diameters of the
telescopes themselves
• Key to the process is the
wave nature of
interference and the
electronic processing of
the waves from the various
telescopes
Observatories
• The immense telescopes and their associated equipment
require observatories to facilitate their use and
protection from the elements
• Thousands of observatories are scattered throughout the
world and are on every continent including Antarctica
• Some observatories:
– Twin 10-meter Keck telescopes are largest in U.S.
– The Hobby-Eberly Telescope uses 91 1-meter mirrors set in an
11-meter disk
– Largest optical telescope, VLT (Very Large Telescope) in
Chile, is an array of four 8-meter mirrors
The Schmidt Telescope
11a
For photography of large areas
of the sky the primary mirror is
made with spherical curvature
and an aspheric `corrector
plate' is placed at the top end
of the telescope tube. There
are three large Schmidt
telescopes in the world with
fields about 6° across (the
Moon's apparent diameter in
the sky is half a degree). The
oldest of these is the Palomar
Schmidt (not to be confused
with the Palomar 200-inch) and
the other two are the ESO
Schmidt in Chile and the
United Kingdom Schmidt in
Australia. These have been
used to produce photographic
charts of the whole sky.
The Horsehead Nebula in Orion. This image, approximately 1.5° across, was
obtained with the UK Schmidt telescope at the Anglo-Australian Observatory.
(Image Credit: David Malin, Anglo Australian Observatory/Royal Observatory
Edinburgh.)
The “OWL” Telescope
Detecting the Light
• The Human Eye
– Once used with a telescope to record observations or make
sketches
– Not good at detecting faint light, even with the 10-meter Keck
telescopes
• Photographic Film
– Chemically stores data to increase sensitivity to dim light
– Very inefficient: Only 4% of striking photons recorded on
film
• Electronic Detectors
– Incoming photons strike an array of semiconductor pixels that
are coupled to a computer
– Efficiencies of 75% possible
– CCD (Charged-coupled Device) for pictures
Nonvisible Wavelengths
• Many astronomical
objects radiate in
wavelengths other
visible
– Cold gas clouds
radiate in the radio
– Dust clouds radiate
in the infrared
– Hot gases around
black holes emit
x-rays
Radio Observatories
Radio Observations
• False color images are typically used to
depict wavelength distributions in nonvisible observations
Gamma Rays Bursts
• Exploring New Wavelengths: Gamma Rays
– Gamma-ray astronomy began in 1965
– By 1970s, gamma rays found to be coming from
familiar objects: Milky Way center and remnants
of exploded stars
– 1967 gamma-ray bursts from space discovered by
military satellites watching for Soviet nuclear
bomb explosions
– Source of gamma-ray bursts is likely due to
colliding neutron stars!
The Crab Nebula
– In A.D. 1054, ancient Chinese noticed bright “new
star” in the sky, which then faded from view in just
over a year
– In 1731, with the help of a telescope, a fuzzy patch
was discovered in the area of the former “new star”
– In 1844, filaments were noticed that gave the fuzzy
nebula the appearance of a crab
– In 1921, comparison of photographs lead to idea that
the nebula was expanding
– By 1928, it was realized that the ancient Chinese had
observed a supernova explosion – the death of a
massive star – and the nebula was the result
Crab Nebula catalogue designations
The Crab Nebula (catalogue designations M1, NGC 1952, Taurus A) is a
supernova remnant and pulsar wind nebula in the constellation of Taurus.
The nebula was first observed by John Bevis, and corresponds to a bright
supernova recorded by Chinese and Arab astronomers in 1054. Located at
a distance of about 6,500 light-years (2 kpc) from Earth, the nebula has a
diameter of 11 ly (3.4 pc) and expands at a rate of about 1,500 kilometers
per second.
At the center of the nebula lies the Crab Pulsar, a rotating neutron star,
which emits pulses of radiation from gamma rays to radio waves with a
spin rate of 30.2 times per second. The nebula was the first astronomical
object identified with a historical supernova explosion.
The nebula acts as a source of radiation for studying celestial bodies that
occult it. In the 1950s and 1960s, the Sun's corona was mapped from
observations of the Crab's radio waves passing through it, and more
recently, the thickness of the atmosphere of Saturn's moon Titan was
measured as it blocked out X-rays from the nebula.
Observations of the Crab Nebula
• Since 1928, Crab has been
investigated at all
wavelengths:
– Powerful source of radio
waves
– Further radio observations
revealed the remnant of the
supernova explosion – a
rapidly spinning “star” (30
times per second)
– Radio waves also indicated
that charged particles are
moving at near the speed of
light
– Visible light indicates
expansion of nebula at
about 1000 km/s
– Source of x-rays
Major Space Observatories
• Why put them in space?
Atmospheric Blurring
– Twinkling of stars in sky,
called scintillation, is
caused by moving
atmospheric irregularities
refracting star light into a
blend of paths to the eye
– The condition of the sky
for viewing is referred to
as the seeing
– Distorted seeing can be
improved by adaptive
optics, which employs a
powerful laser and
correcting mirrors to offset
scintillation
Light Pollution
Space vs.Ground-Based Observatories
• Space-Based Advantages
– Freedom from atmospheric blurring
– Freedom of atmospheric absorption
• Ground-Based Advantages
– Larger collecting power
– Equipment easily fixed
• Ground-Based Considerations
– Weather, humidity, and haze
– Light pollution
Telescopes Summary
Ability to Focus
Bending of Light
Index of Refraction
( Dependent)
Collecting Power
How Bright!
Depends on
Collector Area
Resolving Power
Two Objects Close Depends on size
(Ability to Discern) of collector
area
and quality
Magnification
Image Size/Object Size
Related Concepts
Atmospheric Refraction
and distortion
The Moon Illusion
Going Observing
• To observe at a major observatory, an astronomer must:
– Submit a proposal to a committee that allocates telescope time
– If given observing time, assure all necessary equipment and
materials will be available
– Be prepared to observe at various hours of the day
• Astronomers may also “observe” via the Internet
– Large data archives now exist for investigations covering
certain wavelengths sometimes for the entire sky
– Archives help better prepare astronomers for onsite
observations at an observatory
Computers and Astronomy
• For many astronomers, operating a computer
and being able to program are more important
than knowing how to use a telescope
• Computers accomplish several tasks:
– Solve equations
– Move telescopes and
feed information to detectors
– Convert data into useful form
– Networks for communication
and data exchange