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Telescope – Beginners Guide: Refractor January 1, 2010

Posted by aquillam in Astronomy, telescope beginners guide.
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The white tube to the left and black tube near the middle are amateur refractors. Toward the right is a little orange reflector or the same diameter as the white tube telescope.

Refractors are the simplest telescopes in design. They use a large objective lens to collect the light, and an eyepiece to focus it.

There are two basic types of refractors. The usual type is a Keplerian, or some variation on it. The other type is called Galilean, because this was the design used by Galileo.

Galilean Refractors

A galilean telescope has a diverging lens for an eyepiece, so it must be placed close to the objective, before the focal point. The length of this telescope is actually the difference on the focal lengths of the two lenses, F-f.

In a Galilean refractor, the objective is usually a  long focal length lens, and the eyepiece is placed closer to the objective than the focal length. The eyepiece of a galilean telescope is a diverging lens, so it takes the converging light from the objective and makes it parallel, which makes it easier for our eyes to focus.

This is the usual design for spyglasses and day-time ‘scopes because it produces an upright image.  However, it is a very narrow exit pupil and poor eye relief, so your eye must be perfectly aligned to see anything. Also, it has a very small field of view. Most galilean telescopes can’t fit the entire Moon in the field of view, even at the lowest magnification.

Keplerian and other Refractors

The keplerain telescope has a diverging lens for an eyepiece. The distance between the objective and eyepiece is the sum of the focal lengths of both lenses, F+f, so it is longer than a galilean telescope.

Kepler realized if he switched the  converging lens in the eyepiece to a diverging lens, he could get a wider field of view. This design also gives a wider exit pupil, so it is easier to look through the telescope.  However, the image is upside down, so this makes a poor terrestrial telescope.

Problems Common to all Refractors

All refractors have a few problems in common.  There are the obvious problems, such as imperfections in the glass or the shape of the lens.   Large lenses can also sag under their own weight, so very large refractors are particularly hard to build. Light is also absorbed and reflected by glass (think about trying to look outside at night from a brightly lit room). Thin lenses mitigate many of these problems, but the thinner the lens, he longer the focal length, which makes for really unwieldy telescopes.

Spherical Aberration

Spherical aberration: Light coming through the edges focuses farther from the lens than the light passing closer to the center

The objective is normally a spherical lens (meaning each surface is shaped like a section of a sphere, like the overlapping area in a Venn diagram with two sets.) The focus of a spherical lens isn’t actually a point, but a line. The light passing close to the center of the lens focuses at a shorter distance than light passing farther out, so a sharp image isn’t possible.

The shorter the focal length of the lens, the worse the aberration is, so one solution is to make q very long focal length lens. Either choosing a different type of glass, or  making the lens very thin will increase the focal length. Unfortunately, this will result in a very long tube.  Some early refractors were so long, they took several supports, and several people to operate the telescope.

Another option is to use non-spherical or compound lenses. Non-spherical lenses are very hard to make, so they tend to be prohibitively expensive.  Compound lenses are easier, and can solve the other major problem, chromatic aberration, as well.

Chromatic Aberration

Chromatic aberration: red light bends more than blue, so it focuses closer to the lens than blue light.

The shorter the wavelength (the more blue) the light is, the less it bends when it passes through a lens, so the farther away from the lens it focuses.  This means if you bring the red light into perfect focus, but the green and violet light will be out of focus, and usually results in a blue or purple halo around the image.

Achromatic compound lenses were the first solution to this problem, developed in the 18th century. Each half of an achromatic lens is made of a different material. One half of the lens works best with one color, usually red, and the other half works with another color, usually blue. The two different colors then focus at the same point. Many of the great refractors of the 19th century were this design, and this is the common solution for good refractors.

Apochromatic lenses combine several lenses made of different materials designed to bring three colors, usually red, green and blue, into focus at the same point. This produces a clearer image than the achromats. Most are also designed to correct spherical aberration as well.  However, the extra elements make them undesirable for large refractors. Not only are they usually heavy, but every element will absorb some of the light, making the image slightly dimmer. They are generally found in high-end amateur telescopes, especially those designed for photography. These are frequntly the favorite ‘scope for amateurs who prefer imaging solar system objects.

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