Illuminating the telescope

Astronomers keep their observatories dark, as stray light sources interfere with light passing through a telescope. However, to enable them to read the parts of a telescope used to measure celestial objects, illumination systems are incorporated within an instrument to direct light where needed.

In Macdonnell’s telescope, illumination is reflected through the telescope by small angled mirrors. This system was probably first powered by an oil lamp, but a photograph of the telescope suggests that, after 1906, it was fitted with electrical illumination. The Museum conservation team retained and refurbished this later system but had to make a best guess as to how it was controlled and powered historically.