Key idea about an astronomical telescope

The objective lens of an astronomical telescope is made with a large aperture mainly to collect more light and to improve its ability to distinguish two nearby distant objects.

Resolution of a telescope

The resolving power of a telescope depends on the diameter $D$ of its objective:

$\theta_{\min} \approx 1.22\frac{\lambda}{D}$

where:

• $\theta_{\min}$ = minimum angular separation that can be resolved,
• $\lambda$ = wavelength of light,
• $D$ = aperture diameter.

A larger $D$ makes $\theta_{\min}$ smaller, so the telescope can resolve finer details.

Check each option

A: reduce spherical aberration
Incorrect. Spherical aberration depends on lens shape and design, not mainly on making aperture large. In fact, large aperture can even worsen aberrations unless corrected.

B: have high resolution
Correct. Larger aperture increases resolving power, hence gives high resolution.

C: increase span of observation
Incorrect. Aperture does not primarily determine the span or field of view.

D: have low dispersion
Incorrect. Dispersion depends on the material of the lens, not on aperture.

Derived answer

Therefore, the correct option is:

$\boxed{\text{B: have high resolution}}$

Comparison with stored answer

Stored correct answer = B
Derived answer = B

Hence, they agree.