1. Understand the question

A reflecting telescope uses mirrors instead of a large objective lens. In many practical designs, a secondary mirror is placed near the focus of the primary mirror.

We need to identify its purpose.

2. Recall the role of the secondary mirror

In a simple reflecting telescope:

• The primary concave mirror collects light and forms an image.
• The secondary mirror intercepts the converging beam and redirects it to a convenient location for viewing.

This is mainly done so that the observer can place the eyepiece at an accessible position, often outside or at the side/end of the tube.

3. Evaluate each option

Option A: make chromatic aberration zero

Mirrors do not produce chromatic aberration because reflection is independent of wavelength.

So, in a reflecting telescope, chromatic aberration is absent due to the use of mirrors in general, not specifically because of the secondary mirror.

Therefore, A is incorrect.

Option B: remove spherical aberration

Spherical aberration depends on the shape of the primary mirror and optical design. The secondary mirror is not introduced mainly to remove spherical aberration.

Therefore, B is incorrect.

Option C: reduce the problem of mechanical support

One advantage of reflecting telescopes is that the primary mirror can be supported from the back, reducing mechanical support issues compared to large lenses. This is not the function of the secondary mirror.

Therefore, C is incorrect.

Option D: move the eyepiece outside the telescopic tube

This is exactly the practical use of the secondary mirror: it redirects the light path so that the final image can be observed conveniently, with the eyepiece placed outside or at a convenient location on the tube.

Therefore, D is correct.

4. Final answer

The secondary mirror is used to move the eyepiece outside the telescopic tube.

$\boxed{\text{D}}$

5. Comparison with stored correct answer

Stored correct answer: D

My derived answer: D

They match.