In a reflecting telescope, a secondary mirror is used to:

In a reflecting telescope, a secondary mirror is used to redirect the light collected by the primary concave mirror.

Understand the structure of a reflecting telescope. A reflecting telescope uses a large concave mirror (primary mirror) to collect and focus light. However, the focal point of the primary mirror lies in front of the mirror, which means the observer's head would block incoming light if the eyepiece were placed there.

Role of the secondary mirror. The secondary mirror (a small flat or convex mirror) is placed near the focal point of the primary mirror to redirect the light to a more convenient location. In a Cassegrain telescope, it reflects light back through a hole in the primary mirror. In a Newtonian telescope, it deflects light to the side of the tube.

Evaluate the options. Option A: "Reduce the problem of mechanical support" — The secondary mirror helps avoid the need to place heavy equipment at the prime focus, reducing mechanical support issues. This is correct.

Option B: "Make chromatic aberration zero" — Reflecting telescopes already have zero chromatic aberration (mirrors don't cause dispersion). The secondary mirror doesn't contribute to this.

Option C: "Move the eyepiece outside the telescopic tube" — This is also a benefit of the secondary mirror.

Option D: "Remove spherical aberration" — The secondary mirror doesn't remove spherical aberration.

The primary purpose is to reduce the mechanical support problem by redirecting light so equipment doesn't need to be at the prime focus.

The correct answer is Option A.