Parabolic Mirors: Substrate and Coating

1) Substrate

Only a few types of glass are suitable to serve as substrate for telescope mirrors. It is not only important, that a mirror cools down fast but also that it keeps its exact form while doing so. To achieve that, the coefficient of expansion must be as low as possible.

Considering this coefficient there are huge differences between different substrate materials. For example the glass type „BK7“ that is often used as basis for cheaper telescope mirrors, has an expansion coefficient of 7.1x10e-6/K. In contrast the coefficient for the glass types „Pyrex“ and „Borofloat 33“ only is 3.2x10e-6/K, half the value of „BK7“. Extremely expensive glass materials like „Zerodur“ even show only an expansion coefficient of 0.1x10e-6/K, meaning their coefficient is 30 times smaller than that of „Pyrex“. This is why „Zerodur“ is especially suitable to manufacture highly accurate reference optical components.

Since „Pyrex“ blank disks are difficult to source, Spacewalk Telescopes' mirrors are made from „Borofloat 33“ for mirrors with a diameter of up to 18“, for larger ones between 20“ and 25“ the substrate is „Supremax“. The substrates are sourced from a renowned company in Germany. Both material types used for Spacewalk Telescopes' mirrors have an expansion coefficient that is identical to the one of „Pyrex“, meaning that during the cooling down process at the beginning of each night under the stars, they hold their form twice as good compared to the common, cheaper „BK7“ mirrors (umformulieren auf positiv?) .

Up to mirror diameters of 455mm the edge thickness of the glass blanks used for Spacewalk Telescopes mirrors is 25mm, for larger diameters the thickness is increased to 31mm or 34mm. Other thickness values are available on request.

2) Coating

All mirrors made by "Spacewalk Telescopes" receive an aluminum coating. The coating durability is enhanced by an additional SiO- protective layer.

The reflectance value of the coating is 92% within the UV spectrum (380nm), 92-93% within the visible range of the spectrum (550nm) and slightly decreases to 87% in the near infrared area (800nm). The coating therefore offers a very high reflectance grade over the whole spectrum used within visual astronomy, guaranteeing bright images with a high contrast.