Hello! A handful of students at my Smith’s School of English Franchise in Kotoen are interested in science, and a couple of them have research jobs in pure science fields. It is mainly for them and for potential future students like them that I offer this.
Several previous astronomy articles focused on the subject of optical quality according to glass type. Major high-end glass types for astronomical application include Schott Optical glass from Germany, Ohara FPL-51 and FPL-53 from Japan, OK-4 from LZOS Russia, Hoya from Japan and Pyrex from Corning in the USA. All of these have superb thermal properties with minimal temperature expansion. All exhibit the required capacity to produce a lens or a mirror completely free of chromatic aberration or astigmatism, and can maximize light transmission to the point of near-invisibility. A major control material is fluorite, not a glass at all but lenses manufactured from CaFl2 crystals. It is light-weight, has perfect thermal stability, near-perfect light transmission and superb color correction. Takahashi was the first to discover and implement fluorite for astronomy equipment applications.
Which is the best? Why choose one over the other? Why do so many prism, lens and mirror makers still rely on soda lime glass, borosilicate BAK-4 vs BK-7 glasses, fused quartz and other materials less well suited but still functional? Well the answer is cost and supply. Pyrex is a topic of concern since it’s inventor and sole producer decided to stop making it. This worried makers of large Newtonian telescopes as Pyrex has long been the material of choice, not necessarily the absolute best, but the most readily available with highest cost-benefit. The world class Mewlon mirrors made by Takahashi (above picture) all use Pyrex, but with the material’s gradual disappearance, alternatives have been sought. Well calm is in the air once again as Pyrex is being produced. Other major possibilities include glasses used for lens manufacture, though these have important light transmission properties rather than reflective. Ohara FPL 53 FPL 51 and LZOS OK-4 and quartz are the most common under consideration, but Schott might be better prepared to deliver. Schott is a major producer of ultra-low thermal expansion ceramics like Zerodur and Borofloat and Ohara has recently announced its Clearceram. Zerodur and ULE are other highest-grade reflective options, however resulting in considerable added cost.
So now we debate fused quartz, BK-7 vs Pyrex, OK-4 vs FPL-53, Pyrex vs FPL 53, and finally fluorite CaFl2 from early Takahashi scopes and Schott, the venerable mainstays of all Zeiss and Leica products vs all the others. The Zeiss binocular eyepiece shown in the Mewlon above employs Schott optical glass, and perhaps future Mewlons may use Zerodur!
Martin Werner Zander, Partner in the Smith’s School of English Company
Owner, Smith’s School of English in Kotoen ( Astronomy Course )
Smith’s Kotoen Astronomy Course FPL51 FPL 51 FPL-51 FPL53 FPL 53 FPL-53 are all acceptable writings for the famous optical glass types made by Ohara in Japan.
Kokusai Kohki www.kkohki.com Telescope Store