An important component of reflecting telescopes is the main mirror. It collects the light from the night sky and reflects it, usually to a secondary mirror. It is very important that the large primary mirror is of high quality. In addition to the optical surface quality, the mirror substrate is also very important.

There are various materials used that differ primarily in the fact that they have different coefficients of expansion. A mirror made of window glass case undergoes significantly greater expansion or contraction than a mirror made of Pyrex for example. The following materials are common mirror materials:

1. Window glass 
2. BK7
3. Pyrex
4. Zerodur

Window glass has the largest coefficient of thermal expansion and Zerodur the smallest. Basically, it is only this factor that is important in the mirror material and possibly the density and not so much to do with the quality of the image. This can be just as good with a window glass mirror as with one made from Zerodur glass.

Types of glass such as window glass, BK7 or ​​Pyrex are commonly used in telescopes manufactured in large batches. Zerodur, a development by Schott, is relatively expensive and so is rarely used. Why is Pyrex more often preferred to the slightly cheaper BK7 glass?

BK7 mirror glass has more than twice the thermal expansion of Pyrex. This means in practice that Pyrex delivers a much better image than BK7 the in the cool-down period. A mirror is ground to a specific aspheric or parabolic shape. This shape should be maintained and not change. As the mirror shape will distort with large changes in temperature, BK7 is more problematic here. Pyrex does not distort to anywhere near the same extent and retains its shape better. The cool-down time is only slightly different and may vary according to thickness of the glass. The air temperature can also change in the course of the night, and Pyrex (or other borosilicate glass) mirrors are hence also superior here.

But Pyrex mirrors often only first come into use with larger telescopes, as it is here particularly important that the mirror material is not so temperature sensitive. Another advantage of Pyrex is also that it is harder than the other materials used and can therefore be ground more precisely.

Dobson Skywatcher with Pyrex mirror

400mm Dobson Light Bridge, Meade

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