Astronomy Principles and Practice Fourth Edition.pdf

Reflectors 259
in the reflecting surface as the ambient temperature changes. The figured surface is covered with an
evaporated layer of aluminium to produce a high reflection coefficient.
By the method employed to produce images, reflector telescopes are inherently free from
chromatic aberration. However, they are susceptible to all the other forms of aberration which have
been discussed briefly in relation to refractor telescopes. The effects of spherical aberration, coma,
astigmatism, curvature of field and distortion of field may be apparent in the images which are formed
by the primary mirror.
Since the original application of mirror optics to telescope systems, there have been many
improvements in design to eliminate the various aberrations. For example, the effects of spherical
aberration can be removed by using a mirror which has been figured to be in the form of a paraboloid
of revolution. Simple ray diagrams illustrating the difference between the spherical and paraboloidal
mirror for on-axis objects are drawn in figures 16.23(a) and(b). It may be noted that if the molten
blank from which the mirror is made to spin while cooling, its surface takes on a parabolic profile. This
technique has been used to produce parabolic mirrors of large size. Although the paraboloid effectively
removes spherical aberration, it suffers badly from astigmatism and, therefore, has a severely limited
field. This aberration can be removed by using corrector plates prior to the primary mirror, but the
discussion of this type of system (Schmidt telescope) is reserved until later.
In order to have access to the primary image, it is inevitable that the central part of the collecting
area of the mirror will be ineffective. The fraction of the light which is lost depends on the size of
the apparatus which is placed at the prime focus. For a telescope with a large aperture, the light-loss
may be quite small if an imaging camera is placed in the focal plane. However, the telescope must
be extremely large if the observer wishes to inspect an image by eye at the primary focus. In some
designs, it has been found possible to use the collecting mirror off-axis, so that the primary image is
produced outside the cylinder which contains the rim of the aperture. However, the more usual practice
of producing the image relies on a secondary mirror in the system. This is normally placed along the
optic axis, towards the focus of the primary mirror and, consequently, the small central part of the
collecting area is lost. It is held in position by means of the thin frame (spider) which is attached to the
telescope tube. There are several designs of two-mirror combinations, the most common being those of
the Newtonian and the Cassegrain systems. The general principles of the systems are now described.
16.7.2 Newtonian reflectors
The Newtonian system provides access to the image formed by the primary mirror, without altering
the effective focal length of the system. The essential optical parts are illustrated in figure 16.24. A
flat front-surfaced mirror is placed at 45 ◦ on the optic axis of the primary mirror so that the image is
formed just outside the cylindrical beam which the primary mirror collects. The rim of the secondary
mirror describes an ellipse and the mirror is referred to as the Newtonian flat or elliptical flat.
Adjustment of the position and tilt of the Newtonian flat is fairly easy to control and, from this
point of view, the system is very convenient. For some purposes the position of the focus, which is
towards the open end of the telescope, is inconvenient for direct observation and the use of subsidiary
equipment. For these reasons, large telescopes are not frequently used with a Newtonian focus.
16.7.3 Cassegrain reflectors
The Cassegrain normally consists of a large spherical or paraboloidal primary mirror with a secondary
mirror which is complex or hyperboloidal. The optical arrangement is illustrated in figure 16.25, from
which it can be seen that not only is the central portion of the primary mirror not used but it is also
machined out to allow the converging beam to be brought to focus behind the primary mirror. It can
also be seen that the effective focal length of the system is increased by the secondary mirror. If F p is
the focal length of the primary mirror (a positive value), F s the focal length of the secondary mirror (a