Kapteyn's measuring device.
Jacobus Kapteyn started to measure all the photographic plates from the Cape Observatory at the end of the nineteenth century. For this purpose he constructed a device. It consisted of a small telescope, attached to two rings to accurately measure angles, that could be rotated horizontally and vertically. The plates were placed in a holder that was put at a very specific distance from the telescope: namely a distance that simulated the focal distance of the lens of the telescope with which the plate had been recorded at the Cape Observatory. In this way the angles measured by Kapteyn with his device corresponded one to one with the actual angles in the sky. This clever method avoided extra and complex calculations, thus reducing the probability of mistakes as well as saving time.

Kapteyn and his collaborators worked extremely precisely: all measurements and calculations were done at least twice by independent workers and the results were compared.

The device and the plates are conserved. The plate holder can be seen at the University Museum from the University of Groningen. [website]

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