Why are telescope mirrors made of glass?

FIG 1: A TELESCOPE MIRROR

Telescopes are of different types sizes and qualities. Whatever the price is, you will always find that the mirror is made of glass. There are two reasons behind this. 

1. To avoid distortion

Try to think about what happens if you use a magnifying glass to observe the sun. It will generate a bright point of light on the other side of the sun or on the ground. This is the same way how a telescope works. It directs light to a single point. In a simple magnifying glass, the point does not need to be the focal point. But if it is a telescope, the point has to be precisely the focus.

Have you ever visited any fun house with distorted mirrors that makes distorted images to make everything in front of it funny? When it is a telescope mirror, it has to be completely perfect without any distortion. Again think about the magnification. In a telescope, if you magnify an image 100x, any distortion will also be magnified 100x.

Fig 2: The light path inside a telescope

This is one reason behind making the telescope mirror out of glass. If you carefully handle it, you will be able to ground the accurate focus using abrasives. You can ground it to a correct shape, observe and ground it again until you achieve the correct accuracy. And we know that the glass is robust and stable and it does not change shape over time. But if you think about other materials, they change their shape or formation due to moisture and deterioration.

2. Coefficient of Expansion

The second reason is the coefficient of expansion which depends on temperature. When your telescope is in your house the temperature might be constant at 70-75 degrees Fahrenheit. But when you take it outside, the temperature can be 40 deg or even 100. We know that, when the temperature is less, most of the solids shrink and when the temperature is higher, they expand. This phenomenon is known as the coefficient of expansion. If a telescope mirror shrinks or expands, it will obviously distort the image for sure that no user wants. The coefficient of expansion of glass is very low which means that the change in temperature can not affect it that much. 

Glass is in fact a perfect material to make the mirrors of telescopes as it can be ground precisely and it does not expand or shrink with temperature, which ensures the accuracy of the image at any condition and temperature. One other advantage is glass is not expensive and widely available. 

This is why from the inventors to the developers, all of them chose glass for making telescopes.