OF THE ROGER N. CLARK

VISUAL ASTRONOMY OF THE DEEP SKY
THE EYE AND THE TELESCOPE
when rays of light from a point on a distant
object fail to reach the correct point on the
image (also called the focal surface). Some
such errors are inherent in the system design;
others are added by imperfect construction.
The six main ones are: chromatic aberration,
coma, astigmatism, spherical aberration, distortion,
and Pet::;val curvature.
Chromatic aberration arises because different
colors of light are bent by different
amounts when they pass through a lens. Unfortunately,
no kind of glass known has the
same index of refraction (light-bending
strength) for all colors. Simple lenses suffer
from chromatic aberration the most. Images
formed by a simple lens are surrounded by
obvious blue and red fringes. When two or
more lenses with different index-of-refraction
properties are combined, the chromatic
aberration can be controlled to some extent
because two or more different colors can be
made to focus at the same place.
particular radius from the center of a lens or
mirror.
Distortion is the squeezing or stretching of a
part of an image toward or away from the
center. A common test of distortion uses a
square grid; the lines may bend toward or
away from the center, giving the appearance
of a pincushion or a barrel. Hence the common
names of pincushion distortion and barrel
distortion.
Petzval curvature (or field curvature) results
from the image being not flat but slightly
concave or convex. Typically, the best
focus at the edge of the field lies closer to the
objective than the best focus at the center.
(Astigmatism also affects the surface of best
focus.) Eyepieces too can have field curvature,
so stars at the edge of the view are rarely
as sharp as on-axis - unless the eyepiece
curvature happens to compensate for that
the objective.
Te lescope Types
Newtonian Reflector
Refractor
Coma is named fo r the comet-like images of
stars seen off-axis (away from the center of
the field). The farther the star is from the
objective's optical axis, the worse the coma
will be. Coma also depends on the f/ratio of
the system, and for a given f/ratio and angle
off-axis, it increases with the aperture.
Astigmatism may be caused by errors in the
manufacture of an optical component or may
be inherent in the design. An example might
be a "spherical" lens surface that is not perfectly
spherical but slightly cylindrical, for
instance if the light from the left and right
edges of the lens is focused at a differen t point
than light from the top and bottom edges.
Astigmatism can also be caused by improper
alignment of an optical system.
Spherical aberration is a telescope's inability
to focus all the light at the same distance
from the objective. For example, a beam of
parallel light that encounters a spherical mirror
will not all be focused at the same distance
from the mirror. The light that encounters
the mirror's edge will be focused closer
than light that strikes the center. Parabolic
mirrors do not suffer from spherical aberration
when focused on objects very far away.
Zonal Aberration is spherical aberration at a
TYPES OF TELESCOPES
Telescope design is a compromise between
aberrations, cost, portability, focal length
and field of view, among other things. The
different types of telescopes described below
are illustrated in Figure 3.1.
The refractor
A refractor is a telescope whose objective is a
lens. To reduce chromatic aberration, the
lens is almost always compound, or made
two or more disks of glass (lens elements).
Usually the objective is achromatic (made
two elements), though a few expensive ones
are apochromatic (usually having three elements).
The main defect of refractors is
aberration, but astigmatism and
aberration can also be problems d
on field size, focal ratio, and design. A
focal ratio is required in most achromatic
designs in order to minimize the aberrations.
The commonly accepted f/ratio for
is f/ 15, though if they are small and not
tended for high magnifications, f/8 or even
will give acceptable definition. Binoculars
refracting telescopes with an im
Cassegrain Reflector
Schmidt Cassegrain
Maksutov Cassegrain
Figure 3.1. Basic telescope designs. In each case, starlight arrives from the left and is focused to a
point on the image plane, which is seen in profile as a short line.
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