Chapter 3 Astronomy

Flashcard maker : Lily Taylor
Telescope
A “light bucket”—a device whose primary function is to capture as much radiation as possible from a given region of the sky and concentrate it into a focused beam for analysis.
Optical Telescope
Designed to collect wavelengths visible to the human eye.
Reflecting Telescope
A telescope which uses a mirror to gather and focus light from a distant object.
Primary Mirror
A telescope’s main focusing mirror.
Focal Length
The distance between the primary mirror and the focus.
Determines its angular field of view. A longer ” ” telescope takes in a narrower field of view and a shorter ” “scope takes in a wider field of view.
It is focal length that determines a telescope’s field of view, regardless of the diameter of the objective lens or primary mirror.
Your eyepeice has one too.
The ” ” of the telescope divided by the ” ” of the eyepiece determines the magnification you see.
Prime Focus
The point in a reflecting telescope where the mirror focuses incoming light to a point.
Some secondary mirrors may be placed before the light to intercept it on its path to the focus and redirected it to a more convenient location.
Refracting Telescope
A telescope which uses a lens to gather and focus light from a distant object.
Refraction
The tendency of a wave to bend as it passes from one transparent medium to another.
Image
The optical representation of an object produced when light from the object is reflected or refracted by a mirror or lens.
Chromatic Aberration
The focusing of different colors of light at different distances behind a lens.
Chromatic Aberation, Glass absorption, Lense Weight, Lense Cost
4 reasons to choose mirror instead of lense telescope.
For these reasons, all large modern optical telescopes are reflectors.
Newtonian Telescope
A reflecting telescope in which incoming light is intercepted before it reaches the prime focus and is deflected into an eyepiece at the side of the instrument.
Cassegrain Telescope
A 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.
Longer Focal Point because light travels farther.
Coude Telescope
In a variant on the Cassegrain design, light is first reflected by the primary mirror toward the prime focus and reflected back down the tube by a secondary mirror. A third, much smaller mirror then reflects the light into an environmentally controlled laboratory
Prime Focus Telescope
Radiation from a star enters the instrument, passes down the main tube, strikes the primary mirror, and is reflected back toward the prime focus, near the top of the tube.
Collecting Area
The total area of a telescope that is capable of capturing incoming radiation. The larger the telescope, the greater its collecting area, and the fainter the objects it can detect.
The larger the telescope’s reflecting mirror (or refracting lens), the more light it collects .
Angular Resolution
The ability of a telescope to distinguish between adjacent objects in the sky.
In astronomy, where we are always concerned with angular measurement, “close together” means “separated by a small angle on the sky,”
The minimum angular separation that can be distinguished.
Diffraction
Important factor limiting a telescope’s resolution.
The tendency of waves to bend around corners. The diffraction of light establishes its nature as a wave.
Introduces a certain “fuzziness,” or loss of resolution, into the system.
Waves deform to fill the space behind the object.
The amount of diffraction is proportional to the wavelength of the radiation divided by the diameter of the telescope mirror.
The blurring effects increase in proportion to the wavelength used.
This becomes more significant as the size of the aperture decreases relative to the wavelength of light passing through.
Diffraction affects your image sharpness by limiting Depth of Field and useful Resolution
Diffraction-Limited Resolution
A theoretical resolution that a telescope can have due to diffraction of light at the aperture.
At some aperture the softening effects of diffraction offset any gain in sharpness due to better depth of field.
The best possible angular resolution of that wavelenth for that size of lense.
Seeing
Refers to the blurring and twinkling of astronomical objects such as stars caused by turbulent mixing in the Earth’s atmosphere varying the optical refractive index.
A dancing sharp image of the star has been smeared out over a roughly circular region.
Describe the effects of atmospheric turbulence.
Seeing Disc
Roughly circular region on a detector over which a star’s pointlike images is spread, due to atmospheric turbulence.
Charged Couple Device
Electronic device used for data acquisition, composed of many tiny pixels, each of which records a buildup of charge to measure the amount of light striking it.
2 Advantages of CCD’s
Much more efficient than photographic plates, recording as many as 75 percent of the photons striking them, compared with less than five percent for photographic methods.
Produce a faithful representation of an image in a digital format that can be placed directly on magnetic tape or disk, or even sent across a computer network to an observer’s home institution for analysis.
Active Optics
Collection of techniques now being used to increase the resolution of ground-based telescopes. Minute modifications are made to the overall configuration of an instrument as its temperature and orientation change, to maintain the best possible focus at all times.
Adaptive Optics
Technique used to increase the resolution of a telescope by deforming the shape of the mirror’s surface under computer control while a measurement is being taken, to undo the effects of atmospheric turbulence.
In the next decade, it may well be possible to achieve with large ground-based telescopes the kind of resolution presently attainable only from space.
Radio Telescope
Large instrument designed to detect radiation from space in radio wavelengths.
Interferometry
Technique in widespread use to dramatically improve the resolution of radio and infrared maps. Several telescopes observe an object simultaneously, and a computer analyzes how the signals interfere with one another to reconstruct a detailed image of the field of view.
Interferometer
Collection of two or more telescopes working together as a team, observing the same object at the same time and at the same wavelength. The effective diameter of an interferometer is equal to the distance between its outermost telescopes.
Infrared Telescope
Telescope designed to detect infrared radiation. Many such telescopes are designed to be lightweight so that they can be carried above most of Earth’s atmosphere by balloons, airplanes, or satellites.
Some infrered can be observed from earth. The atmosphere does not block all of it out especially at higher altitudes above water vapor.
Ultraviolet Telescope
A telescope that is designed to collect radiation in the ultraviolet part of the spectrum. The Earth’s atmosphere is partially opaque to these wavelengths, so ultraviolet telescopes are put on rockets, balloons, or satellites to get high above most or all of the atmosphere.
Virtually no ultrviolet can be viewed from earth.
Heat is a problem with these telescopes
High Energy Telescope
Telescope designed to detect radiation in X-rays and gamma rays.
X-rays and gamma rays cannot be reflected easily by any kind of surface. Rather, these rays tend to either pass straight through or else be absorbed by any material they strike.
Arperture
A hole or an opening through which light travels.
The opening that determines the cone angle of a bundle of rays that come to a focus in the image plane.
Airy Disc
The distortion of the wave will create a pattern due to difraction.
The diameter starts to increase with a decreasing aperture size. When the cone (the red and yellow) area is as large as your smallest resolution unit is, you will start to lose sharpness due to diffraction. (diffraction limited resolution)
It’s size is inversely proportional to the size of the telescope objective.
Catadioptric (CAT) Telescopes
Fold the light path back and forth internally and thus are typically much shorter, physically, than their optical focal length. This makes for a relatively short, compact optical tube and gives CAT’s a big advantage in portability.
Seeing
Refraction
The amount by which a propagating wave is bent.
Chromatic Aberration
A type of distortion in which there is a failure of a lens to focus all colors to the same convergence point.
Chromatic Aberration
An optical aberration in which the image has colored fringes.
The lens in a refracting telescope focuses red and blue light differently
– the lens in a refracting telescope focuses red and blue light differently. This deficiency is known as chromatic aberration.
– As light passes through the lens, some of it is absorbed by the glass.
– A large lens can be quite heavy therefore it tends to deform by its weight.
– A lens has two surfaces that must be accurately machined and polished, which can be a difficult task. A mirror has only one surface
List three advantages of reflecting telescopes over refracting telescopes?
Currently, the largest (and highest) optical telescopes are the twin Keck instruments atop Mauna Kea in Hawaii operated jointly by the California Institute of Technology and the University of California.
What are the largest optical telescopes in use today?
Atmospheric turbulence produces continuous small changes in the optical properties of the air between the star and our telescope (or eye).
The light from the star is refracted slightly, again and again, and the stellar image dances around on the detector (or retina).
Astronomers use the term seeing to describe the effects of atmospheric turbulence. The circle over which a star’s light is spread is called the seeing disk.
How does Earth’s atmosphere affect what is seen through an optical telescope?
A telescope placed above the atmosphere, in Earth orbit, can achieve resolution close to the diffraction limit.
It has a 2.4-m mirror and a diffraction limit of 0.05″, giving astronomers a view of the universe as much as 20 times sharper than that normally available from even much larger ground-based instruments.
What advantages does the Hubble Space Telescope have over ground-based telescopes? List some disadvantages.
More Efficient and Digital Format
CCDs are much more efficient than photographic plates, recording as many as 75 percent of the photons striking them, compared with less than five percent for photographic methods.
CCDs produce a faithful representation of an image in a digital format that can be placed directly on magnetic tape or disk, or even sent across a computer network to an observer’s home institution for analysis
What are the advantages of a CCD over a photographic plate?

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