The Memphis Astronomical Society Presents A SHORT COURSE

The Memphis Astronomical Society Presents A SHORT COURSE in ASTRONOMY

CHAPTER 3 OPTICS and TELESCOPES Dr. William J. Busler Astrophysical Chemistry 439

A. Refractor Telescope • Refractors are based on the principle that light is refracted (bent) as it passes through a lens. • This is due to the slowing down of the light waves from 186, 000 mi/sec in a vacuum or air to about 1/2 to 2/3 of that value, depending on the type of glass. • As the waves pass through a convex lens, they are slowed more in the center than at the edges of the lens. This causes the waves to converge to a focus. waves .

A. Refractor Telescope • An image of the distant object is formed at the focal point; it may be projected onto a screen, a piece of film, or a retina, if the lens was the eye’s own lens. • The image may not be viewed directly, since the rays passing through the focal point will diverge; rays must be nearly parallel to be focused by the eye. rays focus

A. Refractor Telescope • The rays may be made parallel by passing them through a concave lens just before the focal point, as in a Galilean refractor. • This type of telescope has low power and a narrow field, and is used in opera glasses and field glasses. • The image is right-side-up. Galilean refractor

A. Refractor Telescope • A better way is to allow the rays to pass through the focal point; a convex lens (in an eyepiece) is then used to straighten the rays back to parallel. • This gives a wider field and can be used for higher powers. The image is inverted.

A. Refractor Telescope • Chromatic aberration: different colors focus at different points, making a terrible image. white light blue red • A compound lens made of different types of glass (crown and flint) is used to minimize chromatic aberration. • This makes refractors expensive—they must have four perfect surfaces, perfectly aligned.

A. Refractor Telescope • Other refractor problems: lenses are somewhat opaque and lose light. • They may have bubbles. • Large lenses sag under their own weight. • The largest refractor in the world, at the Yerkes Observatory, has a 40 -inch objective lens.

B. Reflector Telescope • A concave mirror reflects incoming parallel rays back to a focus. • There is no chromatic aberration, since all colors are reflected together. • The mirror must be parabolic in cross-section (nearly spherical). • It is easy to make large mirrors: there is only one surface to figure, there is no need for transparency, and a mirror can be supported from the back.

B. Reflector Telescope • Small reflectors need a diagonal mirror to divert light out to the side, so the observer’s head won’t block the incoming light (Newtonian design). • An eyepiece is used, as with a refractor.

B. Reflector Telescope • A finder scope is needed, preferably a small refractor of straight-through design. • A variation on the reflector design is the Cassegrain, which has a hole in the parabolic objective mirror through which the light is focused. secondary mirror

C. Magnification (Power) • This is overrated, especially in advertisements for cheap telescopes. • Power is adjusted by changing eyepieces. The shorter the focal length (f. l. ) of the eyepiece (E. P. ), the higher the power ( ) for a given objective. f. l. F. L. Obj. E. P.

C. Magnification (Power) • Example: A telescope has a 48”-focal-length mirror and is used with a 1/2" eyepiece. • Although magnification is theoretically unlimited, the practical limit is about 50 per inch of diameter (lens or mirror). • Example: A 21/4" refractor gives a maximum useful power of 2. 25 50 = 113.

C. Magnification (Power) • Eyepieces come in numerous types, depending on the desired magnification, field of view, budget, etc. • Kellner • Erfle • Ramsden • Orthoscopic • Huygens • Plössl • etc.

D. Focal Ratio (f/number) • Same as in a camera: • Example: An 8" mirror with a 56" focal length is f/7. • Lower f/ telescopes of the same diameter give a wider field of view and lower power with the same eyepiece. (The image is not brighter. ) • These telescopes are shorter than those with higher f/ ratios and therefore are somewhat easier to handle and transport.

E. Exit Pupil • The “exit pupil” is the diameter of the parallel beam of light rays coming out of the eyepiece. f. l. } Exit pupil F. L. E. P. Obj. Therefore, .

E. Exit Pupil • The exit pupil should not be greater than about 1/4", or the eye can’t see it all. Therefore, • Minimum power = = 4 Diam. (in. ) • Example: For an 8" telescope, the minimum power is 8 4 = 32. • (Recall that its maximum power is 8 50 = 400. )

F. Limiting Magnitude • The limiting magnitude of a telescope is expressed by the formula: Mag. lim. = 9 + 5 log Aperture (in. ) • Example: For an 8 telescope, Mag. lim. = 9 + 5 log (8) = 9 + 5 (0. 903) = 9 + 4. 5 = 13. 5.

G. Mountings • Telescope mountings are very important! • Equatorial: The polar axis is aligned with the Earth’s axis; the declination axis is perpendicular to the polar axis; therefore, the telescope can follow stars with one motion as the Earth rotates. • Altazimuth: Up-and-down or back-and-forth. Simple, but not recommended for astronomical use. Requires two simultaneous motions to track an object. • Whatever type of mounting is used, it must be sturdy, or images will be too shaky to view.

H. Relative Merits of Various Types of Telescopes • Refractor: No alignment is usually necessary; high power, limited to small size. High cost. Best for small, bright objects: Moon, planets, double stars. • Reflector: Frequently needs alignment (an easy job), and eventual resurfacing of the mirror. Low cost per inch; large is practical. Best for dim objects: nebulae, galaxies, faint clusters.

H. Relative Merits of Various Types of Telescopes • Hybrid (e. g. Schmidt-Cassegrain): Large diameter and long effective focal length due to convex secondary; high power, bright images. • Good for all types of observing. Main drawback: quite expensive! Many commercial SCT’s are rather poorly figured. Corrector plate “dews up” easily. corrector mirror

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