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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