Astronomy Principles and Practice Fourth Edition.pdf

280 Visual use of telescopes
Figure 17.5. The Ramsden eyepiece.
outside the focal plane of the telescope and it is an easy matter to arrange to have a graticule in this
same plane. The Ramsden eyepiece is superior to the Huygens in that the aberrations are much smaller
and it is more convenient to use because of its greater eye relief.
By using a doublet instead of a single lens for the eye lens, it is possible to utilize the desirable
qualities of the Ramsden eyepiece and, at the same time, remove the chromatic aberration. This type
of ocular is known as a Kellner eyepiece.
In the case of the single element eyepiece, it is a simple matter to measure its focal length and the
expected magnification, when used with a particular telescope, can be obtained from knowledge of the
focal length of the telescope. It is usually more difficult to make a direct measurement of the effective
focal length of a compound eyepiece and the magnification that it produces is best measured when it
is used on the telescope. A simple way to do this is to point the telescope at the daytime blue sky
and measure the diameter of the parallel beam (exit pupil) emerging from the eyepiece. Application of
equation (17.5) allows the magnification to be determined.
By joining simple ray diagrams, illustrating the effect of each of the optical components, it is
easy to demonstrate that an astronomical telescope, using one of the previously described eyepieces,
provides an inverted image. A more complicated eyepiece giving an upright field (a terrestrial eyepiece)
is of no real advantage to the astronomer. In fact, its extra lenses lose more light.
17.6 Micrometer eyepieces
By measurement of the physical separation of images in the focal plane of a telescope, it is a simple
matter to convert the records into angular measure on the celestial sphere (see equation (17.2)). One
way of doing this directly at the telescope was by the use of specially designed eyepieces. Although
no longer in commission, such micrometer eyepieces had their main application in the measurement of
the separations of visual binary stars.
The micrometer eyepiece would perhaps be of Ramsden design but, in the plane that coincides
with the telescope focal plane, a system of adjustable cross-wires was placed. There are many different
arrangements in the design of the cross-wires. However, the main feature is that one of them, or
sometimes two, could be adjusted relative to the others. The adjustment was made by a micrometer
screw and the amount of movement recorded on the scale or drum which was attached to the side of
the eyepiece. The eyepiece, together with the cross-wire system, was fitted into a rotatable tube and the
orientation of the cross-wires could be altered. The essential components of the micrometer eyepiece
are shown schematically in figure 17.6, which also shows the positions of the cross-wires when they
have been adjusted for a determination of the separation of two stars.
In a typical arrangement, as shown in figure 17.6, two wires are fixed exactly at right angles to
each other with their intersection at the centre of the field of view. The orientation of the two wires is
conveniently displayed by placing a pointer along the direction of one of the wires and allowing this