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
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:
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.