Question

Short response: Most astronomical telescopes make use of spherical mirrors for their objectives.

What are some advantages for using these mirrors over a similar lens?

What issues may still plague the image formation by these mirrors?

Answer 1

The objective or primary mirror of a reflecting telescope has a concave shape. The shape may be spherical, parabolic, or hyperbolic. Spherical profile mirrors are inexpensive and easy to make. More complex shapes are used to compensate for distortions in low focal ratio (fast) telescopes. These include shape aberrations towards the edge of the field of view and making the focal plane flat for photography. Plane secondary mirrors are used to direct the image to the eyepiece.

Reason 1

Mirrors are used instead of lenses telescopes. This is because an objective lens can have a maximum diameter of approximately 1 metre before it begins to sag, whereas a mirror of several metres in diameter can be used.

Reason 2

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.

Disadvantage

Corrosion

below is an example of the disadvantage/issue

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 aluminium, 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.

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