Telescope Notes

Part I

A simple lens has two purposes: they bend light and separate the wavelengths. If two prism are aligned opposed to each other, the light will tend to come to focus. A lens can be made by simply smoothing this arrangement of prisms. The point at which the light focuses is the focal point and the distance from the lens of the focal point is the focal length. Johannes Kepler developed the first usable Refracting Telescope. Two lenses are required in the usual refractor. The large lens is called the objective lens and it's purpose is to collect light. The larger the objective, the more light can be collected. The second lens is called the eyepiece lens and it is used to magnify the image.

Telescopes only have three purposes. The first is to gather light. The eye uses the pupil to gather light. But a telescope has the entire area of the objective to use in collecting light. Now the area of the objective goes as the square of the diameter. So by doubling the diameter of the objective, you collect four times as much light and thus can see fainter objects.

The second purpose of a telescope is to magnify objects. Stars cannot be magnified since they are points of light. But planets, the Sun, the Moon, nebulas, gas clouds, and galaxies are all extended objects and will pay to be magnified. Magnification reveals greater detail. The focal length of the objective divided by the focal length of the eyepiece gives the magnification. So to increase the magnification, insert a shorter focal length eyepiece. Very high magnification usually is not usable because of the turbulence of the Earth's atmosphere, through which we must observe.

The third purpose of the telescope is resolving power - how close together can two objects be and still be able to tell that there are two. We measure resolving power in seconds of arc. Larger telescopes have intrinsically better resolving power, but a limit is imposed by the atmosphere, which causes the point star images to spread out and appear to have size. The advantage of the Hubble Space Telescope is that it is above the distorting influence of the atmosphere and therefore has a higher resolving power.

The simple refractor has two major problems: chromatic aberration and spherical aberration. Chromatic aberration exists because different colors bend by different amounts. The reds don't bend as much as the blues so the blue colors focus shorter than the reds. The cheapest way to build a lens is with sides that are sections of spheres. Kepler showed that this is the wrong shape. Rays coming through the center of the lens have a different focal length than the rays coming through the edges of the lens. This problem is called spherical aberration. Early telescopic astronomers found that by extending the focal length, they could minimize both of these problems. The first 100 years of telescope use employed these "long" telescopes to minimize the chromatic and spherical aberrations.

The solution for both of the major aberrations is to use lens combinations. A converging, crown glass lens is used in combination with a diverging, flint glass lens to eliminate chromatic aberration. Careful attention to detail can, at the same time, greatly reduce the effect of spherical aberration. The achromatic doublet allowed astronomers to concentrate on making telescope objectives bigger. As refractors got bigger more problems were uncovered.

· Support of the lens can be done only along the rim

· Imperfections in the pure act as sources of scattered light

· Light is absorbed

· Glass flows over time

Newton knew of these disadvantages for the lens and turned to mirrors for building telescopes. The light is not passing through the mirror so chromatic aberration cannot exist. The technology has long existed to make the mirror the correct shape (parabolic) so that spherical aberration does not exist. However, there are problems associated with the Reflecting Telescope. Notice that all reflecting designs block a fraction of the incoming light. This can be minimized in modern designs. Newton worked with a large variety of materials for the objective mirror and settled on "speculum metal" - literally mirror metal. But metals reflect only about 16% of the incident light compared with 80% light transmitted for the early lenses. Metal mirrors also corrode easily and give the stars the characteristics yellow color of the metal, so that all of the color information is lost. Not until vacuum technology developed in the 19^th century would astronomers turn to reflectors in large numbers.

Large reflecting telescopes have none of the problems of large refractors. Support is done along the back side of the mirror. Light does not pass through the mirror, so imperfections in the pour or absorptions are not problem.

The major designs of reflectors in use today are:

· Prime Focus - used for monitoring programs and camera. Only useful on very large telescopes.

· Newtonian - used for visual work or small cameras

· Cassegrain - moderately large instruments placed here - photometers, spectrographs, cameras

· Coudé - only used on very large telescopes with very large instruments