Telescopes Refractors Reflectors Catadioptrics

Transcription

1 Optical Aberrations As indicated by the history of telescopes, limits of design were encountered as the instrument was developed. Here are diagrams to better explain each aberration:

2 Telescopes Refractors In a refractor telescope, light passing through an objective lens is curved (refracted) in such a way that it is focused to the eyepiece. Early refractors suffered from color separation problems because light being refracted through a lens behaves much in the same way as light being refracted through a prism. The angle that the light refracts depends on the wavelength of light. Since white light is composed of the entire spectrum of colors (colors are merely different wavelengths of light) a simple lens tends to break light up into its different component wavelengths. This results in the various focusing at different points instead of all the same point. The result in the eyepiece is rings of color called chromatic abberation. Modern refractors use sophisticated designs that utilize multiple lenses and coatings in an attempt to minimize these effects. Reflectors In a reflecting telescope, such as a Newtonian, light enters the tubeand is focused to the eyepiece by a series of mirrors. Light striking the primary mirror is reflected off of its concave parabolic surface and is then relayed to the eyepiece by the secondary mirror. The principleof reflection is not dependent upon the wavelength of light, therefore all of the rays will reflect at the same angle regardless of wavelength. This provides for a well color corrected image. Reflectors however tend to require more maintenance than their refracting counterparts. Catadioptrics Catadioptric telescopes (Cats) use a combination of lenses and mirrors to gather light and compensate for the spherical abberation inherent in all-spherical optics. This tends to be a good compromise between the two older designs. Telescopes of this design are very popular today since they offer portability and large apertures e.g. Celestron, Meade, Bausch & Lomb, and Questar.

3 A few telescope designs

4 More Telescope designs Comparison of Refractor vs. Reflector Telescopes Reflector PRO totally achromatic ease of portability wide field of view ease of optic fabrication ease of mount fabrication lower initial costs Refractor PRO closed tube unobstructed aperture high contrast images holds collimation well no maintenance no coma CON central obstruction optical coatings deteriorate easily misaligned more complex optic mounts field restricted by coma tube air currents CON not portable over 4-inch aperture needs heavier mount chromatic aberration high f/ ratios very high initial costs difficult optic manufacture restricts photometric use

5 PLEASE NOTE - The above information applies to standard designs of either telescope (a Newtonian or Cassegrain reflector, or a doublet refractor). The newer catadioptric reflector telescopes (say a Schmidt-Cassegrain) and exotic design of refractors are generally free of many of the defects mentioned above save one - MUCH HIGHER COST!! If price is no object, then you can usually name your "specs" and get it. Still, caveat emptor is the best advice in all cases. Telescopic Principles Telescope Resolution The ability of a given telescope to resolve detail can easily be determined by a mathematical calculation. This is referred to as the Dawes Limit, or "D": "D" = 4.56 aperture of telescope in inches Example : a 6-inch telescope can, by this calculation, resolve detail on the moon (or just split a double star) that subtends 0.76 arc-seconds. Telescope Magnification The ability of a telescope to magnify the image of an object is directly related to the focal length of both the telescope and its eyepiece. Magnification for a given telescope-eyepiece combination is calculated by the formula: "M" = telescope focal length eyepiece focal length PLEASE NOTE!!! The units of focal length for both MUST be the same for this formula to work!! Remember, one inch equals 25.4 millimeters. Example : a telescope of 50 inches focal length with an eyepiece of 1/2 inch focal length yields a magnification of 100X. Example : a telescope of 2000 millimeters focal length used with an eyepiece of 20 millimeters focal length yields a magnification of 100X as well. Limiting Magnitude The faintest star that a telescope can permit an observer to see visually is directly proportional to its clear aperture. M l = log A, where: A is the aperture in mm m is the limiting magnitude Telescope Mountings

6 Altazimuth mounting - a mounting consisting of two axes set at right angles; one is horizontal, one vertical, requiring dual axis adjustments for tracking celestial objects anywhere on earth except at the poles. Variants - Dobsonian mounts. Advantages - Ease of construction. - portability. Disadvantages - difficulty of tracking objects at high magnification. field rotation. Equatorial mounting - a mounting consisting of two axes set at right angles; one axis is pointed to the celestial pole (called the polar or right ascension axis) and moves the telescope east-west, the other axis permitting north-south movement (the declination axis). Only the polar axis need be driven to follow a star. As seen in the diagram, the telescope is supported in a two-axis mount, the axes at right angles to each other, and the center of gravity of the telescope at their intersection. One axis is pointed at the celestial pole (either north or south); this angle between the vertical and the axis defines the local latitude.in this way, an object in the sky can easily be tracked by simply turning this one axis (the POLAR or RIGHT ASCENSION AXIS). The telescope may thus follow an object from East to West. If North or South motion is desired, the telescope is turned on the remaining axis (the DECLINATION AXIS). In this way, the entire sky visible from a given site is accessible, and tracking is simplified. A Selection of Telescope Mountings

7 "A picture is worth a thousand words", as the sages say. Here are some of the mounts that have been (and are) used today.

8 Eyepieces Eyepiece design doesn't affect magnification, only its focal length does. For example, a wide field of view is of no benefit when observing planets or double stars, which are quite small. Orthoscopics and Plossls are the most popular. The theoretical limit of useful power, under ideal conditions, is approximately 50 to 60 power per inch of diameter (aperture). Additional magnification is useless. Much lower powers will usually show more, as conditions are rarely ideal. Many subjects require low magnification because of their low surface brightness. KELLNER The three element Kellner gives very sharp, bright images at low to medium powers. At high powers Kellners have uncomfortably short eye relief (the distance ones eye can be away from the eyepiece and still view comfortably) and narrow fields of view. ORTHOSCOPIC The four element ortho is one of the most popular eyepieces. Excellent sharpness, contrast, and color correction. Long eye relief compared to Kellners. Very good overall performance and moderately wide field of view. Great for planetary and lunar work. ERFLE The six element Erfle eyepiece is optmized for the extra wide field of view, at the expense of edge sharpness. Very big, "picture window" images are most impressive for deep sky viewing. Best when used at low to medium powers. PLOSSL The four element Plossl is an extremely sharp design. Inexpensive versions have narrow views, but good Plossls have wider fields than Orthos, with pinpoint images right to the edge. Good contrast, color correction. Excellent for planetary, lunar, and deep sky observing.

9 ULTRA WIDE TYPES Various six to eight element designs provide fields so wide, some people cannot see the whole view at one time without moving the eye around. Expensive, large, and heavy, but very impressive specialty eyepieces. Greater number of elements results in more light loss due to absorption than more mainstream designs. for further reading... The History of the Telescope by H.C. King (Dover). This is a reprint of the classic 1955 treatise which is considered to be a landmark text. Astronomy Through the Telescope by Richard Learner (Van Nostrand). A somewhat overillustrated treatment of the subject, it nevertheless conveys a feel of the times - and personalities - of the people and their discoveries. The Perfect Machine by Ronald Florence. The finest book on the trials and tribulations of the manufacture of the Palomar 5-meter telescope (and you thought opticians had it easy?) Tools of the Astronomer by G.R. Miczaika and W.M. Sinton (Harvard). Out of print, but still available in most larger libraries. It is slightly dated, but worth reading. How to Make a Telescope, 2 ND ed. by Jean Texereau (Willman-Bell). The most recent edition of the classic work gives the reader a feel for the work involved in optical fabrication.