William Herschel began his telescope-making career in 1773 by experimenting with relatively small refractors (small in aperture, but certainly not in length – one of them being 30 feet long!). As these were then still optically primitive compared to today’s instruments, he soon turned his attention toward reflectors. These could be made in larger sizes and without concern for the quality of optical glass, for they used mirrors instead of lenses. But these were not the familiar telescope mirrors of today, as silver-on-glass optics did not appear until long after Herschel’s death. Instead, they were made of speculum metal – a brittle and hard casting composed mainly of copper and tin. He first made several mirrors for a 5.5-foot Gregorian reflector, but then turned to the simpler Newtonian form. All of his telescopes from that point on were long-focus Newtonians of ever-increasing size, culminating in the great 40-foot reflector (see below).
KeywordsOptical Glass Secondary Mirror Royal Astronomical Society Orion Nebula Diameter Mirror
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