![]() The cool-down time is only slightly different and may vary according to thickness of the glass. Pyrex does not distort to anywhere near the same extent and retains its shape better. As the mirror shape will distort with large changes in temperature, BK7 is more problematic here. This shape should be maintained and not change. A mirror is ground to a specific aspheric or parabolic shape. This means in practice that Pyrex delivers a much better image than BK7 the in the cool-down period. Why is Pyrex more often preferred to the slightly cheaper BK7 glass?īK7 mirror glass has more than twice the thermal expansion of Pyrex. ![]() Zerodur, a development by Schott, is relatively expensive and so is rarely used. Types of glass such as window glass, BK7 or Pyrex are commonly used in telescopes manufactured in large batches. This can be just as good with a window glass mirror as with one made from Zerodur glass. 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. Window glass has the largest coefficient of thermal expansion and Zerodur the smallest. The following materials are common mirror materials: A mirror made of window glass case undergoes significantly greater expansion or contraction than a mirror made of Pyrex for example. There are various materials used that differ primarily in the fact that they have different coefficients of expansion. In addition to the optical surface quality, the mirror substrate is also very important. It is very important that the large primary mirror is of high quality. It collects the light from the night sky and reflects it, usually to a secondary mirror. An important component of reflecting telescopes is the main mirror.
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