View of the telescope at Slough, Berkshire, 1839

View of the telescope at Slough, Berkshire, 1839. Circular salted paper photographic print by the English astronomer, mathematician and chemist Sir John Herschel (1792-1871), of the telescope built by his father Sir William Herschel (1738-1822). John Herschel introduced 'hypo' (thiosulphate) as a photographic fixative and discovered the cyanotype or blueprint process in 1842. This 40-foot telescope was constructed by William Herschel in the grounds of his home in Slough, and completed in 1789. On the night of 28th August that year, the first time he had used the telescope, Herschel discovered two new moons of the planet Saturn. National Media Museum, Bradford ©SSPL/National Media Museum