Concept:Normal adjustment of an astronomical telescope means the final image is formed at infinity, so the eye is fully relaxed.
Explanation:In a telescope in normal adjustment, the objective and eyepiece are arranged so that their principal foci coincide.
This means parallel rays from a distant object are brought to focus by the objective at the common focal point of the eyepiece.
The eyepiece then recollimates these rays to produce a final image at infinity.
Because the final image is at infinity, the observer's eye views it without any accommodation, making the eye relaxed.
Therefore, normal adjustment is specifically defined by the final image being at infinity.
Answer:D. final image is at infinity.