Transcript Optical telescopes

TELESCOPES
INTRODUCTION
Telescope – a device that permits distant and
faint objects to be viewed as if they were much
brighter and closer to the observer.
 Typically used to observe the skies.
 Used to learn about
 A. distant stars,
 B. nebulas and
 C. galaxies.

INTRODUCTION
Telescopes
 Used to tackle scientific questions about the:
 a. birth of the universe,
 b. emergence of structure in the early universe,
 c. formation of evolution of stars, galaxies and
planetary systems and
 d. conditions for the emergence of life itself.

INTRODUCTION
Optical telescopes – work by collecting and
magnifying visible light that is given off by stars
or reflected from the surface planets.
 Focus – a point in space where all the light rays
converge.
 Eye piece – small magnifying lens placed at the
focus allows the image to be viewed.
 Cameras – instruments placed near the focus
make a precise recording of the light gathered by
a telescope.

INTRODUCTION
Objective lens - the lens or combination of
lenses in an optical instrument nearest to and
facing the object being viewed.
 Focal length - distance from lens to focal point.

REFRACTING TELESCOPE
INTRODUCTION




Observatories – Earth based telescopes located around
the world. Only radio waves, visible light and some
infrared radiation can penetrate Earth’s atmosphere.
Space telescopes – can collect waves from other
regions of the electromagnetic spectrum.
Spectroscopy – visible light is divided into component
wavelengths through the use of prism or diffraction
grating.
Decodes starlight to yield information about an object’s
temperature, motion and other dynamics, chemical
composition and presence of magnetic fields.
KITT PEAK NATIONAL OBSERVATORY
KITT PEAK NATIONAL OBSERVATORY
Kitt Peak National Observatory is silhouetted
against the night sky.
 Located near Tucson, Arizona, Kitt Peak houses a
variety of instruments, including a large McMath
solar telescope.
 The telescope’s main mirror is 150 cm (59 in) in
diameter and has a focal length of 91.5 m (300
ft).
 Kitt Peak also houses a 400-cm (157-in) reflecting
telescope and a 11-m (36-ft) radio telescope.

HUBBLE SPACE TELESCOPE
HUBBLE SPACE TELESCOPE
OPTICAL TELESCOPES







II. Optical Telescopes
A. Refracting telescopes or refractors – use a glass lens to bend, or
refract, starlight and bring it to a focus. The convex lens bends light
at the edge of the lens to a greater angle than light coming through
the center so all of the rays converge to a focus.
Focal length – distance between the lens and the place where the
rays converge.
Light gathering power is proportional to the size of the objective and
the ratio of the focal lengths of the objective lens and the eyepiece.
Chromatic aberration – causes different colors of light to come to a
different focus because every color has its own degree of refraction.
A fundamental limitation of refractors is that lenses beyond 40
inches are impractical because they weigh more than half a ton and
sag under their own weight.
Cannot be supported from behind compared to optical mirrors.
REFRACTING TELESCOPE
REFRACTING TELESCOPE





Refracting Telescope
The simplest refracting telescope has two convex
lenses, which are thicker in the middle than at the
edges.
The lens closest to the object is called the objective
lens. This lens collects light from a distant source and
brings it to a focus as an upside-down image within the
telescope tube.
The eyepiece lens forms an image that remains
inverted.
More complex refracting telescopes contain an
additional lens to flip the image right-side up.
OPTICAL TELESCOPES
B. Reflecting Telescopes – uses precisely curved
mirror instead of a lens to collect starlight. The
concave mirror brings reflected light waves to a
focus at a point above the mirror. Useful for
gathering light from dim objects.
 Galileo is considered to be the first person to use
telescopes for astronomical observations.
 His design was capable of magnifying objects 30
times.

REFLECTING TELESCOPES
KECK TELESCOPE





The twin Keck telescopes are the world’s largest optical
telescopes.
They sit in the Mauna Kea Observatory on Mauna Kea, a
dormant volcano in Hawaii.
The Mauna Kea Observatory is located at an altitude of
4,205 m (13,796 ft), which permits excellent nighttime
viewing with a minimum of interference from artificial light
sources.
The Keck telescopes are both reflecting telescopes that use
a giant mirror consisting of 36 individual segments.
The segments collect light and bring it to a common focus.
On both mirrors, each of the mirror’s 36 segments measure
about 2 m (6.6 ft) across.
REFLECTING TELESCOPES
REFLECTING TELESCOPES
The first reflecting telescope was designed by Sir
Isaac Newton in 1668.
 A reflecting telescope uses a curved mirror to
focus the light.
 Light from distant objects such as stars and
galaxies enters the telescope tube in parallel rays.
 These rays are reflected from the concave mirror
to a diagonal plane mirror.
 The diagonal mirror reflects the light through a
hole in the side of the telescope tube to a lens in
the eyepiece.

REFLECTING TELESCOPES
Reflecting telescopes can be made larger than
refracting telescopes because the curved mirror
can be supported along its entire surface, while a
large lens can only be supported at its edges.
 A large surface can collect more light from distant
stars than a small surface.
 Modern reflecting telescopes include the 508-cm
(200-in) reflector at the Palomar Observatory in
California and the 400-cm (158-in) reflector at
Cerro Tololo Inter-American Observatory near La
Serena, Chile.

OPTICAL TELESCOPES






> Reflecting telescope’s light sensitivity increases with the square of
the diameter of the telescopes mirror. Doubling mirror’s diameter
increases light-gathering power by a factor of four.
> Larger optical telescopes can see faint objects better and
faster.
> Mirror has a hyperbolic or parabolic shape bringing light to a
precise focus. It can also be a sphere.
> Mirror could be made with Pyrex glass, borosilicate glass and
glass-ceramic composite.
> Huge mirrors are expensive and difficult to make, challenging
to move while tracking celestial
objects and heavy. E.g. The 200 inch Hale telescope on
California’s Mount Palomar weighs 14 tons.
OPTICAL TELESCOPES




Twin Keck telescopes in Mauna Kea, Hawaii broke the mirror
size barrier by combining 36 hexagonal 72 inch mirror with a
light collecting power equal to that of a 400 in mirror.
Mirror weight has been reduced by sandwiching a honey
comb pattern of glass ribs between a thin, but rigid concave
mirror and a flat back plate.
Meniscus mirrors too thin to support their own weight were
developed. Adjustable framework supports the meniscus
mirror.
Servomechanical actuators – controlled by the computer,
continually adjust the shape of the mirror as it tracks
celestial targets.
OPTICAL TELESCOPES


C. Resolution – the ability to see fine detail. Increases with mirror or lens
size. Turbulent atmosphere blurs incoming starlight.
Adaptive optics- distorts mirrors to remove the blur of starlight. E.g. Mauna
Kea outfitted with this technology takes pictures 20 times better. Can
resolve a coin size at distance of more than 50 kilometers.




D. Optical Interferometry – combines signals from telescopes in separate
locations so that resulting image is equal to that received from one giant
telescope. E.g. Very Large Telescope (VLT) in the Acatama Desert in northern
Chile.
> VLT combines light from four 323 inch telescopes and several smaller
telescopes to produce an image equivalent to that of a 630 inch telescope.
> Use to resolve bright, closely paired objects like double stars. May be
used to observe Earth-size planets orbiting stars.
END OF PRESENTATION