HYE!

It had been a long time since I updated any status....
I was kinda busy with study and homework.For today I will just give out an information about "Telescope".

What is Telescope

Example of some type of telescope.

  • The refractor telescope, which uses glass lenses.

  • The reflector telescope, which uses mirrors instead of the lenses.

Whatever the telescope, it’s most important spec is its aperture: the diameter of its main, light-gathering lens or mirror. (This lens or mirror is called the telescope's objective.) The bigger the aperture, the sharper and brighter the view will be.

­To understand how telescopes work, let's ask the following question. Why can't you see an object that is far away? For example, why can't you read the writing on a dime when it is 150 feet (55 meters) away with your naked eyes? The answer to this question is simple: the object does not take up much space on your eye's screen (retina). If you want to think about it in digital camera terms, at 150 feet the writing on the dime does not cover enough pixels on your retinal sensor for you to read the writing. If you had a "bigger eye," you could collect more light from the object and create a brighter image, and then you could magnify part of that image so it stretches out over more pixels on your retina. Two pieces in a telescope make this possible:

  • The objective lens (in refractors) or primary mirror (in reflectors) collects lots of light from a distant object and brings that light, or image, to a point or focus.

  • An eyepiece lens takes the bright light from the focus of the objective lens or primary mirror and "spreads it out" (magnifies it) to take up a large portion of the retina. This is the same principle that a magnifying glass (lens) uses; it takes a small image on the paper and spreads it out over the retina of your eye so that it looks big.

When you combine the objective lens or primary mirror with the eyepiece, you have a telescope. Again, the basic idea is to collect lots of light to form a bright image inside th­e telescope, and then use something like a magnifying glass to magnify (enlarge) that bright image so that it takes up a lot of space on your retina.

A telescope has two general properties:
  • how well it can collect the light
  • how much it can magnify the image
­A telescope's ability to collect light is directly related to the diameter of the lens or mirror -- the aperture [fine holes on the camera (where light enters)] -- that is used to gather light. Generally, the larger the aperture, the more light the telescope collects and brings to focus, and the brighter the final image.
The telescope's magnification, its ability to enlarge an image, depends on the combination of lenses used. The eyepiece performs the magnification. Since any magnification can be achieved by almost any telescope by using different eyepieces, aperture is a more important feature than magnification.
To understand how this actually works in a telescope, let's take a look at how a refractor telescope (the kind with lenses) magnifies an image of a distant object to make it appear closer.
Refractors are the type of telescope that most of us are familiar with. They have the following parts:
  • a long tube, made of metal, plastic, or wood
  • a glass combination lens at the front end (objective lens)
  • a second glass combination lens (eyepiece)
               Diagram of a refractor showing the light path inside.
The tube holds the lenses in place at the correct distance from one another. The tube also helps to keeps out dust, moisture and light that would interfere with forming a good image. The objective lens gathers the light, and bends or refracts it to a focus near the back of the tube. The eyepiece brings the image to your eye, and magnifies the image. Eyepieces have much shorter focal lengths than objective lenses.

Achromatic refractors use lenses that are not extensively corrected to prevent chromatic aberration, which is a rainbow halo that sometimes appears around images seen through a refractor. Instead, they usually have "coated" lenses to reduce this problem. Apochromatic refractors use either multiple-lens designs or lenses made of other types of glass (such as fluorite) to prevent chromatic aberration. Apochromatic refractors are much more expensive than achromatic refractors.

Refractors have good resolution, high enough to see details in planets and binary stars. However, it is difficult to make large objective lenses (greater than 4 inches or 10 centimeters) for refractors. Refractors are relatively expensive, if you consider the cost per unit of aperture. Because the aperture is limited, a refractor is less useful for observing faint, deep-sky objects, like galaxies and nebulae, than other types of telescopes.

Reflectors

Isaac Newton developed the reflector about 1680, in response to the chromatic aberration (rainbow halo) problem that plagued refractors during his time. Instead of using a lens to gather light, Newton used a curved, metal mirror (primary mirror) to collect the light and reflect it to a focus. Mirrors do not have the chromatic aberration problems that lenses do. Newton placed the primary mirror in the back of the tube.
Because the mirror reflected light back into the tube, he had to use a small, flat mirror (secondary mirror) in the focal path of the primary mirror to deflect the image out through the side of the tube, to the eyepiece; otherwise, his head would get in the way of incoming light. Also, you might think that the secondary mirror would block some of the image, but because it is so small compared to the primary mirror, which is gathering a great deal of light, the smaller mirror will not block the image.
In 1722, John Hadley developed a design that used parabolic mirrors, and there were various improvements in mirror-making. The Newtonian reflector was a highly successful design, and remains one of the most popular telescope designs in use today.
Diagram of a Newtonian reflector showing the light path inside.
Rich-field (or wide-field) reflectors are a type of Newtonian reflector with short focal ratios and low magnification. The focal ratio, or f/number, is the focal length divided by the aperture, and relates to the brightness of the image. They offer wider fields of view than longer focal ratio telescopes, and provide bright, panoramic views of comets and deep-sky objects like nebulae, galaxies and star clusters.
A view inside the barrel -- note the primary mirror, and the image of the secondary mirror reflected back onto the primary.
Dobsonian telescopes are a type of Newtonian reflector with a simple tube and alt-azimuth mounting (see "Telescope Mounts"). They are inexpensive to build or buy because they are made of plastic, fiberglass or plywood. Dobsonians can have large apertures (6 to 17 inches, 15 to 43 centimeters). Because of their large apertures and low price, Dobsonians are well-suited to observing deep-sky objects.
The reflector is simple and inexpensive to make. Large aperture primary mirrors (greater than 10 inches or 25 centimeters) can be made easily, which means that reflectors have a relatively low cost per unit of aperture. Reflectors have large light gathering capacities, and can produce bright images of faint, deep-sky objects for visual observing as well as astrophotography. One disadvantage of reflectors is that you occasionally have to clean and align the mirrors. Also, slight errors in grinding the mirrors can distort the image. Here are some of the common problems:
  • Spherical aberration - light reflected from the mirror's edge gets focused to a slightly different point than light reflected from the center.
  • Astigmatism - the mirror is not ground symmetrically about its center (it might be slightly egg-shaped, for example); star images focus to crosses rather than to points.
  • Coma - stars near the edge of the field look elongated, like comets, while those in the center are sharp points of light.

In addition, all reflectors are subject to some light loss, for two reasons: First, the secondary mirror obstructs some of the light coming into the telescope; second, no reflective coating for a mirror returns 100 percent of the light striking it -- the best coatings return 90 percent of incoming light.

Compound or Catadioptric Telescopes

Compound or catadioptric telescopes are hybrid telescopes that have a mix of refractor and reflector elements in their design. The first compound telescope was made by German astronomer Bernhard Schmidt in 1930. The Schmidt telescope had a primary mirror at the back of the telescope, and a glass corrector plate in the front of the telescope to remove spherical aberration. The telescope was used primarily for photography, because it had no secondary mirror or eyepieces -- instead, photographic film was placed at the prime focus of the primary mirror. Today, the Schmidt-Cassegrain design, which was invented in the 1960s, is the most popular type of telescope; it uses a secondary mirror that bounces light through a hole in the primary mirror to an eyepiece.

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