What are the differences between the Pyrex and ULS Quartz mirrors (besides the price)?
On average, the flatness of the quartz parts is better than Pyrex for a given size. Quartz is a zero expansion glass, while Pyrex is low expansion, which makes it possible to achieve a better surface before removing the parts from the polishing block. (The Pyrex parts will “bounce back” slightly after being allowed to cool to room temperature.) This is why most reference flats are made of quartz (or a derivative of quartz). Quartz is also a harder glass than Pyrex, which is why it takes a smoother micro polish than Pyrex and other softer glasses. This property is especially important for laser applications where low laser beam dispersion is desired.
