Refracting telescopes, also known as refractors, use a large objective lens to gather and focus light. The objective lens is the primary element that determines the resolving power and light-gathering ability of the telescope.

When a large objective lens of a refracting telescope has a long focal length, it provides several benefits, including:

High Light Gathering Power: A large objective lens of a long focal length allows more light to enter the telescope. This enables the telescope to capture faint objects that would be invisible to the naked eye or smaller telescopes with smaller lenses.

Higher Magnification: The large focal length of the objective lens allows for higher magnification, which makes it possible to observe details of distant celestial objects.

Reduced Chromatic Aberration: Chromatic aberration is an optical problem that occurs when different colors of light have slightly different focal points. This results in a blurred or distorted image. Longer focal lengths can help reduce chromatic aberration, making the images sharper and more accurate.

Reduced Spherical Aberration: Spherical aberration is an optical problem that causes spherical mirrors and lenses to produce distorted images. Longer focal lengths help reduce spherical aberration, making the images clearer and sharper.

In summary, large objective lens of a long focal length provides a high-quality image with more detail and clarity, making it an ideal choice for astronomical observations.