The tarnished mirror used in Herschel’s 40-foot reflector on display at the London Science�Museum.
Image credit: geni/Wikimedia Commons
Your backyard Dobsonian has a mirror that far outperforms the best telescopes of yore.
This week, we thought we�d break the �all-Pluto all the time� mantra for July to bring you a classic from our archives.
Those olden mirrors of yore didn�t use silver or aluminized glass like the reflecting telescopes we have today. Instead, they relied on a polished metal alloy known of since antiquity for its reflective properties. This combination of tin, copper with just a pinch of added arsenic was known as speculum. The good folks over at the Periodic Table of Videos show us a fine example of speculum in Newton�s first reflecting telescope at the Royal Society of London:
With a mirror just 2 inches in diameter, you can see just how tiny Newton�s early tabletop reflector was. Polished tin has a blue tint, whereas polished copper is reddish in hue. A optician had to hit a sweet spot to balance out the amalgam to get the desired colorless mirror.
Such metal mirrors of yore had some definite drawbacks, which are the reason they�re no longer in use. They would begin to tarnish and corrode as soon as they were exposed to the damp night air. William Herschel was said to have kept a spare mirror for his massive 49.5� telescope, and was constantly re-figuring one, while the other corroded in the tube.
And such mirrors only reflected 40% of the light falling on them when they were new. �It�s no small wonder that you only see speculum in the museum today. Still, it might be a fun project to construct a speculum metal mirror telescope, just to gain insight into how those astronomers of yore did it. You really had to be part astronomer, part metallurgist, �and part alchemist to make the magic of a speculum metal mirror work.
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Wonderful video David! Ernesto & I really enjoyed it. Thanks for sharing.