The capacity of a telescope to focus light is dependant on many features such as the aperture, magnification, focal length and ratio, not to mention wave error and resolution. These are the important features to bear in mind when purchasing a telescope. Purchasing quality telescope is important as it vastly improves the quality of the images and pictures you will produce.

Basically, an aperture is a hole or an opening through which light is admitted. But more importantly an aperture determines the quality of the light waves admitted into the telescope. The narrower the aperture the sharper, but darker the resultant image maybe. Thus the aperture relates to the telescopes ability to focus light. The focusing also depends on the size of the lens or mirror. The bigger the lens used, the brighter the image. Aperture is hugely important, but bigger isn’t always best, so take other features into consideration, too.

Magnification depends on a variety of things namely the lenses and distance to the eyepiece. As a guide, 40-60x per inch of aperture is sufficient. Low magnifications give the best light in most cases.

Focal length. The longer the focal length - the distance the light travels from the lens to eyepiece, the higher the magnification. Focal length is not the same as the length of the telescope. Rather, it is the optical length of the telescope's lens. Compound telescopes actually fold the path of the light making the optical length longer than the length of the tube. Eyepieces have focal lengths, also (typically between 4-50mm). To calculate a telescope's magnification power, you divide the focal length of the telescope by the focal length of the eyepiece.

Focal ratio tells you the relative brightness and field of view. The focal ratio is defined as the relationship between the focal length and the aperture. A 10-inch scope with the focal length of 100 inches would have the focal ratio of 10. This would be expressed as f/10 or f10. Similar to cameras, the lower the focal ratio the faster (brighter) the optical system. Lower focal ratios have gentler curve to their optical elements and produce less optical aberrations.

F/10 and above is for viewing the moon, planets and stars. F/8 meets most general purpose requirements and F/6 and below is for viewing far distant objects.

The wave error gives you a measurement of the the quality of the mirror or lens in the telescope. A wave error occurs when the light's wavelength is shifted or othe distortions occur. The wave error is epxressedas a fraction and the smaller the number, the more perfect the quality. The overall rating number for a telescope represents the accumulated wave error numbers of each lens or mirror; an absolute minimum is one-fourth. A wave error of 1/4 is typical of many mass-produced telescopes.

Resolution. The better resolution that a telescope has the better the clarity of the details of the images. Resolution also reflects the telescopes ability to separate two close objects - say two moons of Jukopitor whose orbits are crossing.

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