What makes the glass reflective?
After the glass surface is polished to a very precise figure, it is turned into a front-surface mirror by applying a very thin metallic coating. The function of the glass substrate is to hold the shape of this thin metal layer which reflects the light. Aluminum is most frequently used, although silver or gold is used in some applications. Aluminum is the preferred coating since it has a high reflectivity over the whole visible spectrum whereas the reflectivity of silver decreases significantly shortward of a wavelength of ~ 400 nm. The thickness is typically ~ 100 nm ( 4.0 x 10-6 inch) thick and weighs only a few grams. Aluminum is applied in a vacuum chamber by evaporating a small amount of metal and allowing it to bond to the clean glass surface. For astronomical telescopes, the mirror coating (aka aluminization) is normally done at the mountaintop observatory site. • Why is borosilicate glass used for the castings? Borosilicate glass is chosen for the casting of the honeycomb mirror
