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Is a telescope difficult to make? Does glass have to be polished and shaped very precisely? Or is a device using two or more lenses to magnify things just not obvious?

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    $\begingroup$ It might seem obvious to us, but people were making spectacles for centuries before the telescope was invented. From space.com/21950-who-invented-the-telescope.html "One story goes that he got the idea for his design after observing two children in his shop holding up two lenses that made a distant weather vane appear close". And then it took a year or so before a telescope was used (by Galileo) to view celestial objects. $\endgroup$
    – PM 2Ring
    May 27 at 8:44
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    $\begingroup$ As a very minor addition to James K’s excellent answer, keep in mind that to make a refracting telescope (the one that uses lenses), you need perfect optical-quality glass, plus you need to perfectly polish not one but two surfaces for each lens. Mirrors, on the other hand, don’t require perfect-quality glass, and only one surface needs to be polished. But there, too, progress was slow, and optical-quality mirrors were late in coming. $\endgroup$ May 27 at 17:04
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    $\begingroup$ You can check History of Science and Mathematics Stack if you are more interested. $\endgroup$ May 28 at 12:00

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Ancient glass was opaque or at most translucent. It was also often full of bubbles. Glass was not even suitable for windows until the first century AD, and even then it was as a means of letting some light in, it wasn't useful for looking through.

Lenses were made in Mesopotamia, but using quartz crystal instead of glass, and were costly. Nero is said to have had glasses made from emerald. Only an emperor could afford such luxury.

Transparent glass and the development of techniques to make flat planes of glass date from about 1000 AD and was developed in Venice. This is just an example of slow evolution of technology. Making transparent glass requires a pure source of silica and soda ash (or potash, but that comes with other issues), chemistry to supply various oxides that improve transparency, more advanced metallurgy, better furnaces and so on. Transparent glass is said to be the discovery of Angelo Barovier in the middle of the fifteenth century.

Glass had developed to a quality that made is a suitable substitute for quartz in lens making by the 16th century, but it was still secret knowledge that few held. The process of grinding and polishing glass to a suitable degree takes time, and needs the development of fine abrasives (or you just end up with lens that is so scratched to be opaque again).

It was only after the printing press had been developed that the theory of lens-making became widely known, and the first telescopes were designed. Even then, the idea of using a telescope at night wasn't immediate. The people making lenses weren't the same as the people who tracked the planets.

Lenses for telescopes need to be an order of magnitude better in quality than lenses for correcting long-sightedness. The glass does have to be shaped and polished very precisely. The technology and skills required for this were not fully developed and shared before 1600.

sources:

Ancient glass
lenses
telescope Angelo Barovier

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    $\begingroup$ Anyone doing photography with a real camera would understand the issues of quality. You can see specks of dust on your lens as a giant blob. $\endgroup$
    – Nelson
    May 29 at 1:03
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    $\begingroup$ When you say it was secret knowledge in the 13th century, does that mean that it was knowledge that only a few experts had so most people didn't know it, or that the relevant group of experts was actively keeping it a secret and not sharing outside their group? $\endgroup$
    – terdon
    May 29 at 11:05
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    $\begingroup$ @terdon Guilds had a strong tendency to keep knowledge secret in the era before patents, and in some cases was main reason they existed. Especially easy in this case where there are 'magic' ingredients where the workers know the mix white powders into the sand sand but not know what those materials are or most importantly, how they were assessed for quality. Similarly preparing the materials for grinding glass can be kept a guild 'mystery' but the results used (and consumed) by outsiders. $\endgroup$ May 29 at 11:56
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    $\begingroup$ @terdon not 100% certain in this case, but I'd bet that it was actively kept secret. This would not be difficult in a time when there was no printing press, and knowledge tended to be hoarded and passed only from master to student. But even with the knowledge, the skill and materials required would have prevented many copies. $\endgroup$
    – James K
    May 29 at 15:21
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    $\begingroup$ @terdon Not exactly lens making but kind of adjacent and it illustrates the lengths to which guilds would go to protect their trade secrets: barbaraathanassiadis.com/history/venetian-glassmakers-in-paris $\endgroup$
    – biziclop
    May 29 at 21:36

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