... lenses (greater than 4 inches or 10 centimetres) for refractors. Refractors are relatively expensive, if you consider the cost per unit of aperture. Because the aperture is limited, a refractor is less useful for observing faint, deep-sky objects, like galaxies and nebulae, than other types of telescopes. Reflecting telescopes Reflecting telescopes use mirrors instead of using lenses. The reflector is simple and inexpensive to make. Large aperture primary mirrors (greater than 10 inches or 25 centimetres) can be made easily, which means that reflectors have a relatively low cost per unit of aperture. Reflectors have large light gathering capacities, and can produce bright images of faint, deep-sky objects for visual observing as well as astrophotography. One disadvantage of reflectors is that you occasionally have to clean and align the mirrors. Also, slight errors in grinding the mirrors can distort the image. Here are some of the common problems: * Spherical aberration - ...

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