Astronomy Principles and Practice Fourth Edition.pdf

Chapter 17
Visual use of telescopes
17.1 Magnifying power
In order to make a visual inspection of the images which the telescope collector provides, some kind
of eyepiece must be used. The effect of the use of an eyepiece is illustrated in a simplified way in
figure 17.1, where the virtual image is at a position on or beyond the near point of the eye. (The actual
final image is formed on the retina of the eye.) Under this circumstance, it can be seen that the image
produced by the collector lies inside the focus of the eyepiece. The figure illustrates that, with this
simple eyepiece, the viewed image is inverted. The magnifying power, m, of the overall optical system
is defined as the ratio of the angle subtended by the virtual image at the eye, α e , and the angle, α c ,
subtended by the object at the collector. Thus,
m = α e
α c
. (17.1)
For astronomical observations, it is usual practice to adjust the position of the eyepiece so that
the virtual image appears to be at infinity. It is obvious that in this adjustment, the eye lens must be at
the exact distance of its focal length from the image produced by the collector in its focal plane. This
situation is represented in figure 17.2. The collector aperture acts as the entrance pupil and the image
of the collector aperture formed by the eyepiece acts as the exit pupil. The distance from the eye lens
(or the last lens of a compound eyepiece) to the exit pupil is known as the eye relief. As all the rays
from all parts of the field which can be viewed by the telescope pass through the exit pupil, it is at this
position that the eye should be placed. The ray passing through the centre of the collector is known
as the chief ray. It is not deviated at the collector aperture and it can be seen from figure 17.2 that it
passes through the centre of the exit pupil.
From figure 17.2, it can be seen that
tan α e = h v
and
tan α c = h u
where h is the distance of the chief ray from the optic axis when it arrives at the eyepiece, u and v
are, respectively, the object and image distances corresponding to the entrance and exit pupil distances
from the eyepiece. As both α c and α e are small angles,
α e ≈ h v
(17.2)
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