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TELESCOPES

5

refracting
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A CLOSER LOOK AT REFRACTORS

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Achromatic Refractors

apochromatic refracting
The refracting telescope is particularly good for seeing detail in the moon
and planets. It's long focal ratio (dimensions) give it a high magnification
capability.
Refractors gather more light than reflectors of the same size, but because of their cost mainly, they are not made to the same sizes as reflectors. So most refractors won't gather as much light as big reflectors, but this is not necessary for viewing the planets because they are already very bright. The other advantage the refractor has over a reflector is it has no secondary mirror. A reflector's secondary mirror acts as an obstruction because it's in front of the telescope's main mirror. This obstruction degrades the image slightly, especially if the telescope has a short focal ratio. The shorter the focal ratio, the larger the secondary mirror is in relationship to the main mirror.
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Refractors have comparatively little maintenance compared to reflectors. They're more like looking after a pair of binoculars.
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The first refractors were known for their chromatic aberration. This is where the white light is broken up into it's different colours it's made up from much like a rainbow. They are also known for their spherical aberration. This is where the curvature of the lens causes differences in distance between the center of the lens to the focus point, and the outer edges of the lens to the focus point, once again giving a poor image.
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The development of the refractor has brought about lenses of different materials being glued together which help correct the aberrations. Different types of materials defract (bend) light at different angles. This system of gluing the lenses together is called an achromatic lens, hence the name achromatic refracting telescope.
Apochromatic Refractors
The lensing system in a apochromatic refractor eliminates just about all aberration. They are the best telescope you can buy for observing fine detail in the planets and the Moon. Unfortunately they are also the most expensive type of telescope per millimeter of aperture (size). Apos, as they are known, use multiple elements (up to four) in their main lens made form extra low dispersion (ED) glass and/or fluorite. Because apos can be corrected for aberration so well, they can be manufactured in shorter focal ratios.
refracting
Apos, because of their sealed tubes and multi-element glass with fluorite, can take longer for them to adjust to different temperatures when taking them from inside to outside. The different temperature will cause the glass to expand at different rates which will give a poor image until the glass is the same temperature as outside.
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