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THE MOST FREQUENTLY SEEN ABERRATIONS IN TELESCOPES

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E. CHARACTERISTICS OF TELESCOPES (OPTICAL TERMS)

THE MOST FREQUENTLY SEEN ABERRATIONS IN TELESCOPES

As wonderful as modern telescopes are, they are not perfect. Though many aberrations may not be immediately noticeable, the view through the eyepiece can exhibit a few problems. Let’s talk about what they are, and how to solve them:

Coma is an aberration present in the very shortest focal length scopes, but not seen in longer ones. This is usually seen in reflectors, but can be seen in short f/ratio refractors if the focal length is short enough. In all scopes of f/5 ratio or shorter, coma can be seen as the outer stars in the field of view being smeared out into fan shapes pointing along radial lines toward the center of the field. It is not a flaw, per se, but merely a characteristic of the short focal length mirror or lens. It can be fixed, if objectionable, by a coma corrector lens inserted before the eyepiece. Lumicon and TeleVue both make this lens, so coma is user-correctable. It should be noted that most telescopes are not short enough in focal ratio to require this and many people consider this essential only for photography with the short f/ratio scope.

Chromatic Aberration has several causes, but the most prevalent cause is when, in a refractor, the front lens does not focus all colors of light to the same point. Long ago, optical designers realized that if the front lens had two elements (parts) it could focus two colors perfectly and the other colors would focus closely. The exception is purple, which focuses at a different place. This is why achromatic refractors often display a violet fringe around objects viewed. The following illustrates how only two colors are perfectly focused:

The eye is very insensitive to the red end of the spectrum (especially at night), so it is the violet end we see as a fringe in the image. This can be corrected by the use of a Minus Violet Filter or Violet Fringe Filter (Lumicon makes one of the best ones), or by adding another lens or two.

With three elements, chromatic aberration can be reduced to negligible, or non-visible, levels. This more-corrected type of refractor is referred to as Apochromatic:

Note that now 3 colors come to perfect focus and the total deviation in the visible spectrum is very small.

In recent years, exotic glasses (called ED or SD) and fluorite crystal lenses have allowed better color correction with the simpler, 2-element, lens design. This design is more technically termed semi-apochromatic, but is often advertised as apochromatic. Color correction is excellent with this 2-element type, and the simpler designs offer the advantages of lower cost.

Note that mirrors (as in reflectors) focus all colors together, so are completely free from color fringing.

Astigmatism is an aberration caused by a tilted element. Though it can be caused by a lens or mirror that has a different focal length in the up-and-down direction than the side-to-side direction, this is unlikely in modern optics. Typically, this can be corrected by collimation (alignment) of the optical parts, or the freeing of a pinched edge on one of the optical elements. It is most often caused by the eyepiece. It exhibits itself as a change in the star image from round to a distorted + or – sign. If it is combined with coma, the star images at the edge of the field look a lot like seagulls. Astigmatism is rarely seen in quality eyepieces, or in eyepieces used in longer focal length telescopes. But the wrong match of eyepiece to short f/ratio telescope can make it visible. It is easily, though not always inexpensively, cured.

Field curvature is the last of the most frequently seen aberrations in nighttime viewing. This is where the stars in the center of the field do not focus at the same position of the focuser as the stars in the outer edge of the field. If the star images are in focus in the center but the images at the edge appear round, and just a bit of movement of the focuser is required to focus them, then the telescope has field curvature. This is caused by the eyepiece. If it is slight, and your eyes can accommodate a little diopter shift (deviation from the perfect 20/20), a compromise focus can be found that produces generally sharp star images over the entire field. But if your eye cannot accommodate to the amount of curvature present, the cure is a change of eyepiece. Many designs have flatter fields than others. Sometimes experimentation is required.

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