PACE WORK FORM 7 22/04/20

Тема: Телескопи. Види телескопів

T.: Now, let's learn some new items on the topic.

I'll give you the cards (card#1) with new words. You have to read and translate them.

v  Telescopic -  sight gunsight consisting of a telescope on a firearm for use as a sight

v  Telescope -  sight gunsight consisting of a telescope on a firearm for use as a sight

v  Telescopically - n in a telescopic manner

v  Telescopic - n visible only with a telescope

v  Telescoped -  shortened by or as if by means of parts that slide one within another or are crushed one into another

v  Telescopium -  a small constellation

v  onvex lens:  lens with a thick middle and thin edges; light rays bend toward each other

v  concave lens:  lens with edges thicker than the middle; light rays bend away from each other

v  telescope:  a tool used to make far away things look close

v  reflecting telescope: a telescope (tool used to make far away things look close) that can see things by bouncing back light off of mirrors

v  refracting telescope:  a telescope (tool used to make far away things look close) that can see things by bending light through a lens.

Reading

Kinds of Telescopes

A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation (such as visible light). The first known practical telescopes were invented in the Netherlands at the beginning of the 17th century, using glass lenses. They found use in terrestrial applications and astronomy.

There are many different kinds of telescope.

In the refracting telescope, the objective lens is usually composed of two lenses, made out of different kinds of glass. Such a lens is called an achromat. A glass prism can be used to produce a rainbow of colors from white light. This is because glass bends different colors of light to different degrees.

When one wants to have a clear and sharp image of something one is looking at, this effect becomes an annoyance, known as chromatic aberration. An achromat is designed to cancel this effect, by using lenses made of two different kinds of glass. One lens is convex, made of crown glass. The other is concave, made of flint glass, which is denser, and which bends light more strongly, if made into a lens of the same shape, than crown glass. However, not only does it bend light more strongly, but the difference in how it bends lights of different colors is also more pronounced, even in proportion to the increased amount of bending.

Thus, two lenses close together, one of flint glass, and one of crown glass, bending light in opposite ways, can be made so that the difference in bending different colors cancels out, but the lens itself still performs a net function of bending the light that goes through it in one direction.

However, because the behavior of light of different colors in glass doesn't follow a simple fixed law, this cancellation can only be exact for two colors. This still gives a great improvement over a plain lens. But sometimes a larger improvement is desired, and then a lens is designed out of three elements of three different kinds of glass. Such a lens is called an apochromat, and these are used as objective lenses on some more expensive telescopes.

The objective lens on a telescope, instead of being like an ordinary magnifying glass lens in profile, equally fat on both sides, usually has a crescent-shaped profile, and such a lens is called a meniscus lens.

This is done to minimize another aberration, called spherical aberration. The bending of light by a lens is due to the mathematical law called Snell's law, and is due to the fact that light travels more slowly in glass than it does in air.

A spherical surface is relatively easy to produce when grinding lenses, but it is only an approximation to the shape of a surface that would focus incoming rays of light to a single point in an image.

Sometimes, particularly when many lenses are being manufactured for a fixed purpose, by being molded from plastic, it is worth the expense to make the mold the exact shape needed to produce the ideal surface for bending light to form an image. Lenses like this are called aspheric lenses. Sometimes such lenses are even made from glass for special purposes, but such lenses are expensive, and thus not commonly used.

The term aspheric, because it means "not spherical", is sometimes applied to other kinds of lenses which are not as difficult to make. They still have curved surfaces which are circles instead of the complicated curves needed to make perfect images. For example, you may have seen cylindrical lenses that can make a line of printing taller, even though they don't make it wider. Such lenses can make optical instruments that do one thing in one direction, and a different thing in another.

Another application of this is eyeglasses. The lenses in eyeglasses are usually toroidal rather than spherical, so that they can correct not only for a wrong overall focal length in the lens of the eye, but for differences in that focal length in different directions, or astigmatism.

Usually, telescope objective lenses in refracting astronomical telescopes don't use aspheric elements.

A refracting telescope is illustrated below:

A thin lens has less spherical aberration than a thick one. Even after correcting for chromatic aberration has made the two elements of the objective considerably thicker, spherical aberration is still fairly low. Making the objective a meniscus lens minimizes it, because then the overall shape of the lens follows the curve of the surface where the incoming rays of light would be bent into their new desired direction while retaining a uniform spacing between them. (Of course, why that should make a difference is complicated.)

But the next common type of telescope, the Newtonian telescope, does usually make use of an aspheric element. In the Newtonian telescope, the place of the objective lens is taken by a concave mirror, which can magnify and form images in much the same way as a convex lens. An extra mirror, a small flat mirror called the diagonal, is used to keep the head of the person using the telescope out of the way of the incoming light.

This type of telescope is illustrated below:

The fundamental types of reflecting telescopes are illustrated in the diagram below:

The fundamental types of reflecting telescopes are illustrated in the diagram below:

T.: Please, answer the questions:

What are the kinds of telescope?

What kind of telescope is a reflecting telescope?

What kind of telescope Ptolemy use?

Who invented telescopes & when?

 What are their uses or applications?

 What are the different kinds of telescopes?

 How are they made?

 How far can we see with telescopes?

 What are the best locations for telescopes?

 What & where is the largest, or most powerful telescope?