Reconstructing the polar axis

When a telescope is first set up in an observatory, its polar axis must be positioned to point to the celestial pole. This aligns it with Earth’s axis of rotation and ensures that it rotates in the same plane as Earth. As the telescope rotates at the same rate as Earth, but in the opposite direction, the sky seen through the telescope appears stationary.

While inspecting Macdonnell’s telescope, the conservation team found that the polar axis was not connected to the mechanism used to set sidereal time. This meant that the telescope could not be set to rotate at the same rate as Earth. The telescope manufacturer’s papers from the 1880s suggested that a small pin should connect the two mechanisms, and close inspection revealed that on Macdonnell’s telescope it had broken off. The repair was relatively simple, but required the complete dismantling of the polar axis assembly.