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BK7 Glass is a borosilicate crown glass that is used extensively for lenses, prisms and mirror substrates.
It is relatively hard, does not scratch easily, and performs well in chemical tests. It also has excellent
visible light transmittance.
Pyrex Glass is a low-expansion borosilicate glass (LEBG) made by Corning and is well suited for applications
in which high temperature, thermal shock, or resistance to chemical attack are primary considerations. On the
other hand, Pyrex is typically less homogeneous and contains more striae and bubbles than optical glasses such
as BK7. This material is well suited for application as mirror substrates, condenser lenses for high-power
illumination systems, and or windows in high-temperature environments.
Plate Glass (Float Glass) Basically, glass is sand-a very high quality silica sand, to which other materials are added. The resulting mixture is called a batch. Some of the other materials included in the batch are salt cake, limestone, dolomite, feldspar, soda ash and powdered cullet. Cullet is broken glass. Glass that is left over from a previous batch or from the edges that remain after a batch of glass has been formed and cut to size. Adding cullet helps the batch melt more easily. Glass is made by melting and cooling the batch. As the batch cools, it becomes solid without forming crystals. Crystals are three-dimensional building blocks that make a substance internally rigid. The lack of crystals makes glass technically a liquid, not a solid. It also makes glass transparent.
Plate Glass Primary Mirrors: In an effort to reduce the weight and cost of primary mirrors, manufacturers have determined over time that
they can maintain the quality of their products yet reducing the diameter thickness ratio to between 8:1 to
10:1 from the historical 6:1 thickness ratio. In addition, some of the manufacturers have switched from Pyrex
to plate glass without sacrificing the quality to maintain on certain product lines within cost targets. As
in use of thin Pyrex blanks, telescope manufacturers have given more attention to reducing the risk of
flexure (which cause image distortion) by redesigning their mirror cells.
Transmission coatings include primary and secondary mirrors coated with aluminum enhanced
with a coating silicon dioxide (SiO2), the thickness of the layer precisely controlled. The
result is a dramatic increase in mirror reflectivity across the entire visible spectrum reflectivity is
increased from 89% to 92%.
Wavefront error: Theoretically, a "perfect" lens has 0 wavefront error. A wavefront
error of 1/3 wavelengths is good. It's about
the error of a decent astronomical telescope. A really good telescope may have only 1/10 wavelength
of wavefront error. At 1/2 wavelengths of wavefront error image degradation becomes quite noticeable.
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