Give several reasons why many research telescopes use mirrors instead of glass.

1. glass can distort light over time, as it is actually a liquid.
2. glass is very, very heavy as it gets larger and larger. This is one reason why very large lenses are so impractical to make.
3. with glass, the lense tends to get larger and larger in size to get larger and larger magnifications. Whereas, mirrors can be many, small and spaced apart, and their reflections combined to increase magnification.
4. A larger reflecting telescope can be made of multiple high-quality mirrors spaced close together, each built seperately and at a relatively low cost to one very large lense.
5. Mirrors are lighter, and can be manipulated more easily than a large lense. This is not only helpful for viewing from the ground, but especially if you want to take a large telescope up into space (heavy things are very, very expensive to take to space!)
6. Finally, when size, weight and expense begin to matter a great deal like this, the telescope is used primarily for research. Thus, mirror telescopes tend to be the telescopes of choice for researchers.

Is that enough reasons for you? :)

You will recall that passing white light through glass can cause it to break up into it's component spectrum. Big thick lenses and thick mirrors are to be avoided !

Historically with the traditional telescope (Refractor ) using lenses throughout there was significant optical aberrations (distortion) when looking at white light (The moon, stars etc). These telescopes produced a slightly blurred image with distortion around coloured edges on the image at high magnification. The object can appear yellowish eg Toy telescopes show a blue / purple ring around the moon.

Isaac Newton knew this and designed telescopes with a reflecting Parabolic mirror which reduces the effects of chromatic aberrations (light separation). This design is called a Newtonian Telescope.
Other aberrations can be apparent too.

As time went on designs of Newtonian reflectors were further improved by front coating the mirrors. This was to reduce the aberration effect of light having to move through the air-glass transition on the mirror front surface..