Which one of these two fine Japanese-made telescopes do you think sport the finest optical glass in the world at this time? It may come as a surprise to many but the very best optical glass currently produced is not from Germany or America but from right here in Japan, by a small but prestigious Kanto firm called Ohara.
Of course there are great instruments out there using other kinds of glass, Pyrex from the USA, Schott from Germany for example or CaFl2 Calcium Fluorite. Other types of glass are known as BAK-4 and BAK-7. But the last word in light transmission, contrast and near-perfect color correction for lens-based telescopes goes to FPL53 from Ohara.
To date only a handful of very special instruments contain FPL53 and all are smallーaperture apochromats. Ohara glass is currently found in apertures up to a maximum of 130mm, namely in the Takahashi TSA-102 and TOA-130, William Optics FLT-98, TMB Signature 92 and two from StellarVue and Synta. At some point in the near future, small aperture apochromats containing FPL53 will be common and Ohara will be obliged to start making it for apochromats in the much larger 140 to 200mm aperture range. While this will be very interesting, these instruments will indeed need to be very rare and expensive.
Actually, neither of the two world-class Takahashi scopes pictured above use the FPL-53 glass. The Mewlon, which is the Dall-Kirkham Cassegraine on the left has an optical mirror made from Pyrex, generally accepted as the best material for the fabrication of optical mirrors, and the little scope on the right held by my good friend the owner of Kokusai Kohki linked below, has lenses made from Calcium Fluorite. FPL-53 cannot be used to make mirrors, and the Mewlon on the left is a mirror-only design of 180mm aperture, significantly larger than 130mm. The little darling lens-based scope on the right was designed in 1991 when fluorite was considered to be the state-of-the art lens material. It’s still great!
Anyone who is interested in making an astronomy equipment purchase are invited to contact me for advice. I’m an equipment monitor in Japan and can advise you on how to get the most for whatever budget you decide on. It is possible to put together a functionally satisfying, astronomical telescope system for less than $1500. Others are not content until they’ve spent at least a USD million!
Martin Werner Zander, Smith’s Partner
For a look at what I do as an astronomy equipment monitor in Japan:
Also, see my Astronomy and Natural Science course at my Smith’s School in Kotoen:
月謝制のスミス英会話 甲東園 校 仁川 校 逆瀬川 西宮市