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The diagonal mirror should be offset away from the focuser in a Newtonian telescope.
To understand the reason for this, compare the two telescope drawings at the right.
The slope of the light cone has been greatly exaggerated by making the primary mirror
an f2 (a very short focal length).
The upper drawing has the diagonal centered in the tube. We can see how the light
cone partly misses the diagonal at the bottom, and part of the diagonal is wasted at
the top. This is because the lower part of the light cone is wider where it
intersects the diagonal than the top half. The top half has to travel farther toward
the front of the telescope and therefore gets narrower before it intersects the
diagonal.
The solid line is the actual light cone from the primary mirror. The dotted line at
the bottom shows the portion of the light cone which actually hits the diagonal. The
result is that the entire primary mirror is not being used. Some of the light coming
from the bottom of the mirror is being wasted because it misses the diagonal.
In the lower drawing, the same size diagonal has been offset downward, away from the
focuser. Now the light cone is completely intercepted.
Note that the diagonal has also been moved closer to the primary mirror. Instead of
simply moving the diagonal down, it was moved toward the lower right, sliding in a
direction parallel to it's reflective surface.
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