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Message: <blockquote data-quote="Binastro" data-source="post: 3207514" data-attributes="member: 111403">. Hi Gijs,. It's late for me at the moment, so I will have to look for references another day.You are correct that Digges is involved with the Elizabethan telescope. The Oxford lecturer on astronomical history mentioned that this telescope probably was made and kept as a military secret. I think that it was part reflecting telescope.My suggestion that a telescope was known very early on is conjecture, but lenses were known, as you say a very long time ago. And they were made in different strengths. It does not take much initiative to put two of these together and see what happens. That is why I believe that simple telescopes were made a long time ago. It may be that they were not very good but at least they magnified.Just because, there are no western written records of telescopes, does not mean they were not made.As you were saying about the Dutch invention of the telescope, only a month later a binocular was made.I did not know about the Ceylon lenses made from 1340 until early this week. But I showed my optician the paper on this and he confirmed that the details of the European lens maker are correct and that the firm involved with setting up the optical surface shop in Ceylon is still operating as a large concern.The old man making the lenses from nearly clear locally sourced high quality amethysts, had a variety of stone tools to make different strengths of lenses. The curves on these tools only provided plus surfaces on both sides of the Amethyst, but the focal lengths produced varied a lot.If you put a weak and a strong lens together you would make a telescope producing an inverted image.I don't know if negative lenses where  ever made in historical times. Presumably the stone tools needed would have to have convex surfaces rather than concave surfaces. To produce a simple upright image telescope you would need a negative lens.I cannot remember if the Digges telescope produced an inverted or upright image.If you have a long focus lens, just one, you can use your eyes alone to achieve magnification.And if you look at the details of the Visby lenses, they seem to be made in the 11 to 12th century or maybe quite a lot earlier. Some of these have aspheric surfaces. I would think that the original lens maker in Ceylon in 1340 may well have put a weak and a strong lens together and known what would happen. And his craft originated even earlier in India.Of course I don't have any written references to early telescopes but it seems very likely to me that telescopes existed well before the independent Dutch discovery.</blockquote>

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