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. focus rays

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. 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. white light blue red

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. Obj. F.L. f.l. 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 2 1 / 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. Obj. F.L. f.l. E.P. } Exit pupil 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. = log Aperture (in.) Example: For an 8  telescope, Mag. lim. = log (8) = (0.903) = = 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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