How Large a Diagonal part 2
The location of the diagonal inside this cone must now be
established, and it is obvious that the closer it is to the focal
plane the smaller it can be. With a well-designed focusing eye-piece holder, the distance of the secondary focal plane (fp) outside the tube can be kept at a minimum, about 1^".
But the amateur, using a telescoping drawtube and a bulky support, will require about 3", which added to the radius of the tube (next to be established) gives the distance inside of focus for the diagonal.
Now, it is desirable for constructional reasons to have the tube extend to the primary focal plane, where the oblique rays FA and PB, proceeding from points on the edge of the wide field, are found to be 3½" distant from the axis of the mirror.
Evidently, then, the inside diameter of the tube need be no larger than 7". But what
if it is smaller, as is the case with the aluminum tube shown in the frontispiece? That tube has an outside diameter of 7", with a wall thickness of 1/16", and a cork lining l/8" thick. It therefore intrudes 3/16" into those field rays, obstructing a peripheral zone of similar width on the mirror, resulting in an insignificant loss of illumination at the edges of the field of the low-power eyepiece.
The major portion of the field, ¾° in extent, is unaffected. With higher-power eyepieces, which may take in a field of this size or less, there is no hindrance by that tube. Actually, the inside diameter of the tube may be as little as 6½", but it should not be less than that, as some space must be provided around the mirror
to allow ventilation.
The position of the diagonal inside the focal plane is now fixed, in this case, at 6½" (the radius of the tube plus the distance outside to fp). That is assumed to be the position of the diagonal in Fig. 47, where D2 is the major axis, and D1 may be taken for the minor axis. These lengths can be measured from a scale drawing;
or the width Dj is given by the formula,
c(M-v)
D> = F + v,
where v is the linear width of VV, c is the distance of the diagonal from the focal plane, F is the focal length of the mirror, and M its diameter. D2 multiplied by 1.4 equals D2. This gives, for the above example, a diagonal of dimensions 1.7" x 2.4". But in making the analogy with the star diagonal of a refractor, we did not take into account that the diagonal of the reflector is placed in front of the mirror, forming an obstruction to the incoming light.
So before adopting the above size, consideration should be given to the effects of a central obscuration. In Fig. 48, which is not drawn to scale, an elliptical diagonal,
forming a circular obstruction, is shown 41½" from a 6-inch f/8 mirror. Parallel rays from an axial star, and from two other stars each 36 minutes of arc distant from the axis, are shown approaching the mirror.
For the axial star a central area of the mirror equal in size to the area of the obstruction is blocked off. For the field stars, overlapping areas of similar size are also obstructed. It is seen that a small central area of the mirror, the black spot in the diagram, will never be used. Furthermore, the gray zone, a trifle more than 2/5" wide when the low-power eyepiece is used, contributes only to the edge of the field of that low-power eyepiece. In the case of a single star at the edge, less than half
of the gray zone is used.
How large the diagonal part 3
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