detecting, collecting & focusing light
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DETECTING, COLLECTING &
FOCUSING LIGHT
TelescopeThe main purpose of a telescope is to capture
more light and see fainter objects than our
eyes allow us to see.
Telescopes come in many different varieties but
they all boils down into two major principles.
Refracting and Reflecting of light.
Refracting TelescopeA refracting telescope uses a convex mirror
(the center is thicker than the edges) to focus light.
The refraction properties of glass (the way it bends light) are used to gather parallel rays of light and
focus them to a point. The image is created at the
focal point of the telescope.
Reflecting telescopeA reflecting telescope uses a concave mirror
instead of convex mirror to collect and focus light.
A large concave mirror (the center is thinner
than the edges) collects and reflects the light to
make an image.
Reflecting telescopes can see objects that are a
millionth or a billionth the brightness of the faintest
star that can be seen by the human eye alone.
Objective/Primary Mirror
The large, concave, light-gathering mirror in a reflecting telescope.
Diagonal/Secondary Mirror
In a reflecting telescope, a mirror that directs the light from the primary
mirror to a focal position.
Cassegrain TelescopeA type of reflecting telescope in which
incoming light hits the primary mirror and is then
reflected upward toward the prime focus,
where a secondary mirror reflects the light back down through a small hole in the main mirror into a detector
or eyepiece.
Newtonian TelescopeA reflecting telescope designed
so that the focused light is reflected by a small secondary
mirror out to the side of the telescope, where it can be
viewed.
ApertureThe opening in a telescope or other optical instrument that determines how much
light it collects.
The size of the diaphragm opening in a telescope’s lens
regulates amount of light passes through onto the film
inside the camera the moment when the shutter curtain in telescope opens during an
exposure process.
LensAn optical instrument, made
of glass or some other transparent material, shaped so that parallel rays of light
passing through it are bent to arrive at a single focus.
FilterColor filters are usually
constructed using transparent pieces of dyed glass, plastic,
lacquered gelatin (e.g. Wratten filters) that have been treated to selectively transmit the desired wavelengths while restricting
others
The two most common types of
filters in use today
Absorption FiltersAbsorb unwanted
wavelengths.They are used to create special
effects in a number of photography applications and
are widely employed in the cinema industry.
In addition, absorption filters are commonly found in signs and traffic signals and as directional signals
on automobiles, boats, and airplanes.
Interference FiltersRemove selected
wavelengths by internal destructive interference
and reflection.
These filters differ from absorption filters in the fact that they reflect and
destructively interfere with unwanted wavelengths as
opposed to absorbing them.
CCDCharged-coupled device:
an electronic device that records the intensity of light falling on it. Have replaced film in most astronomical
applications.
When you take a picture with a digital camera, the
image is recorded by a sensor, called a "charged coupled device" or CCD
Focal PlaneThe surface where the
lenses and/or mirrors of a telescope form an image of
a distant object.
FocusAlso called an image
point, a point in an optical system in which light rays are brought together; the location where an image forms in such systems.
CoherentA relationship indicating
that electromagnetic waves are in synchronization with each other. Waves that are coherent exhibit persistent
interference effects.
Chromatic AberrationA distortion of an
optical system that results in different colors not
focusing in the same way, often producing color
fringes or distorted shapes.
DispersionThe spreading of light
or other electromagnetic radiation into a spectrum. A rainbow is an example of
the dispersion of light caused by raindrops.
False-color imageA depiction of an
astronomical object in which the colors are not the object's real colors.
Instead, they are colors arbitrarily chosen to
represent other properties of the body, such as the intensity of radiation at other than visible wave
lengths.
Photoelectric EffectEmission of electrons
from a material when light of a high-enough frequency
strikes it.
Regardless of the brightness of the light, no
electrons are emitted unless the photons' energy is greater than a value that depends on the material.
PixelThe term "pixel" is
actually short for "Picture Element." These small
little dots are what make up the images on computer
displays.
The screen is divided up into a matrix of thousands or even millions of pixels. Typically, you cannot see
the individual pixels, because they are so small.
Surface BrightnessThe overall brightness
of an extended astronomical
object such as a galaxy, star cluster, or nebula.
Ex: a galaxy with a high density of stars will have a
high surface brightness, but one with the same number of stars more
widely spread out will have a lower surface brightness.
THE END!
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