If you’ve ever used cheap binoculars or telescopes, you will have noticed that when looking at something with a sharp edge between bright and dark, such as the Moon, you see coloured fringes. This problem, called chromatic aberration, which gets worse when you try to use higher magnifications, has been the bane of telescope designers for centuries. The cause of the problem is simple; a glass lens bends blue light more strongly than it does red light, so different colours come to a focus at different distances.
Newton solved the problem completely by designing a telescope using a concave mirror to collect the light and form the images. His “reflecting telescope” is the ancestor of large, observatory telescopes such as the Canada France Hawaii Telescope, and also the larger telescopes used by amateurs. However, it is hard to make efficient small reflecting telescopes, so designs using lenses, called “refracting telescopes”, continue to be heavily used today. The centuries fighting chromatic aberration led to some very innovative telescope and lens designs, so that today it is no longer much of a problem, except in those unusually cheap binoculars and telescopes.
The aficionado of exotic telescope design must be Johannes Hevelius, an astronomer in 17th Century Poland. He found that the chromatic aberration problem is less serious for lenses that are less sharply convex, that is, of longer focal length, so he made some of the longest telescopes in history. His most ambitious was 45m long, supported on a huge mast and adjusted by many guy wires. However, he found that while reducing one problem he ran into a new and equally serious one. His telescope sagged under its own weight and flexed in the wind. Surprisingly though, he did manage to make useful observations with the telescope, but few took up his solution to the chromatic aberration issue.
In the 18th Century optical engineers found that by combining a convex lens made with one type of glass and a convex lens made of another it was possible to form an image with much of the chromatic aberration cancelling out for the main problem colours, red and blue. They called these lenses “achromats”. This invention made useful telescopes available for the first time, for sailors and others as well as astronomers. Even today there are many “achromatic” telescopes and binoculars in use. However, the engineers kept working and eventually came up with something even better. They managed to combine lenses in such a way that they largely cancelled chromatic aberration at three wavelengths: red, blue and green. This was another big improvement, and these lenses, known as “apochromats”, are now widely used in the best telescopes and binoculars. Thanks to modern computer design and manufacturing techniques, apochromats are getting better and cheaper. It’s worth remembering though that until recently we did not have these tools. Innovations in lens design involved experimenting with different glass types, calculations on pieces of paper and finally making the lenses by hand.
Mercury lies quite close to Mars, low in the southwest after sunset. Jupiter is high in the southwest most of the night. Saturn rises about 1am. The Moon will reach first quarter on the 17th.
Ken Tapping is an astronomer with the National Research Council’s Dominion Radio Astro-physical Observatory, Penticton.