why the mirror of a telescope need to be so heavy and thick ?
In: Engineering
They aren’t, they are very thin and light. Weight is the enemy when it comes to telescope elements because it means you need more structure and more robust and powerful mechanisms.
That said, there are a couple of things that put a lower limit on the thickness of a mirror or element. The first is maintaining the shape of the mirror as the telescope moves and so the relative direction of gravity changes. This causes the mirror to sag in varying directions, so clearly the overall sag needs to be small. The biggest telescopes use *active optics* to mitigate this issue. The second factor is that the mirror needs to be machined to shape. It will deflect due to the cutting forces during this operation and then spring back afterwards. If the deflection is excessive then the final shape will be wrong.
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