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A telescope is an instrument that collects and focuses light. The nature of the optical design determines how the
light is focused. Some telescopes, known as refractors, use lenses, .and other telescopes, known as reflectors
(Newtonians), use mirrors.
Developed in the early 1600s, the refractor is the oldest telescope design. It derives its name from the method it
uses to focus incoming light rays. The refractor uses a lens to bend or refract incoming light rays, hence the name
(see Figure 3-1). Early designs used single element lenses. However, the single lens acts like a prism and breaks
light down into the colors of the rainbow, a phenomenon known as chromatic aberration. To get around this
problem, a two-element lens, known as an achromat, was introduced. Each element has a different index of
refraction allowing two different wavelengths of light to be focused at the same point. Most two-element lenses,
usually made of crown and flint glasses, are corrected for red and green light. Blue light may still be focused at a
slightly different point.
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To focus your telescope, simply turn the focus knob located directly below the eyepiece holder (see Figures 1-1).
Turning the knob clockwise allows you to focus on an object that is farther than the one you are currently observing.
Turning the knob counterclockwise from you allows you to focus on an object closer than the one you are currently
observing.
Note: If you wear corrective lenses (specifically glasses), you may want to remove them when observing with an
eyepiece attached to the telescope. However, when using a camera you should always wear corrective lenses to
ensure the sharpest possible focus. If you have astigmatism, corrective lenses must be worn at all times.
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You can change the power of your telescope just by changing the eyepiece (ocular). To determine the
magnification of your telescope, simply divide the focal length of the telescope by the focal length of the eyepiece
used. In equation format, the formula looks like this:
Focal Length of Telescope (mm)
Figure 3-1
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