Types of Telescopes
All telescopes fall into one of three optical classes. The relative advantages of each of these telescope designs will be made clear below.
 In the refracting telescope (a) light is collected by a 2-element objective lens and brought to a focus at F. By contrast the reflecting telescope (b) uses a concave mirror for this purpose. The mirror-lens, or catadioptric, telescope (c) employes a combination of both mirrors and lenses, resulting in a shorter, more portable optical tube assembly. All telescopes use an eyepiece (located behind the focal point, F) to magnify the image formed by the primary optical system.
In the refracting telescope (a) light is collected by a 2-element objective lens and brought to a focus at F. By contrast the reflecting telescope (b) uses a concave mirror for this purpose. The mirror-lens, or catadioptric, telescope (c) employes a combination of both mirrors and lenses, resulting in a shorter, more portable optical tube assembly. All telescopes use an eyepiece (located behind the focal point, F) to magnify the image formed by the primary optical system.
Refracting Telescopes use a large objective lens as their primary light-collecting element. Meade refractors, in all models and apertures, include achromatic (2-element) objective lenses, in order to reduce or virtually eliminate the false color (chromatic aberration) that results in the telescopic image when light passes through a lens. Example: Meade NG-60, NGC-60, ETX-70AT, DS-2070AT.
Reflecting Telescopes use a concave primary mirror to collect light and form an image. In the Newtonian type of reflector, light is reflected by a small, flat secondary mirror to the side of the main tube for observation of the image. Example: DS-2114ATS, DS-2130ATS.
Mirror-Lens (Catadioptric) Telescopes employ both mirrors and lenses, resulting in optical configurations that achieve remarkable image quality and resolution, while housing the optics in extremely short, highly portable optical tubes.
Example: Meade ETX-90AT, ETX-105AT, ETX-125AT, LX200GPS Series.
Achromatic Refractors
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 neccassary 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 seconday 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.
Basics
Mounts
Light Gathering Power
Closer Look At Reflectors
Dimensions
Refractors have comparitively little maintenance compared to reflectors. They're more like looking after a pair of binoculars.
Balancing
Eyepieces
Cleaning
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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Build your own telescope
Build your own telescope
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Binoculars
The development of the refractor has brought about lenses of different materials being glued together which help correct the abberations. Different types of materials defract (bend) light at differnt 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 abberation. 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 aperature (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 flourite. Because apos can be corrected for abberation so well, they can be manufactured in shorter focal ratios.
Apos because of their sealed tubes and multi-element glass with flourite, 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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