Help
Add this page to your book
Show collection (0 pages)
Suggest pagesGeneral Astronomy/Telescopes
From Wikibooks, the open-content textbooks collection
The word telescope originates from the Greek language (τηλεσκόπιο), and it means literally far-seeing. It is an instrument that is intended to gather information, usually in the form of light or other energy, about a distant location that can not be perceived directly by the unaided eye. Telescopes let you magnify faint stars, and see finer detail. This allows you to, for example, distinguish one star as two stars that are very close (such as Mizar A and B).
Telescopes are used by astronomers to gather visual and other information the human eye cannot see alone. They do so by means of a large collector surface that then directs the incoming energy photons into a smaller focusing lens. This large surface also allows the resolution of fine details that are too close together for the human eye to discern. It was through his usage of the telescope that Galileo discovered that the celestial spheres weren't perfect, as had been thought. He could see sunspots, and the lunar "seas."
Some difficulties with telescopes are chromatic aberration (different colors focused at different points) and spherical aberration (light isn't focused at a single point). Spherical aberration can occur when the reflecting mirror in a telescope isn't a true parabola.
Adaptive optics are used to compensate for atmospheric distortion which is caused by small changes in the refractive index of the atmosphere, which is responsible for the "twinkling" effect.
The Light Gathering Power of a telescope is measured by the area of the lens divided by the area of the eye.
Large telescopes are housed inside observatories. One of the most famous is the Lick Observatory on Mt. Hamilton, named after James Lick, a streetcar baron, who made donations to have the observatory built. When he died, he was buried underneath it.
Light pollution makes stargazing more difficult. This is caused by fixtures (such as streetlamps) that allow light to go up, instead of directing it down.
Contents |
[edit] The spectrum
Our sense of sight is based upon particles of electro-magnetic energy that are emitted by objects and then collected by our eyes. These particles are vibrating quanta of energy that are known as photons. Every photon oscillates at a particular frequency, and the range of all possible frequencies is called the spectrum.
Human eyes are attuned to a particular frequency range of photons that lie within the visual portion of the spectrum. We can not directly view photons that vibrate at frequencies above or below this range, although measuring instruments have been constructed to measure these parts of the spectrum.
All the information we have about the planets in our solar system, and the stars and galaxies comes to us from photons that have traveled that great distance. It is almost exclusively by means of these photons that astronomers have been able to assemble our current knowledge of the universe beyond the Earth.
(Some people are working on a device to try to detect "gravitons" that have traveled that great distance.)[citation needed]
[edit] Optical Telescope
Behind this lens could be a camera device or else an eyepiece for an observer.
Optical telescopes increase the apparent brightness and magnification of an object. Infrared telescopes look for heat signatures. There are also huge radio telescope dishes, as well as ultraviolet and x-ray telescopes that have been launched into orbit.
There are two main types of optical telescope - reflectors and refractors. Reflectors use a mirror to collect the light, whilst refractors use a lens. Generally, reflectors are better for deep sky objects whilst refractors are useful for planetary observations. Some telescopes use a combination of lenses and mirrors to use best the advantages of each.
Refractor telescopes require a large lens at the front to direct the incoming light to the lens. This lens can be made of more than one piece, and the differing refractive properties of the components serve to reduce aberration at the edge of the image. The lenses can also be coated to reduce internal reflections.
Catadioptrics are short, wide telescopes that use both mirrors and lenses. The price range is between reflectors and refractors.
Large lenses suffer from the disadvantage of sagging under their own weight. This causes the shape to become distorted and so the image will be irregular. For this reason the largest refracting telescope ever built only has a diameter of 41". All larger telescopes instead use a mirror are the primary collecting surface.
In the optical spectrum, great precision is required in crafting the lenses and mirrors used in a telescope. The standard of precision for a telescope mirror is accuracy to 1/4 the wavelength of light, an exceedingly small quantity. But even greater accuracy is generally preferred, especially for space-based telescopes. Special tools have been developed for measuring the accuracy of a telescope mirror and lens, and the procedure for finishing the surface is very exacting.
[edit] Gravity Telescope
[edit] Radio Telescope
The first "radio astronomer" was Karl Jansky, who studied mysterious radio interference he picked up with his antenna. The signal repeated every 23 hours and 56 minutes. He finally determined that it was caused by our Milky Way Galaxy. The signal is strongest at the center of the galaxy, in the constellation Sagittarius.[1]
The Very Large Array (VLA) is a radio telescope system in New Mexico.
The James Webb Space Telescope (JWST) is a large, infrared-optimized space telescope, scheduled for launch in 2013. JWST will find the first galaxies that formed in the early Universe, connecting the Big Bang to our own Milky Way Galaxy. JWST will peer through dusty clouds to see stars forming planetary systems, connecting the Milky Way to our own Solar System. JWST's instruments will be designed to work primarily in the infrared range of the electromagnetic spectrum, with some capability in the visible range.
JWST will have a large mirror, 6.5 meters (21.3 feet) in diameter and a sunshield the size of a tennis court. Both the mirror and sunshade won't fit onto the rocket fully open, so both will fold up and open only once JWST is in outer space. JWST will reside in an orbit about 1.5 million km (1 million miles) from the Earth. http://www.jwst.nasa.gov/
[edit] X-Ray Telescope
[edit] Gamma Ray Telescope
[edit] Solar Telescope
Solar telescopes are used for studying the sun. They are quite large. The biggest is the McMath-Pierce Solar Telescope, with a diameter of 1.6 meters. It is housed in a structure that is 100 feet tall. [2]
[edit] Cutting Edge
Some new telescopes use multiple or segmented mirrors (usually hexagonal).
- New Technology Telescope (NTT)
- Multiple Mirror Telescope (MMT)
- Keck Telescope (segmented, hexagonal mirrors)
