Transcript Document

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Chapter 4 “Telescopes”
John Swez
Instructor
Physics 360/Geol 360
1.
This page was copied from Nick Strobel's Astronomy Notes. Go to his site at www.astronomynotes.com for the updated
and corrected version
1
Men and women have looked up at the sky and wondered about the things they see
up there for as long as humans have lived on our Earth. Long ago, the Sun and Moon
were mysterious objects that could be seen in the day and night. But the planets and
stars were even more mysterious probably because they are so far away that we
could only see them as points of light. Unlike the things on the Earth that we can
study up close, handle, listen to, smell, and taste, the only thing ancient watchers of
the sky had to learn about things in space was their eyes and imaginations. Only very
recently in the history of humanity have astronomers been able to extend the reach of
our eyes (and our imaginations!).
Galileo pioneered modern explorations in the early 1600's by using a device originally invented for
naval operations to explore the heavens. The device he used, of course, was the telescope, an
instrument used to gather and focus light. Our atmosphere prevents most of the electromagnetic
radiation from reaching the ground, allowing just the visible band, parts of the radio band, and small
fractions of the infrared and ultraviolet through. Our eyes can detect the visible (optical) band, so the
early telescopes were all built to observe in that part of the electromagnetic spectrum. It wasn't until
the 1930's that astronomers began observing with another part of the electromagnetic spectrum---the
radio band. The development of space technology has enabled astronomers to put telescopes above the
atmosphere and explore all of those places out there using the full range of the electromagnetic
spectrum
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Tip of the Day: (1) How sunrise to sunset is defined. Sunrise is time from just when the top of the
sun clears the horizon to sunset when the last bit of sun disappears.
(2) Astronomy Magazine Sept. 2002 issue defines the faintest naked eye star at 6.5 apparent
magnitude.
“Apparent Magnitude” was defined by Hipparachus in 150 BC. He devised a
magnitude scale based on:
However, he underestimated
the magnitudes. Therefore,
many very bright stars today
have negative magnitudes.
Magnitude
Constellation
1
(Orion)
2
Big Dipper
6
Star
Betelgeuse
various
stars just barely seen
Magnitude Difference is based on the idea that the difference between the
magnitude of a first magnitude star to a 6th magnitude star is a factor of 100.
Thus a 1st mag star is 100 times brighter than a 6th mag star. This represents a
range of 5 so that 2.512 = the fifth root of 100. Thus the table hierarchy is the
following.
Absolute Magnitude is defined
Magnitude Difference of 1 is 2.512:1, 2 is
2.5122:1 or 6.31:1, 3 is 2.5123 =
15.85:1 etc.
as how bright a star would appear
if it were of certain apparent
magnitude but only 10 parsecs
distance.
3
The Physics of Light
Later, Diffraction will
have a direct link to
resolving power
Left: Picture depicts
the Relationship of the
Intensity versus the
inverse square of the
distance
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Chapter 4, Telescopes
Ability to Focus
Bending of Light
Index of Refraction
( Dependent)
Collecting Power
How Bright!
Depends on
Collector Area
Resolving Power
Two Objects Close
(Ability to Discern)
Depends on Quality
of Collector Area
Magnification
Image Size/Object Size
Related Concepts
Atmospheric Refraction
The Moon Illusion (page 122, text)
Alteration of the Sunset/Sunrise Time hence the equinox
(SAME PAGE)
5
More on the Physics of Light
An example of the “second order bending of light” Left
Credit for photo on lower left
http://www.glenbrook.k12.il.us/gbssci/phys/
Class/light/u12l1a.html
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
7
This slide and is
copied verbatim from
from the SommersBausch Observatory's
"APAS 1010
Laboratories Introduction to
Astronomy" lab
manual, 1996, by
Keith Gleason. Via
website
http://lyra.colorado.ed
u/sbo/astroinfo/coords/
coordinates.html
Angular Measure is Important in Astronomy
“In order to specify a direction by angular measure, you need to know just how
"big" angles are. Here's a convenient "yardstick" to use that you carry with you at
all times: the hand, held at arm's length, is a convenient tool for estimating angles
subtended at the eye:” It is convenient to remember that the width of your
knuckles when the arm is extended out is about 8 degrees. Remember, there are
360 degrees to a full circle.
8
Basics of how a simple refracting telescope works
A simple refracting two lens telescope (right) showing aperture objective
and eyepiece. (left and below) A diagram depicting chromatic aberration
Images courtesy of Nick Strobel's Astronomy Notes. Go to his site at
www.astronomynotes.com for the updated and corrected
version.
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.
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A classical Cassegrain reflecting telescope.
(Image by Duncan Kopernicki.)
In the classical Cassegrain telescope the primary mirror takes a paraboloid
shape. This 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. This is used on
big telescopes to take pictures of small areas of the sky. This used to be
done using photographic plates but these have largely been replaced by
more efficient digital detectors, called Charge Coupled Devices (CCDs).
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Basic Type of Telescopes
Basic Diagram of Schmidt-Cassegrain Technology
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.)
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Resolving Power
•A telescope’s ability to resolve two objects (stars) close to
each other
•Is limited by the nature of wave light (Diffraction)
•Two points separated by an angle  (measured in seconds)
cannot be observed as separate sources unless D > 0.02 /
where D is the telescope diameter in centimeters,  is the
wavelength of light in nanometers and  is the angle of
separation (seconds) [Equation on page 128, text]
• Example: to resolve two stars separated by 0.1 seconds of
arc when observing with visible light you need a 1 meter
diameter telescope**
•** Unfortunately the atmosphere seriously blurs fine details
degrading the resolving power to earth based telescopes to
below their diffraction limits
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Mathematical Expression for Resolving Power
D is expressed in centimeters (cm) of the
aperture.
Example: Problem 1, page 143. Compare the collecting power of a telescope
with a 10 cm (about 4 inch) diameter mirror to that of a human eye. (Take the
diameter of the pupil of the eye to be about 5 millimeter)
Solution. Part (a). Telescope. Solve the above equation for  to get  = 0.02  / D.
Then substituting in the numbers solve for (use  = 500 nm)  = 0.02 (500) / 10 = 1
second of angular separation.
For Part (b). Eye. Again, solve the same equation.  = 0.02 500 / .5 = 20 seconds
of angular separation. Use this result to also solve Problem 3.
14
The pictures clearly show
the increase in sharpness
as the objective size is
increased. The size of each
of the blobs is the size of
the smallest detail that can
be seen with that telescope
under ideal conditions.
Atmospheric distortion
effects (smearing of the
binary star images to a
blob the size of the entire
frame) and obscuration
and diffraction by the
secondary and its supports
are NOT shown here.
Figure and Text from
http://www.astronomyn
otes.com/ Nick
Strobel’s Astronomy
Notes
15
Collecting Power
“The area of the objective is the determining factor. Since most telescope
objectives are circular, the area = p × (diameter of objective)2/4, where the
value of p is approximately 3.1416. For example: a 40-centimeter mirror
has four times the light-gathering power as a 20-centimeter mirror [(
p402/4) / ( p202/4) = (40/20)2 = 4]. “
Figure and Text from http://www.astronomynotes.com/ Nick Strobel’s
Astronomy Notes
16
Magnifying Power (not discussed in detail in text)
“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. “
Figure and Text from http://www.astronomynotes.com/ Nick
Strobel’s Astronomy Notes
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Why Reflecting Telescopes are Preferred over Refracting
• A large mirror can be thin but a large lens must be thicker
thus heavier.
• A lens has two surfaces that must be cleaned and polished;
a mirror only has one;.
• Glass absorbs light! The thicker the light the more
absorption.
• Lenses need to be supported only around the outside;
mirrors can be supported by the back
• For large lenses, glass deforms under its own weight; thus
changing the lenses’ properties.
• In a lens, different colors are refracted by different
amounts. (Chromatic Aberrations). Lenses are corrected
for chromatic aberrations and are called achromats.
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Recording Images
•For many years the naked eye was used; sketches were
produced
•Photographic Film became in use about the turn of the
last century
•Low efficiencies occur with photographic film (~ 4%,
thus much patience must be spent with clock drive
mechanisms)
•CCD (Charge coupled detector arrays) are used today
with efficiencies of 75%
•CCD’s are used in digital cameras today
19
Advances in Observing
Observing in the Infrared, UV, Gamma Rays and Radio
Waves
The Hubble Space Telescope (public pictures at)
http://oposite.stsci.edu/pubinfo/pictures.html
The Chandra X-ray Observing Telescope
We (students and teachers) can observe
The personal computer
Image Processing
Interferometer Telescopes (resolution is not set by the size
of the individual mirrors but by their distance of
separation (the 100 x 100 rule); exp. Twin Keck
telescopes
20
http://oposite.stsci.edu/pubinfo/pictures.html
21
The black hole in
globular cluster M15
[left] is 4,000 times
more massive than
our Sun. G1 [right], a
much larger globular
cluster, harbors a
heftier black hole,
about 20,000 times
more massive than
our Sun.
Hubble Discovers Black Holes in Unexpected
Places
22
Stargazers Pub at http://www.stargazers-pub.net gives a
very nice treatment of telescopes; especially if you are
interested in purchasing one.
•If you're thinking of buying a telescope, the best way to choose one is
to go to a local astronomy club meeting or star party. Most clubs have
public viewing evenings every month, and these are most helpful to the
interested newbie. Nothing beats actual experience with a variety of scopes
when you're trying to decide what to spend your money on.
•Try to stay away from 'department store' telescopes. You know, the ones
you find a the local SUPERSTORE (I'm not going to name names, but we all
know the kinda stores I'm talking about..). They usually come in brilliantly
colored boxes with amazing pictures of Saturn and the Andromeda Galaxy
on the top and claim to be able to magnify your views by 500x or more.
They might look nice on the shelf, but do a little more research into
telescope buying & optics before you shell out for one of these snoozers.
You're *MUCH* better off saving your money for another couple months
and buying a scope from a reputable astronomical company, such as Orion,
Celestron, or Meade.
•Etc.
23
10-meter Keck Telescope at the W.M. Keck Observatory.
1.
This page was copied from Nick Strobel's Astronomy Notes. Go to his site at
www.astronomynotes.com for the updated and corrected version.
24
This page was copied from Nick Strobel's Astronomy Notes. Go to his site at www.astronomynotes.com for
the updated and corrected version.
25
The Very Long Baseline Array is a huge interferometer that uses ten telescopes placed in sites from Hawaii to
the Virgin Islands. This telescope is the 8,600 kilometers across and has a resolution as good as 0.0002 arc
second! With a resolution about 50 times better than the Hubble Space Telescope, it is able to detect features as
small as the inner solar system at the center of our galaxy, about 26,000 light years away.
This page was copied from Nick Strobel's Astronomy Notes. Go to his site at www.astronomynotes.com
for the updated and corrected version.
26
Radio Telescope Image (Top) and Visible Image (below)
1.
This page was copied from Nick Strobel's Astronomy Notes. Go to his site at
www.astronomynotes.com for the updated and corrected version.
27
“The Hubble Space Telescope orbits far above the distorting effects of the
atmosphere, about 600 kilometers above the Earth. This perch gives
astronomers with their clearest view ever, but it also prevents them from
looking directly through the telescope. Instead, astronomers use Hubble's
scientific instruments as their electronic eyes.” Upper Left: Closer View
Photo and text courtesy of http://hubble.nasa.gov/
Hubble Telescope with corrective optics
28
M 100 a few days before (left) and after (right) the corrective optics (COSTAR) were installed in December 1993.
1.
This page was copied from Nick Strobel's Astronomy Notes. Go to his site at
www.astronomynotes.com for the updated and corrected version.
29
Credit for picture and text: NASA
“This color image of Saturn was taken with the HST's
Wide Field and Planetary Camera (WF/PC) in the wide
field mode at 8:25 A.M. EDT, August 26, 1990, when
the planet was at a distance of 1.39 billion kilometers
(860 million miles) from Earth.”
30
Courtesy for
picture and text:
NASA
“This enlargement of the Saturn image reveals unprecedented
detail in atmospheric features at the northern polar hood.
Saturn's north pole is presently tilted toward Earth by 24
degrees”
31
Build a Hand Held Hubble: http://hubblesite.org/fun_.and._games/handheld_hubble/materials.shtml
Photo and
text courtesy
of
http://hubble.
nasa.gov/
NASA's Hubble Space Telescope has obtained the clearest pictures
ever of our solar system's most distant and enigmatic object: the
planet Pluto. The observations were made with the European
Space Agency's Faint Object Camera.
32
View of a colliding galaxy dubbed the "Tadpole" (UGC10214):
Photo Courtesy NASA Hubble
33
Astronomy 360
The slides on celestial coordinates
may be covered at a later date.
34
From Astronomy
Notes on Web;
Ref. Noted below.
Learning Celestial Coordinates: Part I
Figure from:
http://csep10.phys.utk.edu/astr161/lect/time/coordinates.html
Study pages 65
– 67 in your
text (Thomas T
Amy); in
particular the
section on
“Celestial
Coordinates”
“In the celestial
coordinate system
the North and
South Celestial
Poles are
determined by
projecting the
rotation axis of
the Earth to
intersect the
celestial sphere,
which in turn
defines a Celestial
Equator. “ **
35
Celestial
Coordinates: Cont.
“The celestial equivalent of latitude is called declination and is measured in degrees North
(positive numbers) or South (negative numbers) of the Celestial Equator. The celestial
equivalent of longitude is called right ascension. Right ascension can be measured in
degrees, but for historical reasons it is more common to measure it in time (hours,
minutes, seconds): the sky turns 360 degrees in 24 hours and therefore it must turn 15
degrees every hour; thus, 1 hour of right ascension is equivalent to 15 degrees of
(apparent) sky rotation. “ from
http://csep10.phys.utk.edu/astr161/lect/time/coordinates.html
36
Celestial Coordinates
This slide and the next six slides are
copied verbatim from from the SommersBausch Observatory's "APAS 1010
Laboratories - Introduction to
Astronomy" lab manual, 1996, by Keith
Gleason. Via website
http://lyra.colorado.edu/sbo/astroinfo/coor
ds/coordinates.html
“The alt-azimuth (altitude - azimuth) coordinate system, also called the horizon system, is a useful and
convenient system for pointing out a celestial object. One first specifies the azimuth angle, which is the
compass heading towards the horizon point lying directly below the object. Azimuth angles are measured
eastwardly from North (0 deg azimuth) to East (90 deg), South (180 deg), West (270 deg), and back to North
again (360 deg = 0 deg). The four principle directions are called the cardinal points. Next, the altitude is
measured in degrees upward from the horizon to the object. The point directly overhead at 90 deg altitude is
called the zenith. The nadir is "down", or opposite the zenith. “
37
More Important: The
Equatorial Coordinate
System
Measurement of
"celestial latitude" is
given the name
declination (DEC),
“ If we extend the Earth's axis outward into space, its intersection with the celestial sphere
defines the north and south celestial poles; equidistant between them, and lying directly over the
Earth's equator, is the celestial equator. Measurement of "celestial latitude" is given the name
declination (DEC), but is otherwise identical to the measurement of latitude on the Earth: the
declination at the celestial equator is 0 deg and extends to ±90 deg at the celestial poles. “
38
“The ecliptic crosses the equator at two
points; the first, called the vernal
(spring) equinox, is crossed by the
Sun moving from south to north on
about March 21st, and sets the moment
when spring begins. The second
crossing is from north to south, and
marks the autumnal equinox six
months later. Halfway between these
two points, the ecliptic rises to its
maximum declination of +23.5 deg
(summer solstice), or drops to a
minimum declination of -23.5 deg
(winter solstice). “
“The east-west measure is called right ascension (RA) rather than "celestial longitude",
and differs from geographic longitude in two respects. First, the longitude lines, or hour
circles, remain fixed with respect to the sky and do not rotate with the Earth. Second, the
right ascension circle is divided into time units of 24 hours rather than in degrees; each
hour of angle is equivalent to 15 deg of arc
The Earth orbits the Sun in a plane called the ecliptic. From our vantage point, however,
it appears that the Sun circles us once a year in that same plane; hence, the ecliptic may be
alternately defined as "the apparent path of the Sun on the celestial sphere".
39
“The fundamental purpose of all
timekeeping is, very simply, to enable
us to keep track of certain objects in the
sky. Our foremost interest, of course, is
with the location of the Sun, which is
the basis for the various types of solar
time by which we schedule our lives. “
“As with longitude, there is no obvious starting point for right ascension, so astronomers have assigned one:
the point of the vernal equinox. Starting from the vernal equinox, right ascension increases in an eastwardly
direction until it returns to the vernal equinox again at 24 h = 0 h.
The Earth precesses, or wobbles on its axis, once every 26,000 years. Unfortunately, this means that the Sun
crosses the celestial equator at a slightly different point every year, so that our "fixed" starting point changes
slowly - about 40 arc-seconds per year. Although small, the shift is cumulative, so that it is important when
referring to the right ascension and declination of an object to also specify the epoch, or year in which the
coordinates are valid. “
40
This slide and the previous six slides are copied
verbatim from from the Sommers-Bausch
Observatory's "APAS 1010 Laboratories Introduction to Astronomy" lab manual, 1996,
by Keith Gleason. Via website
http://lyra.colorado.edu/sbo/astroinfo/coords/co
ordinates.html
“Time is determined by the hour angle of the celestial object of interest, which is the angular distance from
the observer's meridian (north-south line passing overhead) to the object, measured in time units east or west
along the equatorial grid. The hour angle is negative if we measure from the meridian eastward to the object,
and positive if the object is west of the meridian.
For example, our local apparent solar time is is determined by the hour angle of the Sun, which tells us how
long it has been since the Sun was last on the meridian (positive hour angle), or how long we must wait until
noon occurs again (negative hour angle).
If solar time gives us the hour angle of the Sun, then sideral time (literally, "star time") must be related to the
hour angles of the stars: the general expression for sidereal time is Sidereal Time = Right Ascension + Hour
Angle which holds true for any object or point on the celestial sphere. It's important to realize that if the hour
angle is negative, we add this negative number, which is equivalent to subtracting the positive number. “