Telescopes

There are several disadvantages with a refracting telescope.

1. Chromatic aberration is one, where all colours are not focused at the same point.

2. The mass of the lens, if large, could cause it to change shape as it is moved.

3. It is essentially impossible to have an optically perfect lens surface, and any minute imperfection causes 'confusion' of the light ray passing through that point.

4. Any lens surface will cause some reflection of light - light lost to the detector at the end point. You must have seen reflections of yourself, or of some landscape, from an ordinary transparent window glass. These reflections will re-reflect within a lens system, causing some optical confusion - loss of quality of the image.

Large modern reflector type telescopes avoid these problems by minimizing the number of glass surfaces between the incoming light and the recording surface. These days, the recording surface is usually a cmos, or similar, solid state detector.

To find use the following equation

object size = distance x (smallest angle/57.3°)

Since Arecibo is on Earth, to find the distance take the distance between the sun and Earth (in AU) and subtract the distance between the sun and Venus (in AU). Then multiply the AU by 1.5 x 108 (or the number of kilometers in 1 AU)

1 AU - 0.72 AU = 0.28 AU

0.28 AU x 1.5 x 108 = 4.2 x 107 km

For 1'

object size = 4.2 x 107 x (1/60°/57.3°)

object size = 12216.4 km

For 1"

object size = 4.2 x 107 x (1/3600°/57.3°)

object size = 203.6 km

How is a telescope made?

A telescope is a device used to form images of distant objects. The most familiar kind of telescope is an optical telescope, which uses a series of lens-1 or a curved mirror to focus visible light. An optical telescope which uses lenses is known as a refracting-telescope or a refractor; one which uses a mirror is known as a reflecting telescope or a reflector. Besides optical telescopes, astronomers also use telescopes that focus radio waves, X-rays, and other forms of electromagnetic radiation. Telescopes vary in size and sophistication from homemade spyglasses built from cardboard tubes to arrays of house-sized radio telescopes stretching over many miles.

The earliest known telescope was a refractor built by the Dutch eyeglass maker Hans Lippershey in 1608 after he accidentally viewed objects through two different eyeglass lenses held a distance apart. He called his invention a kijker, "looker" in Dutch, and intended it for military use. In 1609, the Italian scientist Galilei built his own telescopes and was the first person to make astronomical observations using them. These early telescopes consisted of two glass lenses set within a hollow tube and were rather small; Galileo's largest instrument was about 47 inches (120 cm) long and 2 inches (5 cm) in diameter. Astronomers such as Johannes Kepler in Germany and Christian Huygens in Holland built larger, more powerful telescopes throughout the 1600s. Soon these telescopes got too large to be easily controlled by hand and required permanent mounts. Some were more than 197 feet (60 m) long.

The ability to construct enormous telescopes outpaced the ability of glassmakers to manufacture appropriate lenses for them. In particular, the problems caused by chromatic-aberration (the tendency for a lens to focus each color of light at a different point, leading to a blurred-2 image) became acute for very large telescopes. Scientists of the time knew of no way to avoid this problem with lenses, so they designed telescopes using curved mirrors instead.

In 1663, the Scottish mathematician James Gregory designed the first reflecting telescope. Alternate designs for reflectors were invented by the English scientist Isaac Newton in 1668 and the French scientist N. Cassegrain in 1672. All three designs are still in use today. In the 1600s, there was no good way to coat glass with a thin reflective film, as is done today to make mirrors, so these early reflectors used mirrors made out of polished metal. Newton used a mixture of copper, tin, and arsenic to produce a mirror which could only reflect 16% of the light it received; today's mirrors reflect nearly 100% of the light that hits them.

It had been known as early as 1730 that chromatic aberration could be minimized by replacing the main lens of the telescope with two properly shaped lenses made from two different kinds of glass, but it was not until the early 1800s that the science of glassmaking was advanced enough to make this technique practical. By the end of the 19th century, refract telescopes with lenses up to a meter in diameter were constructed, and these are still the largest refracting telescopes in operation.

Reflectors once again dominated refractors in the 20th century, when techniques for constructing very large, very accurate mirrors were developed. The world's largest optical telescopes are all reflectors, with mirrors up to 19 feet (6 m) in diameter

The Hubble Space Telescope (HST) was launched into space to allow it to view the heavens without the distortion of images caused by looking through the atmosphere itself, the degradation of images caused by the dust in the atmosphere, and the interference caused by ambient light on the surface of the earth. All those problems can be partially overcome by a careful selection of a location for a telescope on earth, but putting the telescope in space completely eliminates all of them, except space dust.

The improvement of the image quality allowed astronomers to look at small objects relatively close to Hubble, or huge objects much farther away.

For example Hubble was used to study the dwarf planets in our solar system Pluto and Eris.

Hubble was also used to study galaxies BILLIONS of light years away. A light year is a measure of distance. It is the distance light travels in one year. Light travels 5,860,000,000,000 miles in one year, so Hubble can see galaxy sized objects billions of times a light year away.

The distance, however, is not the important part of that statement. The important part of the statement "billions of light years away" is "billions of years". Because Hubble can see so far it is, in fact, showing something NOW that happened billions of years ago. That is because the light had to travel for billions of years to reach Hubble. It is allowing astronomers and scientists to observe the universe as it was billions of years ago. This is producing a multitude of theories and scientific papers.

that pretty much covered everything i said except that hubble was last luanched to get to the begginning of time, it's a big gamble for sientists...

i don't even have to write more now :P