How Large a Diagonal
How Large a Diagonal?
Whereas each point image in the primary focal plane would be illuminated by the full aperture of the mirror (in the absence of the tube), only a part of the secondary plane surrounding the axis will receive the benefit of full aperture, the size of the fully illuminated field there being governed by the
size of the diagonal. Fig. 46 illustrates the convergence of rays by the mirror to form a single point image on the axis.
It can be seen that, wherever the diagonal is placed in that cone of rays, the plane of the cone intersected by its reflecting face is in the shape of an ellipse. As the angle is 45°, the lengths of the minor and major axes of the ellipse may, for all practical purposes, be regarded as in the proportion of one to the square root of
two, or 1 to 1.4.
The diagonal may be either rectangular in shape, which is simplest to make but needlessly obstructs light and introduces added diffraction, or elliptical. If rectangular, the shape of the diagonal obstruction, when viewed from a point on the axis, is that of a square; if elliptical, the apparent shape is circular. The need for secondary reflection in the Newtonian telescope has its analogy in the star diagonal of the refractor.
This is usually a prism placed a little distance ahead of the focal plane to deflect
the image to a more comfortable viewing position. The prism is of such size as to permit the unobstructed passage of rays from the objective to all parts of the largest field to be viewed. This field, measured linearly, is of approximately the same diameter as the field lens of the lowest-power eyepiece. For a starter we might try
applying the same reasoning to the problem of the size of the Newtonian diagonal.
In general, the least power that will be used on the reflector will be had from an eyepiece of l½" focal length, the field lens of which will be about 1" in diameter. Accordingly, a field of view of this size, VV in Fig. 47 (not drawn to scale), is marked off, V2" on either side of the optical axis, in the primary focal plane FP. The angular size of this field, VMV, can be found as follows:
An arc of a circle equal in length to the radius, and called a radian, subtends an angle of 57°.3. As the focal length of the mirror, 48" for the 6-inch f/8, is the radius here involved, a 1" length of the arc will contain 1°.2, or 72 minutes of arc.
Two
stars that are separated by this angular distance may be imaged at V and V, and can just be fitted into the field of view of the low-power eyepiece. If the principal rays MV and MV have been drawn, only two more rays, AV and BV, need be added to complete the representation of image formation of the two stars. (To avoid confusion in the diagram from the use of many lines, rays BV and
AV are shown in part.) The truncated cone ABVV is thus seen to contain all the rays that flow from the mirror to illuminate every point within the field VV.
How large the diagonal part 2 |
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