Astronomy Principles and Practice Fourth Edition.pdf

Transmission efficiency of the reflector 261
Figure 16.24. The Newtonian system.
mirror lies somewhere between two and five, according to the individual design. The secondary mirror
must be sufficiently large to accept all the light of the convergent cone from the primary. The diameter
needed and hence the fraction of the central part of the primary mirror which is not used, depends
on the magnification of the secondary mirror. Some small adjustment of the position of the secondary
mirror is usually made available by push-button control and the position of the final focus can be altered
by this means. This is particularly convenient when a range of equipment is available for fixing to the
base plate of the telescope.
One of the modified forms of the Cassegrain system is known as the Ritchey–Chrétian telescope.
In this telescope, both coma and spherical aberration are removed. Astigmatism and field curvature
are greatly reduced at the expense of having a larger diameter of secondary mirror than is usual for
an ordinary Cassegrain telescope. Thus, although the light-grasp is not as great as the same size of
Cassegrain telescope, the image quality is greatly improved.
16.8 Transmission efficiency of the reflector
Not all the light which is collected by the aperture of a reflecting telescope is able to pass through
the system without loss, with the result that the images in the focal plane are less intense than might
be expected. The causes of the loss of energy are the secondary mirror which blocks out the central
cylinder of the light beam and the inability of mirror coatings to act as perfect reflectors.