Saturday, 14 March 2009
Wednesday March 14, 1894
Centred crown lens of 28in O.G. in photographic position, the holes having been drawn to allow of this. The inner flange of crown cell is now very exactly concentric with outer flange of flint cell to which it is bolted. Wrote to Sir H. Grubb on subject of 26in photo-equatorial.
William Christie, Astronomer Royal
RH says..... In order to correct colour distortions, telescope lenses are composites of two different kinds of glass - crown glass and flint glass. Because the corrections for photographic use or visual use are different, and the 28-inch telescope was intended to work both ways, the two glass cells had to be painstakingly adjusted - as Christie's experiments over the last few months demonstrate.
The image accompanying this post was taken with the 28-inch telescope in Greenwich - with the assistance of modern equipment. It shows a clearer and more colourful image than Christie would have managed to produce.