Astronomy Principles and Practice Fourth Edition.pdf

240 The optics of telescope collectors
Figure 16.1. A schematic representations of the function of a telescope.
Figure 16.2. A simplified ray diagram illustrating the separation of images in the focal plane according to the
angular separation of the point objects.
is lost either by reflection or absorption. From the design of a telescope it is possible to predict the
fraction of the energy found in an image in relation to the amount of energy that is collected by the
aperture. This fraction defines the transmission efficiency of the system and this is one of the factors
which must be considered in the design of telescopes and which must be taken into account when
brightness measurements are being determined absolutely.
As all the objects to be investigated are effectively at infinity, the distance of the primary images
from the collecting aperture defines the focal length, F, of the telescope. A plane through this point
and at right angles to the optic axis is defined as the focal plane. Telescopes of the same diameter
need not have identical focal lengths. This latter quantity is engineered according to the purpose for
which the telescope is primarily designed. Thus, the focal length is another parameter which is used to
describe the optical properties of any telescope.
It is often convenient, especially when describing a telescope’s ability to act as a camera, to
describe a telescope in terms of its focal ratio, f , which is defined as the ratio of the focal length of
the collector to its diameter. Thus,
f = F D . (16.1)
This definition is synonymous with that of the speed of an optical system when applied to general
photography. (It may be remembered that, in everyday photography, the exposure time is proportional
to f 2 .)
When a telescope is directed to the sky, an image of a part of the celestial sphere is produced in the
focal plane. The relationship between the size of this image and the angular field which is represented
by it is governed by the focal length of the telescope. In figure 16.2, rays are drawn for two stars