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Optics Basics
All telescopes are based upon two basics properties of optics
Mirror: An object which is not transparent to EM waves
Lens: An object which is transparent to EM waves.
* Reflection: The bouncing of light of surfaces - mirrors
* Refraction: The bending of light through transparent materials - lenses
Optics Basics - Reflection
The angle of
incidence ΘI is equal
to the angle of
reflection Θr
Θi
Θr
Optics Basics - Reflection
Optics Basics - Reflection
Image
Focal Length
Focal Length (f)
identifies the
distance from
the mirror where
an image of the
object will form if
the object is very
far away.
Optics Basics – Refraction
An electromagnetic wave traveling through a transparent material (not a
vacuum) travels at a speed less than 3 x 108 m/sec
The amount of slowing is characterized by the index of refraction of the
material, n.
c
n=
Speed in the material
The larger the index of refraction, the slower the em wave travels
Optics Basics – Refraction
Θi
Θr
Optics Basics – Refraction
Θi
Θr
The bending of the light ray due to the change in speed of the light is called
refraction.
Optics Basics – Refraction
Image
Focal Length
Focal Length (f)
identifies the
distance from
the lens where
an image of the
object will form if
the object is very
far away.
Optics Basics – Light Gathering
The large mirror or lens that is the “primary” collector of EM energy is called
the objective mirror or objective lens.
The number of photons collected by the telescope is directly proportional to
the area of the objective.
Since the area of the objective is proportional to the diameter of the
objective squared,
Number of photons collected is proportional to d2
Finally, the brightness of the image is proportional to the number of photons
collected,
Image brightness is proportional to d2
Optics Basics – Refraction
Image
Eyepiece
A short focal
length lens, called
an eyepiece, is
used as a simple
magnifier to
examine the
image.
Refractors
Refractors
Reflectors
Newtonian Reflectors
Catadioptric Reflectors
Also Known as Compound Telescopes
Compound telescopes “fold” the path of the light back upon itself. The result is a
short barrel telescope that is very convenient for amateur astronomers because
of the telescopes transportability.
The two most popular designs of catadioptric telescopes are SchmidtCassegrain and Maksutov-Cassegrain. They each use differently shaped
lenses and mirrors to achieve a similar quality aperture in an easy-to-carry size.
For people who like to take their telescope to a remote location and track distant
objects across the night sky, a compound telescope is a great fit.
Catadioptric Reflectors
Schmidt – Cassegrain Telescope
Catadioptric Reflectors
Maksutov – Cassegrain Telescope
Cheap Telescope design – the Dobsonian Telescope
Newtonian Reflector
See the links in the PHY250 website for building instructions
Other Applications
Radio Telescope
National Radio Astronomical Observatory at Green Bank, Va.
Radio Telescope
National Radio Astronomical Observatory at Green Bank, Va.
Radio Telescope
Radio Wave Emission from the Center of the Milky Way
Radio Continuum (408 MHz)
Intensity of radio continuum emission from high-energy
charged particles in the Milky Way,from surveys with
ground-based radio telescopes (Jodrell Bank Mark I and
Mark IA, Bonn 100-meter, and Parkes 64-meter). At this
frequency, most of the emission is from electrons moving
through the interstellar magnetic field at nearly the speed
of light. Shock waves from supernova explosions
accelerate electrons to such high speeds, producing
especially intense radiation near these sources.
Emission from the supernova remnant Cas A near 110°
longitude is so intense that the diffraction pattern of the
support legs for the radio receiver on the telescope is
visible as a cross shape.
Frequency: 408 MHz
Angular resolution: 51 arcminutes
Jodrell Bank Lowell
Telescope
Infrared Telescope
Infrared Emission from the Center of the Milky Way
Infrared
Composite mid-and far-infrared intensity observed by the
Infrared Astronomical Satellite (IRAS) in 12, 60, and 100
micron wavelength bands. The images are encoded in
the blue, green, and red color ranges, respectively. Most
of the emission is thermal, from interstellar dust warmed
by absorbed starlight, including star-forming regions
embedded in interstellar clouds. The display here is a
mosaic of IRAS Sky Survey Atlas images. Emission from
interplanetary dust in the solar system, the "zodiacal
emission," was modeled and subtracted in the production
of the Atlas.
Frequencies: 3.0 x 103-25 x 103 GHz
Angular resolution: 5 arcminutes
Infrared Astronomical Satellite
Orion – Optical Wavelength Image
Infrared Telescope
Infrared Telescope
Stars bright in different
lightThis infrared portrait of
the Orion starbirth region was
taken by the European
Southern Observatory’s new
VISTA telescope, the world’s
largest wide-field-of view
telescope. The image, which
measures about 35 lightyears from top to bottom,
records radiation with about
twice the wavelength of light
visible to the human eye.
Many of the red features just
above the center are young
stars and the high-speed
streams of gas they eject.
These stars are completely
hidden by dust in visible light
but can be seen at dustpenetrating infrared
wavelengths. Feb 10, 2010
Optical Telescopes
Visible Emission from the Center of the Milky Way
Optical
Intensity of visible (0.4 - 0.6 micron) light from a photographic survey.
Due to the strong obscuring effect of interstellar dust,the light is
primarily from stars within a few thousand light-years of the Sun,
nearby on the scale of the Milky Way. The widespread bright red
regions are produced by glowing, low-density gas. Dark patches are
due to absorbing clouds of gas and dust, which are evident in the
Molecular hydrogen and Infrared maps as emission regions. Stars
differ from one another in color, as well as mass, size and luminosity.
Interstellar dust scatters blue light preferentially, reddening the
starlight somewhat relative to its true color and producing a diffuse
bluish glow. This scattering, as well as absorption of some of the light
by dust, also leaves the light diminished in brightness. The panorama
was assembled from sixteen wide-angle photographs taken by Dr.
Axel Mellinger using a standard 35-mm camera and color negative
film. The exposures were made between July 1997 and January
1999 at sites in the United States, South Africa, and Germany.
Frequency: 460 x 103 GHz
Angular resolution: 1 arcminute
Hubble Space Telescope (HST)
Ultraviolet Telescopes
Ultraviolet
You might notice that missing from the list of images is
the ultraviolet region of the electromagnetic (EM)
spectrum. Ultraviolet radiation begins just past the
blue/violet region of the visible (optical) spectrum, and
ends when X-rays take over. The boundaries between
named regions can get a little blurred, especially if the
broad-band regions (example: infrared) are further broken
into sub-regions (example: near infrared and far infrared).
One reference on EM waves says microwaves extend
from about 1 millimeter to about 10 centimeters. In that
case, the map of molecular hydrogen would fall into the
category of microwaves.
The sky has been observed at ultraviolet wavelengths
with various detectors [links to EUVE, IUE, etc. will be
listed]. An all-sky image has not been presented here
because it is relatively featureless. You may view an allsky survey map in the extreme-ultra-violet at
http://archive.stsci.edu/euve/images/jbis_map.gif. The
image at that site comes from data obtained with the
Extreme Ultraviolet Explorer (EUVE).
Extreme Ultraviolet Explorer
X-ray Telescopes
X-Ray Emission from the Center of the Milky Way
X-Ray
Composite X-ray intensity observed by the Position-Sensitive
Proportional Counter (PSPC) instrument on the Röntgen
Satellite (ROSAT). Images in three broad, soft X-ray bands
centered at 0.25 , 0.75, and 1.5 keV are encoded in the red,
green, and blue color ranges, respectively. In the Milky Way,
extended soft X-ray emission is detected from hot, shocked
gas. At the lower energies especially, the interstellar medium
strongly absorbs X-rays, and cold clouds of interstellar gas
are seen as shadows against background X-ray emission.
Color variations indicate variations of absorption or of the
temperatures of the emitting regions. The black regions
indicate gaps in the ROSAT survey.
Frequency: 60-360 x 106 GHz
Angular resolution: 115 arcminutes
ROSAT
Gamma Ray Telescopes
Gamma Ray Emission from the Center of the Milky Way
Gamma Ray
Intensity of high-energy gamma-ray emission observed by
the Energetic Gamma-Ray Experiment Telescope (EGRET)
instrument on the Compton Gamma-Ray Observatory
(CGRO). The image includes all photons with energies
greater than 300 MeV. At these extreme energies, most of
the celestial gamma rays originate in collisions of cosmic
rays with hydrogen nuclei in interstellar clouds. The bright,
compact sources near Galactic longitudes 185°, 195°, and
265° indicate high-energy phenomena associated with the
Crab, Geminga, and Vela pulsars, respectively.
Frequencies: >2.4 x 1013 GHz
Angular resolution: ~120 arcminutes
CGRO
Milky Way Object Finder
Finder Diagram
Major structural features of the Milky Way (red), optical H II regions (blue), radio
sources (green), and OB associations (purple) are labeled in the finder chart. The
image in the finder chart is derived from the IRAS 100 micron map of intensity with
contours from the COBE DIRBE 3.5 micron map overlaid. The axes of the finder
diagram are labeled in degrees of Galactic longitude and latitude.
Then mosaics in the previous transparencies were taken from the website of the
Astrophysics Data Facility (ADF) at the NASA Goddard Space Flight Center
(GSFC).