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Astronomical instruments

This is the basis of the REFRACTING telescope. The light rays are bent or refracted by the lenses.

Galileo’s telescopes were quite small: Here is a photograph of it on the left, and, to show you the size, the photograph of a replica of it on the right.

and here is Galileo's drawing of the moon:

In 1672, Sir Isaac Newton realized that magnifying could be done by a mirror system instead of a lens system. The way it works is shown here:

In this arrangement the main mirror has a concave shape. This shape reflects parallel light rays to a point F1, called the focal point.

But to get the system to work, a flat (or plane) mirror reflects the rays to the side where they are made parallel again by the eye lens.
This arrangement is called a reflecting telescope. Here is what Newton’s telescope looked like. It was 6 inches in diameter.

This means that there are two basic types of telescope;

(1) the refractor which bends the light to a focus by lenses; and

(2) the reflector which brings light to a focus by mirrors.

There are problems with both types of telescope. These problems have to be overcome in special ways.

Problems with the refractors.

First, refractors will normally have a rainbow of colors or at least several colors around the object being looked at.   This is because light is broken up into its colors when it is bent.  When rain does this, and the sun is shining through it, we see a rainbow.  Prisms do the same things

This is what happens with this type of telescope.

The colored fringes around everything are called chromatic (color) aberration. When you look at something through this system, here is the sort of effect you see.

It is overcome by using lenses made of special sorts of glass. The refractors with special glass are called a-chromatic refractors. But they usually still have one color fringe - usually violet like this:

The very best corrected lenses are called apo-chromatic refractors and they overcome the problem completely.  For telescopes this is very expensive. 

The second problem is more practical.  The glass lenses in refractor telescopes are very heavy. In addition, the telescopes must be very long. These two things limit the maximum size of this type of telescope. The largest refractor telescope that has ever been constructed is at the Yerkes Observatory in Wisconsin. It is 40 inches wide, 60 feet long, and weighs 20 tons.  Alvan Clark started building this telescope in 1893 and completed it two years later.   It was installed in 1897 and still in use. It is currently operated by the University of Chicago.  They had to build a special observatory for a telescope this large.  When the telescope is raised the floor is constructed so it will drop down  to allow the telescope room to move. Here are some of photos of the great Yerkes telescope:

Problems with reflectors.

Reflector telescopes have no problems with color (or chromatic) aberration. In fact, it was because Sir Isaac Newton was looking for a way to overcome this problem that he invented the reflector. He knew that this type of telescope would be completely free from the color problem. However, these telescopes have problems of their own. All the early primary mirrors were made so that their reflecting surface was part of a sphere. But a surface which is part of a perfect sphere does not bring all the incoming parallel light rays to the same focus. Here is what the mirror does:

 

 

This effect is called spherical aberration.  The result is a blurry image, like this one on the left from the Hubble Space Telescope before it was corrected. The image on the right was after its correction for spherical aberration.

 

 

The way of overcoming the problem is to deepen the curve on the primary mirror so that it is “parabolical” instead of “spherical” as shown below.

In actual practice even a parabolic mirror is not actually perfect, so there are a number of ways of overcoming this problem. A specially shaped glass plate can be used at the entrance to the telescope to completely overcome the problem. This was done with the Schmidt Camera telescope. It has a spherical mirror surface but a corrector plate corrects for all spherical aberration. It looks like this

The other practical problem with reflectors is that the eyepiece is usually near the front of the telescope, which is difficult to get to when it is pointing straight up. This is called a Newtonian reflector after Newton. In order to overcome this problem, an additional mirror is inserted in the tube which reflects the light back down to a hole in the mirror. The eyepiece is then at the bottom of the telescope instead of at the top. This type of reflector is called a Cassegrain telescope. Its length is drastically reduced compared with its diameter (width). Ordinarily, the larger the diameter, the longer the telescope must be. But the Cassegrain design overcomes this problem. The result is that Cassegrain telescopes can be very compact. Here is a comparison:

Above is the Newtonian telescope

Below is the Cassegrain telescope

The Best Observatory Telescopes:

The brightest image is needed.

In order to get the brightest image, the diameter of the telescope must be as great as possible. The greater the diameter of either a refractor or reflector, the more light it gathers and so the brighter the image appears. This means that you can magnify the image more and still see it as being bright. The brighter the image is, the more clearly you can see the detail. This is why the world’s best telescopes are as large as possible.  This means that they are all some form of reflector. The largest refractor (Yerkes) is 40 inches across. The largest reflectors are the two Keck telescopes on Mauna Kea, Hawaii, both at 33 feet diameter and standing 279 feet apart.

 

To overcome the twinkling problem, there is a sensor telescope which points at a guide star in the field of view that the main scopes are pointed at. This sensor scope monitors the twinkling of the guide star and sends the information to a computer which corrects for the twinkling. If a guide star is not available, a laser beam is sent up from the sensor and this does the same thing.

The photograph above was taken by John Bunyan, a friend of ours, when he visited the Keck Observatories. This shows the laser beam being used as a guide.

When the beam is aimed up, the sensor and its computer correct for the twinkling in a fascinating way. Each of the 36 smaller segments of the main mirrors is on a motorized support which is computer activated. The computer changes the shape of the main mirror by adjusting the position of the 36 segments 670 times per second. In this way a very stable, sharp image results. In other words, we can get images from the Kecks which are almost as good as those from space.

 

 

Other Telescopes

Some reflecting telescopes use optical system which contain a variety of the elements discussed above. Some of the most popular telescopes among amateurs are a mixture of the Schmidt system and the Cassegrain system. They are called Schmidt-Cassegrains or SC telescopes. They have a spherical mirror and corrector plate (sometimes called a corrector lens) at the front like a Schmidt camera, but also have a folded light path like the Cassegrain. Here is their optical arrangement:

 

 

 

These telescopes are very portable, so can go anywhere, but they are also quite powerful. This makes them ideal for amateurs. They range from 6” to 16” diameter.  Because they are able to ‘fold’ the light path with mirrors, what would normally need to be a five foot telescope can be shortened to this:

 

 

What you can use

It is pretty unusual for a family to go out and buy a really good  telescope.  They are pretty expensive.  But there is something you can afford, and maybe you already have, which is as good as a small telescope:  your binoculars.

 

 

Binoculars are simply small refractors. They allow us to see in stereo. They give superb wide-angle views of star-fields.  Most good binoculars are better than Galileo’s telescopes. In fact, it is now becoming popular to make medium sized telescopes in binocular form as well. The 3-D vision they give is stunning.

The diagram showing what happens with binoculars is here:

 

 

Remember how prisms break up light into rainbows?  Why doesn’t that happen with these prisms?  There are two reasons:  First, the second prism helps ‘put the light back together,’ and second, front lenses (at the bottom in the picture) and the prisms have a special coating to help keep the light from breaking up.  In the picture of the binoculars, you will see the eyepieces look yellow-orange.  This is the coating.

The front, or objective, lenses collect the light which is magnified in the eyepiece system. The wider the objective lenses, the more light they can collect.  The prisms simply fold the light path so that the light comes to a focus in a short distance so the binoculars are not too large.

Here’s a BIG binocular telescope!

 

Challenge Question: We have looked at the types of optical systems that telescopes use. See if you can discover from the internet the types of mounts that the telescopes are supported by. These telescope mounts must allow the telescopes to point to every part of the sky. Hint: there are two main types of mount, and several types less frequently used.

NOTE:
The light we see is only a small part of the entire electromagnetic wave spectrum:

There are other types of telescopes for different wavelengths – like radio telescopes for radio waves; X-ray and gamma ray telescopes for X-rays and gamma-rays.  Since we are doing astronomy here, we are only dealing with the light part of the spectrum, but some of you might be interesting in looking up some of the other telescopes used for other parts of the spectrum.   

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