The principal reason for making modern reflecting telescopes with mirrors made of glass is that: Question 6 options: glass is 'soft' enough that we can push and prod even the very biggest primary mirrors into different shapes, in 'adaptive optics' special "zero-expansion" glass can be used that doesn't change shape (and ruin the images) as the temperature changes glass is extraordinarily cheap, and also very easy to grind and polish to create the perfectly smooth surfaces needed in our mirrors special glasses can be made which are harder than diamond, and hence very robust and long-lasting