Whether looking at distant objects on Earth or at the stars in space, all telescopes operate under the same principles. They gather light from a distant source and reflect or bend it, focusing it into an eyepiece. Telescopes that use lenses are called refracting telescopes, and those that use concave parabolic mirrors are called reflecting telescopes. Each type of telescope has its advantages and disadvantages.
Reflecting telescopes use mirrors instead of lenses to collect light. Because mirrors have only one reflective surface, they are easier to construct than lenses, which are made from optical glass, and cannot have any occlusions because light must pass completely through them. In addition, mirrors have less spherical aberration, which is a scattering of light that occurs when lenses don't perfectly focus on a single point. Mirrors also reflect all wavelengths of light equally, whereas lenses bend light differently depending on its wavelength.
Because mirrors are easier to construct than lenses, they can be made larger and more durable. Additionally, because only one side of the mirror is used to focus the light, the other side can be placed against a surface for support. This allows for the mirror to be extremely large as compared to a lens, and makes them ideal for observing space. The larger the collection device, the more light can be directed to the eyepiece. The largest optical telescopes in the world, including the largest one at the Keck Observatory in Hawaii, are reflecting telescopes.
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Because reflecting telescopes use mirrors instead of lenses made from optical glass, they are less expensive to produce. Lenses are used for smaller telescopes and particularly for observing a point on the Earth's surface. Reflecting telescopes of comparable size cost significantly less to produce. Consumer-grade reflecting telescopes made for backyard stargazing deliver more magnification power for the user's dollar than their refracting counterparts.
Reflecting telescopes are not without their problems. Because of their size and the design of an open tube assembly, the mirror must be cleaned periodically. Whenever it is cleaned, it must also be realigned, which can be expensive. An improperly aligned telescope results in a blurry or marred image.
The single reflective surface in a telescope's mirror allows it to be large, but it is also exposed to the air. Originally, reflecting telescopes used mirrors coated with a silver surface, which tarnished in the open air. These telescopes required regular polishing to keep them clear. Now, reflecting telescopes' mirrors are coated with aluminum, which also oxidizes, but the product is clear and does not require as much polishing. Even with modern telescopes, the metal coating of the reflective surface needs to be replaced after years of service.