Building Large Telescopes: Ii- Reflectors

On the turn of the twentieth century the refractor was rapidly approaching its limits in terms of aperture. The main reason for this was the difficulty in manufacturing discs of crown and flint glass with diameters greater than 100 cm. Manufactures of plate-glass could however cast large disks of ordinary crown glass, of lower optical quality, that were suitable for mirrors. It was also known that the light losses in lenses of more than 100 cm would be much greater that those in mirrors of equal aperture, especially in the blue region of the electromagnetic spectrum where the highest sensitivity of the first photographic plates lay. Lenses with diameters of more than 1 m were also very difficult to mount and flexure was a major drawback. Mirrors were easy to mount, the focal ratios were smaller and there were no residual chromatic aberrations. Astrophysicists aimed at the largest possible aperture and perfect color correction. The reflector was the way to go. The lower focal ratio of the reflector meant shorter tubes, smaller domes and lower overall costs. Reflectors were, in spite of all, considered by the majority of astronomers around the turn of the century, as imprecise and difficult to use instruments. The mounts of the first big reflectors were imperfect and the mirrors sagged under their own weight. For this reason the reflector was being mainly used by amateur astronomers.