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New technology has created chargecoupled devices (CCDs) to gather light more efficiently than photographic plates. These devices produce images with astounding detail. Shown in this picture are the intricate details of the Rosetta Nebula 5000ly away. Ordinary Photography Versus CCDs ORDINARY PHOTOGRAPH NEGATIVE NEGATIVE USING CCDs COMBINED CCD IMAGES USING COLORED FILTERS REFRACTING TELESCOPES Light rays traveling into a transparent medium such as glass bend at the surface. The bending of light rays between two transparent media is called refraction. If the lens is curved, parallel rays will converge at a focal point, just like the rays in a reflecting telescope. 1 Like reflecting telescopes, extended objects produce images in a focal plane. Refracting telescopes use an objective lens to gather light and an eyepiece through which the image is viewed. Limitations of Refracting Telescopes •Different colors of light are refracted differently and have different focal points. Thus, all the colors of the image will not be focused at once. This is called chromatic abberation. •It is difficult to grind a lens into the proper shape to have all parallel rays converge at a single focal point. •The weight of a large lens can cause the lens to sag and distort the image. •Air bubbles in the glass cause unwanted refractions, distorting the image. THE LARGEST REFRACTING •Glass is opaque to certain wavelengths of light, TELESCOPE AT YERKES OBSERVATORY meaning they do not go through the glass. These issues do not affect reflecting telescopes because the light from the stars does not travel through a glass lens before being focused. However, reflecting telescopes do have some problems. One problem is that the secondary mirror used to deflect the light out the side partially blocks the light from the star. 2 Another problem with reflecting telescopes is called spherical aberration. When a sphericallyshaped mirror is used, the light rays hitting far from the center will not converge at the same point. One solution is to grind the mirror into a parabolic shape. Another solution is to use a correcting lens to make all the light rays converge at a single point. Making a Large Parabolic Mirror 40,000 pounds of glass are loaded into a rotated furnace and heated to 1500K. After melting, spinning and cooling, the surface is ready to be coated with a highly reflective material. Ground Based Telescopes are Limited Our atmosphere causes the stars to appear to shift in color and brightness. This cuases the star to appear to flicker. We call this twinkling. GROUND BASED TELESCOPES BLURRED BY TWINKLING TELESCOPES IN ORBIT ABOVE OUR ATMOSPHERE AVOID THIS PROBLEM 3 Light from cities scatters in our atmosphere reducing the visibility of celestial objects. This is called light pollution, and has been an increasing problem in recent years. The view from Kitt Peak National Observatory of the Tuscon, Tuscon, Arizona skyline in 1959 The same skyline in 1972 To avoid adverse effects of our atmosphere, we launch telescopes on satellites that circle the Earth above the atmosphere. THE HUBBLE SPACE TELESCOPE (HST) Using adaptive optics to calculate the amount of twinkling of our atmosphere, as well as changes to the shape of the mirror, we can receive better images from ground based telescopes. IMAGES OF SATURN GROUND BASED WITH NO ADAPTIVE OPTICS GROUND BASED WITH ADAPTIVE OPTICS HUBBLE IMAGE 4