why the mirror of a telescope need to be so heavy and thick ?
In: Engineering
Mostly because of the fact that you need to move it. And there’s a lot of stress that that mirror experiences when being rotated, such as of course the motion itself, but also the fact that, the closer to horizontal it is, the more likely it will fall apart under its own weight. Also, vibrations always happen with machines. This is especially true for space telescopes, as rockets are essentially a way to let a bomb explode somewhat unidirectionally to allow you fly upwards. Vibrations tend to not be very good for the stability of a lot of things.
Finally. theres the fact that, the bigger the mirror, the more you can make out, as the size of the mirror ultimately determines how much light you get.
Basically to avoid warping but there is a limit to how thick it needs to be. There is a trade off in the thicker it is the longer it takes to cool to outside temperature. Amateur telescopes mirrors are thicker per inch than large professional scopes to be able to be held tightly in a smaller scope.
The mirrors’ shape needs to be accurate to within a fraction of the wavelength of light. Just moving from a horizontal to a vertical position, as happens when pointing telescopes at different parts of the sky, puts varying stresses on a mirror which can cause it to flex out of shape. Mounting and supporting the mirror correctly helps but, even so, making it thick helps it keep its shape. These days the largest mirrors are honeycombed at the back to keep the weight down while maintaining the necessary thickness and rigidity.
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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