Transcript prime focus
Methods of Observation A telescope is a “light bucket.” Large telescopes work better than smaller ones because they collect more light. Optical telescopes gather visible wavelengths. Two basic types: Reflectors – curved mirrors. Refractors – curved lenses. Both of them focus light to a point called the prime focus. The distance from the mirror or lens to this point is the focal length. There are five basic problems, unique to lenses, that make refractor telescopes inferior to reflectors. (Especially very large telescopes. Small refractor telescopes work just fine.) Lenses in refractors bend different colors of light different amounts. It splits light into its spectrum to some degree. This undesirable splitting of light is called chromatic aberration. It is a problem with lenses that increases with the size of the lens. Lenses also block much IR and UV radiation. In addition, lenses are heavy and must be at the top of the telescope. Since the lens can only be supported around the edges, the lens tends to deform under its own weight. Finally, a lens has two surfaces that must be polished. Mirrors, on the other hand, (1) do not cause chromatic aberration, (2) do not block any type of light, (3) are put at the bottom of the telescope, (4) can be supported across the entire back so the mirror does not deform, and (5) only have one surface to be polished. Because of these facts, all large telescopes are reflectors. The largest refractor is at the Yerkes Observatory and is 1 meter in diameter. Most large reflectors are between 4 and 6 meters across. There are four different reflector telescope designs. All have a primary mirror; the difference is where the light rays go after this mirror. In a prime focus reflector the instruments are placed at the prime focus. This is not a good system for bulky instruments. In a Newtonian focus, the light is deflected 90° to an eyepiece before it reaches the prime focus. This is the normal design for smaller reflectors. A Cassegrain Focus telescope reflects the light back through a hole in the primary mirror. This allows heavy equipment to be used easily . In a Coudé Focus, a third mirror intercepts the light before it returns to the primary mirror and deflects it to the side. The light then goes to a laboratory called a coudé room. This is used for very large or precise equipment. One telescope that fits most of these types is the Hale 5-meter diameter telescope on Mount Palomar in California. Construction was completed in 1948, and it was the largest telescope in the world for almost 30 years. Observations can be made at the prime, the Cassegrain, or the coudé focus.