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2nd class in PHS 207-02 & -S2 Astronomy Astronomical Tools
Views of Orion nebula at 3 different wavelength ranges
from http://www.rssd.esa.int/SD/INTEGRAL/docs/Publications/Science_In_School_issue20.pdf
OBJECTIVES
What you learn about in this second class entitled astronomical tool are:
 The electromagnetic (EM) spectrum and what part it plays in our understanding of the
universe
 The detection devices such as telescopes at visible and radio frequencies as well as others
and satellites
 The specific parts and applications of the optical telescope
 The limitations of these detection devices including our eyes and
 How digital technology is reducing our limitations
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 Satellite images
 Building your own telescope
 NAAP telescope
simulator http://astro.unl.edu/classaction/animations/telescopes/telescope10.html

Introduction
In this second class we are studying ways by which we detect and study the universe beyond us.
Until robots landed or satellites encircled a heavenly object, practically the ONLY way (besides
meteorites) has been by the signals an object emitted or reflected in the electromagnetic
spectrum.
EM SPECTRUM
The electromagnetic spectrum shown in figure 2.4 on page 34 is much more than the visible light
we see. The higher frequency the faster the wave moves and the higher power required to
generate that wave.
This pictorial shows that we can see that only visible light, sub-millimeter, and radio waves
penetrate our atmosphere and reach ground level. This is the principal reason for our satellites to
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detect what we can not and for optical and other telescopes high in the mountains where the
atmosphere is thinner and hopefully less polluted.
A short video em spectrum http://imagine.gsfc.nasa.gov/Videos/general/spectrum.mov
Short descriptions of each frequency range of the em spectrum
http://science.hq.nasa.gov/kids/imagers/teachersite/UL3waves.htm
Radio waves have the longest wavelength in the electromagnetic spectrum. These
waves carry the news, ball games, and music you listen to on the radio. They also
carry signals to television sets and cellular phones. Your cell phones operate around
3.9 GHz in the radio and microwave spectrum.
Microwaves have shorter wavelengths than radio waves, which heat the food we eat.
They are also used for radar images, like the Doppler radar used in weather forecasts.
There are infrared waves with long wavelengths and short wavelengths. Infrared
waves with long wavelengths are different from infrared waves with short
wavelengths. Infrared waves with long wavelengths can be detected as heat. Your
radiator or heater gives off these long infrared waves. We call these thermal infrared
or far infrared waves. The sun gives off infrared waves with shorter wavelengths.
Plants reflect these waves, also known as near infrared waves. . The night vision
goggles converts the infrared signals that any warm blooded animal emits into
visible light.
Visible light waves are the only electromagnetic waves we can see. We see these
waves as the colors of the rainbow. Each color has a different wavelength. Red has the
longest wavelength and violet has the shortest wavelength. These waves combine to
make white light.
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Ultraviolet waves have wavelengths shorter than visible light waves. These waves are
invisible to the human eye, but some insects can see them. Of the sun's light, the
ultraviolet waves are responsible for causing our sunburns. Ultraviolet is emitted by
very hot gases. The light in a fluorescent light starts as ultraviolet and is converted
by the fluorescent coating on the bulb into visible light. Even in this conversion
fluorescent light is much more efficient than our common incandescent bulbs.
X-Rays: As wavelengths get smaller, the waves have more energy. X-Rays have
smaller wavelengths and therefore more energy than the ultraviolet waves. X-Rays are
so powerful that they pass easily through the skin allowing doctors to look at our
bones and identify different tissues by the amount of x-ray that is absorbed.
Gamma Rays have the smallest wavelength and the most energy of the waves in the
electromagnetic spectrum. These waves are generated by radioactive atoms and in
nuclear explosions. Gamma rays can kill living cells, but doctors can use gamma rays
to kill diseased cells.
A VIDEO EXPLAINING EM SPECTRA
http://www.natgeoeducationvideo.com/film/1178/the-electromagnetic-spectrum
In a later class we will learn that more about signals in each of those portions of the
spectrum tell and about what is likely to occurring to emit a given signal than you
will see in the following video.
2:25 MINUTES FROM NATIONAL GEOGRAPHIC
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SIZE OF WAVELENGTHS THROUGH NATGEO
http://scienceblogs.com/startswithabang/files/2012/10/EM_Spectrum3-new.jpeg
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A SECOND PICTURE
http://pathstoknowledge.files.wordpress.com/20
09/05/electromagnetic_spectrum.png
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http://www.sacta.co.za/Content/images/electro_diag.jpg
Attenuation (reduction in signal) by the atmosphere
Pg 3 in
http://www.rssd.esa.int/SD/INTEGRAL/docs/Publications/Science_In_School_issue20.pdf
CHART OF SOURCE OF EM SPECTRUM
We assume the sources in the universe beyond Earth obey the
same laws as they would on our Earth. We have no reason to
doubt that this assumption is true.
EM spectrum
http://imagine.gsfc.nasa.gov/docs/science/know_l1/emspectrum.html
a fairly general explanation of the different spectrum within the electromagnetic spectrum with
good graphics
http://imagine.gsfc.nasa.gov/descriptions/spectrum.html
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energy levels within frequency range
http://imagine.gsfc.nasa.gov/docs/science/know_l1/spectrum_chart.html
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What is the highest/energy wave?
http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970412e.html
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DETECTION DEVICES – TELESCOPES &
SATELLITES
The satellites which orbit above our atmosphere can detect these signals that we can not detect or
can not detect easily on the Earth. The satellites also are not limited by the horizon and look at a
portion of the sky for many more hours than we can.
We will look at specific satellites of which the Hubble in the most well known
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That is the primary reason for satellites most cases to a level that we As the figure shows The
signals are in the electromagnetic spectrum and range from radio waves to gamma rays in terms
of frequency and power.
OPTICAL TELESCOPE
Refractory or reflectory telescopes
Measures of a telescope (all assuming the telescope is stable or not wiggling around):



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the light gathering power,
the resolving power,
the magnification of a telescope, and
field of view
Background
Notes on AMATEUR TELESCOPE OPTICS from http://www.telescope-optics.net/
On an early autumn day of 1608, Hans Lipperhey, a spectacle maker from Middelburg, in
the Netherlands' coastal province of Zeeland, applied before the States General of The
Hague for a patent on an "instrument for seeing far". By that time, use of small rounded
glass disks to aid the natural eyesight wasn't new. Those bulging out on both sides,
resembling lentil - or "lens" in Latin - have been used to correct for farsightedness since the
mid 13th century. The idea of a device for magnifying distant objects may have been
already grasped for some time as well. But this was the beginning of something else. In the
summer of 1609, Galileo, Harriot, and others, turned the new Dutch invention - the
"spyglass" - toward the night sky. The telescope was born.
NAAP telescope
simulator http://astro.unl.edu/classaction/animations/telescopes/telescope10.html
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The very basic element of a telescope is the diameter of
its aperture. Given optical quality, it is the main
determinant of telescope's capabilities with respect to
light gathering and resolution, thus also of its limits in
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useful magnification. If well made, the eyepiece has no
appreciable effect on the light gathering or inherent
resolution of a telescope. Its main function is
magnification of the real image formed by the objective.
Consequently, the main optical parameters of a telescope
relate to its objective. They are:
● aperture diameter, hereafter denoted by D
● focal length ƒ and
● relative aperture D/ƒ=1/F, with F being the focal
ratio
Thus, telescope consist from a single or or multi-element
objective, and an eyepiece centered around the optical
axis of the objective. The objective forms the focal point
- a point of the highest generated wave energy or,
geometrically, point of ray convergence on its optical axis
- which determines the focal length of a telescope.
Telescope focal length is a distance from the objective
to where it focuses collimated light. That is, when the
light arrives from objects far enough that the wavefront
entering the objective is practically flat, and the light rays
are practically parallel.
2. TELESCOPE FUNCTIONS
The main purpose of astronomical telescope is to make objects from outer space appear as
bright, contrasty and large as possible. That defines its three main function: light gathering,
resolution and magnification. These are the measure of its efficiency. All three are related to
some extent, but also have their individual characteristics and limits.
2.1. LIGHT GATHERING POWER
Light-gathering power of a telescope mainly depends on its aperture diameter. However, it is
the system light transmission that determines how much of the light that entered the
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telescope actually arrives at the final focus. Transmission losses occur due to reflection,
scattering and absorption of light, as well as due to obstructions and diaphragms in the light
path.
Aperture directly determines how much of the light from distant objects is captured. Therefore,
light-gathering gain of a telescope vs. naked eye is primarily due to its larger aperture. The
naked eye pupil opening, at its widest, ranges anywhere from ~4mm to ~8mm in diameter, with
6mm being the most often cited average. Thus - neglecting for the moment transmission and
possible obstructions - telescope of aperture D in mm will gather (D/6)2 times more light than
an average eye.
How much of this light reaches the eye will depend on telescope's light transmission efficiency.
Transmission losses at mirror surface range from ~2% to ~20%, or more.
Light loss in glass elements, therefore, increases with the number of uncoated surfaces and the
in-glass path length. For uncoated doublet objective, it is about 15% due to reflections, plus
nearly 1% per inch of aperture due to in-glass absorption. For coated doublets, it is about 4%
plus the absorption loss. The eyepieces are these days usually multicoated and, unless of
exceptional size, have up to a few percent total light loss.
Finally, most reflecting telescopes have central portion of their main mirror obscured by a
smaller secondary mirror - so called central obstruction. Size of central obstruction is usually
between ~0.15D to ~0.4D, resulting in ~2% -16% light loss.
The true light-gathering power of a telescope is given by the product of its aperture area and
transmission coefficient. At a rough average, light transmission is about 80% for amateur
telescopes, although there are systems as low as ~60%, and those as high as ~95%. (The
difference is often in the cost of the telescope.)
Often times, light-gathering power of a telescope is expressed in terms of limiting stellar
magnitude detectable. The apparent stellar magnitude - usually denoted by m - is a measure of
apparent brightness, with the difference of 5 magnitudes corresponding to 100 times
difference in brightness.
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FIGURE 11: Limiting stellar magnitude as a function of telescope magnification,
expressed as the ratio of the naked eye pupil diameter (E, for the naked eye limiting
magnitude) and the telescope exit pupil X. For E/X=1 (blue line), background brightness
in a telescope is identical as in the naked eye, and it doesn't affect limiting magnitude.
As magnification increases, sky background becomes darker, enabling detection of
fainter stars. As mentioned, choosing 6mm - or any other - naked eye pupil diameter
does not affect the final limiting magnitude, since the naked eye gain/loss is offset by
that with a telescope. But darker skies directly translate into higher limiting magnitude;
in other words, if at this same 6mm eye pupil the limiting naked eye magnitude was
higher, so it would be the limiting telescopic magnitude.
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1.1. Diffraction image ►
1. TELESCOPE IMAGE: RAY, WAVEFRONT and DIFFRACTION
There are two main aspects to how telescopes form images of planets, stars and
galaxies. One is concerned with the physics of image formation and the other with
its geometry. We need the former to determine how light waves behave: where
do they go, and how their interactions determine image brightness, contrast and
resolution. The latter is an interface based on wave behavior that provides
convenient way of determining image location and magnification, as well as the
initial assessment of the size of image aberrations.
By making space objects brighter and larger, telescopes greatly expand our ability
of their detection and observation.
The light energy directed toward focal point is spread into a pattern, setting a
limit to image contrast and resolution. Physical size of diffraction pattern is
inversely proportional to the telescope's relative aperture 1/F, with the first
minima (Airy disc) radius given by rAD=1.22λF, λ being the wavelength of light, and
F the focal ratio F=ƒ/D, ƒ and D being the telescope focal length and diameter,
respectively.
1.1. DIFFRACTION IN A TELESCOPE
Optically, any astronomical object is composed of a countless number of pointsources of light. The telescope forms object's image by imaging each and every of
these point sources in its focal plane. The point-image itself is created by wave
interference around focal point, a phenomenon known as diffraction of light.
Diffraction image of a point-source in a telescope is a bright central disc
surrounded by rapidly fainting concentric rings. What causes the appearance of
this pattern is interference of light waves. Constructive interference is at its peak
in the center of the pattern, which is the center of curvature of near-spherical
wavefront formed by telescope's objective. Farther away from the center point,
constructive interference quickly subsides, resulting in the first bright ring much
fainter than the disc, and every successive bright ring much fainter than the
preceding ring. Size of diffraction pattern in a telescope is proportional to the
wavelength λ; given wavelength, its physical size is proportional to telescope's F16
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number, while its angular size is inversely proportional to the aperture size (FIG.
2).
2.2. TELESCOPE RESOLUTION
Resolution is another vital telescope function. Simply put,
telescope resolution limit determines how small a detail can be
resolved in the image it forms.
In the absence of aberrations, what determines limit to resolution is the effect of
diffraction. Being subject to eye (detector) properties, resolution varies with
detail's shape, contrast, brightness and wavelength.
The conventional indicator of resolving power - commonly called diffraction
resolution limit - is the minimum resolvable separation of a pair of close pointobject images, somewhat arbitrarily set forth by the wave theory at ~λ/D in
radians for incoherent light, λ being the wavelength of light, and D the aperture
diameter (expressed in arc seconds, it is 134/D for D in mm, or 4.5/D for D in
inches, both for 550nm wavelength).
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Resolving two point sources is inevitably dependent on telescope magnification. If the images of two point of light
are to be fully resolved, they need to be separated by at least a single non-illuminated retinal photoreceptor
(presumably cone, since the resolution limit of rods is significantly lower).
The point-source diffraction resolution limit for incoherent light, coherent light with λ/4 OPD
between components and, perhaps, specific cases of partly coherent light, is given by ~λF,
F being the ratio number of the focal length to aperture diameter (F=ƒ/D, with ƒ being the
focal length). It is a product of angular resolution and focal length: λF=λƒ/D. Specifically,
this is the limit to resolution for two point-object images of near-equal intensity (FIG.
12).
2.3. TELESCOPE MAGNIFICATION
Telescope magnification is given by a ratio of the image size produced on the retina when
looking through a telescope, versus retinal image size with the naked eye. As objective to
retina figure shows, image size on the retina in both cases is proportional to the apparent
angle of view, giving telescope magnification …. For sufficiently small (angles) … gives
telescope magnification as:
with ƒO, ƒE being the objective and eyepiece focal length, respectively.
Telescope magnification can be split into two components:
(1) magnification of the objective and
(2) magnification of the eyepiece. Magnification of the image formed by the objective is
either relative to the object imaged (absolute, or optical magnification), or relative to its
apparent size in the naked eye (apparent magnification).
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While there is no single optimum magnification for all types of astronomical objects and
individuals, there is a range of so called useful magnification.
On the low side of this range, the limit is set by the size of eye pupil. It is not to be smaller
than the "exit pupil" of a telescope - an image of the entrance pupil (objective) formed by
the eyepiece.
Limit to magnification increase is set primarily by image imperfections, but also by
dimming, loss of field, vibrations and eye physiology. As we have seen, even perfect optics
will not produce perfect images, due to the effect of diffraction.
The often quoted 50x per inch of aperture is a useful limit to magnification.
Thus for our 20 inch Meade telescope the maximum useful gain is 1000. (20
x 50) Although other effects can limit to the value of ½ or a magnification of
500.
TEST ON THE OPTICAL TELESCOPE
NAAP telescope question summary the following two go together Just how I don't know
http://astro.unl.edu/interactives/misc/Telescope1.html
http://astro.unl.edu/interactives/misc/Telescope1.swf
non-OPTICAL DETECTION DEVICES
RADIO TELESCOPES
basics and scope of a radio telescope
SATELLITES Employing satellites to record the universe eliminates atmospheric
absorption of the electromagnetic spectra.
X-ray 50 years of
http://imagine.gsfc.nasa.gov/docs/features/exhibit/fifty/intro.html
NASA Satellite by spectrum and function
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http://www.rssd.esa.int/SD/INTEGRAL/docs/Publications/Science_In_School_issue20.pdf
European Space Agency satellites – some with NASA
European Space Agency satellites – some with NASA
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Future satellites James Webb Space Telescope
http://www.nasa.gov/externalflash/webb3d/#/instruments/integrated-science-instrument-module/
LIMITATIONS OF DETECTION DEVICES
1st limitation:
Optical -- Most of the signals we receive we can not detected without electronic instrument. For
our eyes can see all of thiese signals because they are outside of the light or optical frequencies.
So we need instruments to detect and collect this energy and recognize and categorize it.
Computers and detection devices like CCD, charge coupled devices, that have been developed in
the last 70 years have enabled us to detected and categorized many of these signals.
2nd limitation -- atmosphere
Our atmosphere that protects us from many harmful signals also prevents us from some of the
signals coming from the universe.
3rd our optical vision – the geometry of the eye
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http://www.ivy-rose.co.uk/HumanBody/Eye/Eye_Image-Formation.php
4th limited viewing time – time available to view
DIGITAL TECHNOLOGY
Compare to human eye, to photographic, and to CCD, charge-coupled devices and digital
technology
NAAP CCD simulator http://astro.unl.edu/classaction/animations/telescopes/buckets.html
Satellite images
http://www.rssd.esa.int/SD/INTEGRAL/docs/Publications/Science_In_School_issue20.pdf
Buying or building your first telescope
Buying your first telescope from YouTube
http://www.youtube.com/watch?v=lH5H9bf9gzo&feature=relmfu
telescope building http://www.atmob.org/
F:\2012-2013\Salem State\PHS 207 Spring 2013 astronomy\Notes from 2012 and for 2013 course\6 inch
home built scope by Paul Domigan
Astronomical references
Astronomical dictionary http://imagine.gsfc.nasa.gov/docs/dictionary.html
THE END
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