Borosilicate (Pyrex®, Borofloat 33) type glasses are not zero expansion over the room temperature range but are still considered low expansion glasses. When transient temperature changes occur around a borosilicate substrate environment (such as in a telescope), the net effect on the mirror is a change in radius of curvature. Cooling temperatures cause a glass, concave mirror to contract and shorten in focal length and the reverse occurs when a mirror is warmed and translates to a change in focus of the image. Unless the rest of the telescope optical tube assembly was designed to be athermal, these same temperature changes that influence a borosilicate mirror, cause the telescope structure to lengthen and shorten depending whether the system is being heated or cooled and thus affects the image. Unless there is tight temperature control, the structure will change the image when using zero expansion materials. The issue of focus change due to temperature variation can simply be addressed with an auto-focuser. Below are two graphs that show a comparison of borosilicate glass coefficient of thermal expansion to other materials at lower temperatures. Note that the coefficient of thermal expansion for borosilicate glass at very low temperatures approaches zero.
Credit: S.F. Jacobs, Optica Acta, 1986, Vol. 33, No. 11, 1377-1388
