Newtonians, like other reflecting telescope designs using parabolic mirrors, suffer from coma, an off-axis aberration which causes imagery to flare inward and towards the optical axis (stars towards edge of the field of view take on a comet-like shape).Combined with short f- ratios this can allow for a much more compact mounting system, reducing cost and adding to portability.ĭisadvantages of the Newtonian design Ī large Newtonian with ladder to reach the eyepiece The eyepiece is located at the top end of the telescope.A short focal ratio can be more easily obtained, leading to a wider field of view.Later achromatic refractor objectives had four surfaces that have to be figured). Since there is only one surface that needs to be ground and polished into a complex shape, overall fabrication is much simpler than other telescope designs ( Gregorians, cassegrains, and early refractors had two surfaces that need figuring.Newtonian telescopes are usually less expensive for any given objective diameter (or aperture) than comparable quality telescopes of other types.They are free of chromatic aberration found in refracting telescopes.The primary mirror makes it possible to collect light from the pointed region of the sky, while the secondary mirror redirects the light out of the optical axis at a right angle so it can be viewed with an eyepiece.Īdvantages of the Newtonian design Secondary mirror can cause diffraction of original incoming light rays causing the "christmas star effect" where a bright object have spikes.A Newtonian telescope is composed of a primary mirror or objective, usually parabolic in shape, and a smaller flat secondary mirror.Require frequent cleaning because the inside is expose to the atmosphere.Optical misalignment can occur quite easily.Easier to make a high quality mirror than lens because mirror need to only concern with one side of the curvature.More cost effective than refractor of similar size.The primary mirror is very stable because it is located at the back of the telescope and can be support in the back.The main advantage is reflector telescope can escape from chromatic aberration because wavelength does not effect reflection.Larger aperture size will yield high quality of image, more crisp and detail image, and increase clarity of the image. The size of aperture also accounts for the resolution power of the telescope. The aperture size is more important because it accounts for the amount of light the telescope receives and the more light, the brighter the image. However, magnification is not the most important optical property for viewing astronomical phenomenon. Typically a long objective focal length combine with a short focal length of the eyepiece lens will yield greater magnification. Magnification of a reflector telescope mainly depends on the distance of the eyepiece lens with regard to the focal point because the focal length determines how large the image get scale by. Refer to the following diagram to understand the components and processes in which light rays travel through a reflector telescope. However this image is also in reverse to the original image. Light rays will continue to travel and refracts at the eyepiece lens and forms an image that is magnified to people's perception. It is from that point where optics of refractors and reflectors are common. Once again a reflection will occur and all the rays will converge to a single point call the focal point. The primary mirror is shaped parabolically so that all incoming parallel rays will reflect off the mirror at their own specific angle and hit the surface of the secondary mirror. However instead of bending the rays through a glass lens immediately, rays are to travel to the back of the telescope where the primary mirror is located. Similar to refractors, light rays are gather through the aperture. Reflector telescopes uses two mirrors (primary and secondary mirror) as well as a glass lens (eyepiece lens) in their lens system. The main difference between a refractor and a reflector is the latter telescope involves both refraction and reflection of light rays. Telescopes - A UBC Math 309 Project - Reflective Telescopes
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