Question

Why does the objective of an optical telescope have a large focal length and large aperture? Also, explain why a reflector gives a better image of a small portion of the celestial body?

Answer 1

Hint: An optical telescope captures and concentrates light mostly from the visible portion of the electromagnetic spectrum to magnify an image for direct visual inspection, take a photograph, or collect data using electronic image sensors.
The diameter (or aperture) of an optical telescope's objective (the primary lens or mirror that collects and focuses light) is proportional to its light-gathering power, and the area of the objective is proportional to its light-gathering power.
The more light the telescope captures and the finer detail it resolves, the greater the objective.


Complete step-by-step solution:
An Optical telescope has a larger aperture because it creates a larger light-collecting area and allows for better detail resolution. The objective's aperture is set to a big value so that it can collect enough light from distant objects.
Because it faces distant objects and bright images of distant objects, the objective of a telescope is a convex lens with a large focal length.
Large magnification \[M = - \dfrac{{{f_{_O}}}}{{{f_e}}}\]
The rays emitted by distant celestial bodies travel across space and through the atmosphere of the planet. A limited quantity of rays penetrates our eyes while we observe with our naked eyes. The resulting image has a low pixel value. A secondary mirror in reflectors produces additional diffraction, limiting the resolution and contrast of planetary images. As a result, we can see a clearer image.

Note:The basic principle is that the objective (the convex lens or concave mirror used to gather incoming light) focuses light from a distant object to a focal plane, where it generates an actual picture.
An eyepiece, which acts as a magnifying glass, can be used to record or see this image. The eye subsequently sees a virtual picture that has been inverted and amplified.
Many telescopes use secondary or tertiary mirrors to fold or divert the light stream. These may be built into the optical design (Newtonian telescopes, Cassegrain reflectors, and other similar sorts), or they may simply be employed to relocate the eyepiece or detector.