Hubble’s mirrors are made of ultra-low expansion glass kept at a “room temperature” of about 70☏ (21☌) to avoid warping. There it is intercepted by pick-off mirrors that pass it into the scientific instruments. This secondary mirror then reflects the light through a hole in the primary mirror to form an image at the telescope’s focal plane. Hubble’s 1,825 pound, 7.8-foot (2.4-meter) diameter primary mirror collects light from its astronomical target and reflects it to a 12-inch (0.3-meter) diameter secondary mirror located in the optical tube. The light is focused on a small area called the focal plane, where it is picked up by its various science instruments. The secondary mirror bounces the light back to the primary mirror and through a hole in its center. Light from celestial objects travels down a tube, is collected by a bowl-like, inwardly curved primary mirror and reflected toward a smaller, dome-shaped, outwardly curved secondary mirror. Hubble is a Cassegrain reflector telescope. The telescope has beamed hundreds of thousands of celestial images back to Earth during its time in space. In addition to observing visible and near-infrared light, Hubble detects ultraviolet light, which is absorbed by the atmosphere and visible only from space. Orbiting high above the Earth, the Hubble Space Telescope has a clear view of the universe free from the blurring and absorbing effects of the atmosphere. Four Successful Women Behind the Hubble Space Telescope's Achievements.Characterizing Planets Around Other Stars.Measuring the Universe's Expansion Rate.Secondary mirror can cause diffraction of original incoming light rays causing the "christmas star effect" where a bright object have spikes.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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