While working to correct an extreme case of myopia (otherwise known as short-sightedness), Dutch optician Hans Lippershey stumbled upon the fact that a particular arrangement of lenses seemed to magnify distant objects.
This was the birth of the modern telescope.
Lippershey's first telescope, or ‘dutch perspective glass' as it was first known, came with a whopping magnification of 3x! Galileo took the design and made a better one which gave 5x magnification -- this may not sound much compared to even the most modest amateur telescope today offering at least 50x, but it was historic.
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Today's amateur astronomers routinely operate at magnifications in excess of 200x or more. The story of the invention of the telescope has been sketchy, but before Lippershey or even Galileo hit the headlines, there were other discoveries long before that were key to its invention.
Anyone who has studied a telescope will know that its main component is a lens or mirror depending on the design. Refracting telescopes use lenses to refract or bend starlight to a focus; reflecting telescopes use mirrors to reflect and focus it.