Astronomical Tools

Explanation: Cosmic wreckage from the detonation of a massive star is the subject of this official first
image from NASA's Chandra X-ray Observatory. The supernova remnant, known as Cassiopeia A,
was produced  when a  star exploded around 300 years ago in this northern sky constellation. It is revealed
here in unprecedented detail in the light of X-rays - photons with thousands of times the energy of visible
light. The tantalizing bright speck near the bubble's center could well be the dense,  remnant of the stellar
core collapsed to form a newborn neutron star. Chandra was launched aboard the space shuttle
Columbia in July of 1999

 


The Basis of Optics

Optics is the study of instruments that can change the direction in which light travels.

Refraction 

Light travels at different speeds in different media, (such as vacuum, air, glass, ...) and
attains its maximum speed of c=300,000 km/s in vacuum. In air light travels slightly
slower than this, while in glass and water light travels slower still. Therefore, if a beam
of light is traveling in air and then enters glass the light beam will slow down. This
slowing down (or speeding up in the other direction) causes the direction of the light
beam to refract (or bend) at the boundary between the two media. This is refraction.

 

Reflection

Light bounces off a polished surface in the same way for all colors.
This is reflection. The angle of reflection is the same as the angle
of incidence.


Mirror, mirror on the wall ...

Optics and Images

Lens

A lens is an optical device that brings light rays together. When the light rays come from
a distant object the lens causes the light rays to come together (or get focused) at a region
called the focal point. The distance between the center of the lens and the focal point is
the focal length. A concave mirror can also be used to bring light rays together at a focal
point, provided that the object is very far away. 

Telescopes
A telescope is an optical instrument that has two basic optical elements:
an objective and an image viewer, such as an eyepiece or a photographic film.

Objective
This is a large lens, or mirror, that collects light from a distant object and creates
an image at the focal point that is a faithful representation of the object.

Eyepiece
The image formed by the objective is usually  too small to see all the detail it
contains. We therefore use an eyepiece -a sophisticated magnifying glass - to
enlarge the image.

    There are two basic types of telescopes:

    1.   A Refractor telescope that uses a large lens and
    2.   A Reflector telescope that uses a large mirror

 Galileo Galilei - although not the discoverer of the telescope - was the first to
 perfect it. Galileo discovered four of the - many - moons of Jupiter by using a
 Galilean moons by

An engraving of Galileo’s observations of the Moon from his book Sidereus Nuncius (The Starry Messenger),
published in 1610. A modern photo appears for comparison.

Powers of a Telescope

Light-gathering power (LGP)

We need to collect a large amount of light because the objects of most interest are very faint.
The larger the

Area=Pi*(D/2)2

of the objective the more light we can collect.
Mauna Kea, with its many huge telescopes. The Keck twin domes with the Subaru dome near them are
at the mid-left; the NASA Infrared Telescope Facility is above them; and the Canada-France-Hawaii,
Gemini North, University of Hawaii 88-inch, United Kingdom Infrared, and University of Hawaii 24-inch
telescopes are on a ridge at top. In the “millimeter valley” at lower right, we see the Caltech 10 m
submillimeter and James Clerk Maxwell 15 m telescopes (Maxwell, one of the three greatest physicists
of all time—after Newton and Einstein—unified electricity and magnetism theoretically), as well as the first to
be installed of eight 6 m telescopes and the assembly building of the Smithsonian Astrophysical
Observatory’s submillimeter array.
 

Magnifying Power
 
This is just the (apparent) increase in size of an object relative to how it looks to the unaided
eye.  The magnification of the telescope is defined as the ratio of the focal length of the objective
relative to the focal length of the eyepiece:

M=Fo/Fe

Resolving Power 

This is the smallest angular separation between two objects that can be seen. The resolution
depends on the wavelength of the light entering the objective as well as on the diameter
of the objective.  If we measure the resolving power in arcsec,  (1 degree = 60 arcmin; 1 arcmin = 60 arcsec), then the resolving power of the  telescope will be given by: 

        A(in arcsec)=2.06x105 [L(in meters)/D(in meters)]

Invisible Astronomy

Radio Telescopes
 
In 1930 Karl Jansky, while working for the Bell Telephone Company, discovered that radio
waves were coming from deep space. In the 1940's Grote Reber made the first maps of the
sky that showed the source of some of these radio waves. Today, radio telescopes are an
important tool for observing astronomical objects.  The big  problem with the use of radio waves, rather than visible light, is that the wavelength of radio  waves is about 100,000 times longer than that of light! Consequently, for the same size of  objective, a radio telescope will have a resolution that is a 100,000 times worse than its visible  counterpart. The only way to achieve better resolution is to make radio telescopes very, very  large. One way to make such large radio telescopes is to make a large number of small ones  and spread them over a large area. Such an array of small radio telescopes is called a  radio interferometer. The size of the area determines the resolution of the interferometer.


Antennas of the VLA: The Very Large Array, on the Plains of San
Augustin, west of Socorro, NM, uses 27 dish antennas, each weighing
230 tons, to make detailed images of the sky. Photo Courtesy NRAO/AUI.



 Jodie Foster usaing radio telescopes to look for exta-terresrtial life in the movie "Contact"

Space Astronomy

 Ultimately, the best way to view the heavens is to move above the Earth's atmosphere. Then the entire electromagnetic spectrum is available for observation. Indeed many instruments have been placed in Earth's orbit to collect different kinds of radiation  thanks to programs such as the Space Shuttle.
 






The Hubble Space Telescope in 2002 as seen from the space shuttle that had just brought astronauts to service it.


 
   SPECTRUM   OBSERVATORY
   Radio    Arecibo Observatory (Arecibo)
  Infrared   The Infrared Astronomical Satellite (IRAS)
  Visible   Hubble Space Telescope (HST)
  Ultraviolet   The International Ultraviolet Explorer (IUE)
  X-Rays   The Chandra X-Ray Observatory (CHANDRA)
  Gamma Rays   The Gamma Ray Observatory (GRO)
Our Window(s) to the Universe
 


Cygnus in the Radio ...


Cygnus in the Visible ...
 


Cygnus in the X-Ray ...
 

The above images shows a small portion of a nebula called the
"Cygnus Loop." Covering a region on the sky six times
the diameter of the full Moon, the Cygnus Loop is actually
the expanding blastwave from a stellar cataclysm - a
supernova explosion - which occurred about 15,000
years ago.