The Ancient Greek writer Hecataeus claimed the Ancient British had a device that
"could bring the moon so near them as to show the mountains and rocks, and other appearances upon its surface."
This is from 'The Veil of Isis', chapter VI. a book that is partly about the secrets of the Freemasons for those that are interested in this sort of thing...
The only way Hecataeus could have seen these details is if the Ancient British did indeed have a type of telescope, let's be clear about this, the surface of the moon appears completely flat to the unaided eye, mottled for sure, but certainly not rocky. It is possible that they could have had a conventional telescope using lenses of carved and polished transparent rock crystal, the ancients certainly had them since about 3000BC or before. However rock crystal would seem not to have been as optically pure as modern glass would be and besides which all the conditions were right in Ancient Britain for mirrors to be used.
This is backed up in the Ancient Welsh histories - the Triads. According to EO Gordon in ‘Prehistoric London’, first published in 1914
The Druids, it is said, by means of a most powerful reflecting mirror of metal called “Dyrch Haul Kibddar,” filled the circle (Stonehenge) with a blaze of glory from on high. This is mentioned in the Triads as the speculum of the all-pervading glance, or the searcher of mystery ; one of “the Three Secrets of the Isle of Britain.”
This would suggest that they had a parabolic reflector; this is the entry for speculum in Encyclopaedia Britannica 1911
SPECULUM, the Latin word for a mirror, employed more particularly for a metallic mirror used in a reflecting telescope. In early instruments metallic mirrors, made from an alloy of copper and tin, with the addition of a little arsenic or other metals to increase the whiteness, were customarily employed, but they have now been displaced by the more convenient silver on-glass mirror. Various forms of specula are used in surgery for examining internal organs.
The Ancient British certainly had enough tin to make this alloy as they had a virtual monopoly in the ancient world's tin supplies.
The mirror at Stonehenge was used (presumably) for the mid-summer sunrise and it illuminated the central area at the moment of sunrise, given how often the sun fails to shine in Britain it seems likely that they didn’t rely on reflecting the sun alone but perhaps used some kind of flare or firework – the Druids were thought to have used gunpowder, particularly in their sacred groves, again from 'The Veil of Isis', chapter VI.
It is worth noting that Herschel when discovering his planets used exactly the same method to make the primary mirrors for his telescopes - out of polished speculum metal. Some of his telescopes had glass lenses but to look at fainter images he would not use the lenses as they reflected too much light. Instead, he turned his back to the part of the sky he wanted to study and by looking into the curved mirror, directly magnified that part of the sky. The body of the telescope and the eye piece are effectively only tubes or supports and the Ancient British would have had no problem making these either out of wood or metal so this type of telescope would also have been potentially available to the Ancient Britons.
The method for casting and grinding the mirror would also have been possible and so would have been the focusing, this is a modern version
Obviously, the ancients would not have had a light bulb but then neither did Herschel and he got by. Modern historians probably think that the Ancient British did not have razors either, but they did as men were clean-shaven, including ALL their body hair except, as Julius Caesar said
They wear their hair long, and have every part of their body shaved except their head and upper lip.
Which is a bit more than most Brtish men bother with now - clearly they were not hairy savages.
Given that the Ancient British had a reflecting mirror it would have been a bit strange if they had not used it to look at the stars, some claim it was the Druids who worked out the Zodiac system but if so that would have been far earlier than the time of Hecataeus who lived from c. 550BC to c. 476BC but that does not mean the telescope could not have been earlier.
It would seem that the idea of a parabolic mirror finally arrived in Ancient Greece by the time of Archimedes and the Siege of Syracuse, c. 214-212BC. Anyone who has seen the US TV programme Mythbusters’ attempt to reconstruct the heat ray supposedly used by Archimedes to destroy the attacking fleet would realise that flat mirrors would not have done the job but highly polished and focused speculum (white bronze) parabolic mirrors (telescope grade) almost certainly would have worked, so myth 'confirmed' and Mythbusters 'busted' for underestimating the sophistication of ancient technologies.
So if the Ancient British had these telescopes what would they have seen? As it would have been entirely possible for the ancients to built a telescope as powerful as Herschel's we can get some idea of what they saw as most modern amature telescopes are based on this design.
A modern amature photo of the moon