Exit Pupil and Eye Relief
In Fig. 67, a cylinder of essentially parallel rays from a star is collected by the objective and converged to a point intersection at the focus, which is also the focus of the eyepiece. Thence the rays diverge to the eyepiece and from this they again emerge parallel, but condensed into a smaller cylinder.
The size of this cylinder is determined by the diameter of the objective, and also depends on the focal lengths of the eyepiece and objective. The diameter of the cylinder leaving the eyepiece is also the diameter of the exit pupil.
The location of the exit pupil, which is in reality an image of the objective and is called the Ramsden disk, will be a short distance back of the eye lens, at the place where the principal ray1 again intersects the axis after refraction by the eyepiece (see Fig. 65).
As all the rays that pass through the objective to make up the image also pass through the Ramsden disk, it is essential that the lens of the eye itself be placed in that plane for most effective viewing. It is desirable to have as large an eye distance (usually called eye relief) as possible, especially with high powers, else one's eyelashes will be rubbing against the eyepiece. In this respect, a single-lens eye-piece gives greater eye relief than the usual two-lens kind.
And because this distance is never very great, spectacle-wearers should remove their glasses while observing and make compensation by refocusing the eyepiece; otherwise the field of view may be greatly curtailed.
The diameters of the exit pupil and of the objective have the same relative proportions as the focal lengths of the eyepiece and the objective, so the magnification of a telescope may also be expressed as:
Diameter of objective
Magnification.= Diameter of exit pupil
By focusing the telescope on a very remote object and then withdrawing the eye to a distance of about a foot, the exit pupil will be seen apparently suspended in air just back of the eyepiece.
It is more conspicuous if, after focusing, the telescope is pointed at the daytime sky. The diameter of the exit pupil can be quite exactly measured by means of calipers or a fine scale held in the plane of the Ramsden disk. A magnifier will aid in obtaining an exact measurement. The above formula will give the magnification
of the telescope, and by the F/f formula an exact determination of the focal length of the eyepiece can be made.
The principal ray passes through or is reflected at the center of the objective. In the reflector, the principal ray is obstructed by the secondary mirror.
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