Transcript Document
Telescopes Chapter 3 Objectives • To know the general types of telescopes and the advantages and disadvantages of each one. • To know the primary parts and functions of each part of a telescope. • To know the importance of the diameter of the objective and to know how the magnification of a telescope is related to the focal lengths of the objective and eyepiece. • To know the advantages and disadvantages of earth and space-based telescopes. Galileo Galilei (1564-1642) History • invented by Dutch lens maker in 1608 • Galileo: small 30X scope • Observed the moon and “began” the modern age of Astronomy where measurement was more important than philosophy Galileo noticed • • • • moons orbiting Jupiter phases of Venus craters on the moon sunspots This was strong evidence that Copernicus was right although Galileo wasn’t willing to die for it. How a telescope works • gathers light through the objective (mirror or lens) – bigger is better because it gathers more light – ability to see faint objects increases proportionally with the square of the radius of the objective • focuses light • viewed through an eyepiece (changing the eyepiece changes the magnification) • magnification is the ratio of the focal length of the objective to the focal length of the eyepiece General types of telescopes • Refracting (objective is a lens) • Reflecting (objective is a mirror) – Newtonian – Cassegrain • Catadioptrics – uses mirrors and lenses – Schmidt-Cassegrain – Maksutov-Cassegrain Refractors (glass lens) Advantages and Disadvantages • Easy to use and reliable • Excellent for lunar, planetary and binary star observing especially in larger apertures. • High contrast images with no secondary mirror or diagonal obstruction. • Sealed optical tube reduces image degrading air currents and protects optics. • More expensive per inch of aperture • Heavier, longer and bulkier than equivalent aperture Newtonians and catadioptrics. • Small apertures • Less suited for viewing small and faint deep sky objects. • Color aberration due to colors of light bending different amounts. Reflectors (mirror) Advantages and disadvantages • Lowest cost per inch of aperture • Reasonably compact and portable up to focal lengths of 1000mm. • Excellent for faint deep sky objects such as remote galaxies, nebulae and star clusters. • Reasonably good for lunar and planetary work. • Low in optical aberrations. • Open optical tube design allows image-degrading air currents and air contaminants • More fragil • Large apertures (over 8") are bulky, heavy and tend to be expensive. • Slight light loss due to secondary obstruction when compared with refractors. Cassegrain reflector Catadioptric telescopes • Best all-around, all-purpose telescope design. Combines the optical advantages of both lenses and mirrors while canceling their disadvantages. • Sharp images over a wide field. • Excellent for deep sky observing or astrophotography with fast films or CCD’s. • Very good for lunar, planetary and binary star observing or photography. • Closed tube design reduces image degrading air currents. • Most are extremely compact and portable. • Large apertures at reasonable prices and less expensive than equivalent aperture refractors. Schmidtt-Cassegrain Problems with earth-based telescopes • Earth’s atmosphere reflects certain wavelengths – x-rays, gamma rays and most UV light is not transmitted by our atmosphere • Earth’s atmosphere blurs images – the bending of light by the atmosphere depends on the temperature of the “air” – “twinkling” (shimmering) effect • “Light pollution” • Solution? Put the telescope in space. QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. http://imagine.gsfc.nasa.gov/docs/science/know_l1/emspectrum.html Disadvantages of space-based telescopes • Expensive to launch and maintain • Difficult to repair • Low lifetime Examples of space-based telescopes • Hubble Space Telescope – 3 times better resolution – can see fainter objects • Chandra X-ray Observatory • Compton Gamma-Ray Observatory