I have been researching the rise and fall of instruments for the measurement of astronomical photographs. Here I consider those instruments used for astrometry, and their related data reduction procedures.
Known as 'measuring machines', these devices for measuring positions on photographic plates were originally developed mainly for the great international Carte du Ciel/Astrographic Catalogue project, established in 1887. Through much of the 20th century, use of such machines and the methods developed for the Catalogue were part of the standard 'tool kit' of most astronomers, much as computers are today.
Parallel to the instrumental development, during the project's first decade astronomers debated practical methods for converting positions on photographic plates to coordinates in the sky. The original plan had been to make only an atlas, and astronomers were surprisingly unprepared for determining accurate angles in the sky for the Catalogue. This did not stop them from finalizing the camera (astrograph) parameters before understanding the consequences for the determination of accurate coordinates.
The Carte du Ciel astronomers were worried about possible emulsion creep (which was in fact negligible) and for this reason decided to expose a 'réseau' of lines onto each plate along with the image of the sky. Not until a few years later did they mention the more problematic 'optical distortion'. The field curvature of the astrographs was only noticed years later. (The Seidel formalism for aberrations, developed in the 1850s, seems to have been unknown to them.)
The basic calculation technique ultimately used for the Astrographic Catalogue - 'standard' ('tangential') coordinates - was proposed only in 1893, six years after the beginning of the project, by H.H. Turner of Oxford. Without this technique the reductions of the Astrographic Catalogue would have been impossible without electronic computers.
Today these instruments and techniques are largely forgotten. Many machines have been discarded. Unlike old telescopes, they do not interest the public. But these tools were a central part of the mid-twentieth-century observatory, often more than the telescopes themselves, which had already largely migrated away from urban areas and would soon be built at high altitude. The lives of many astronomers and their collaborators, working in university observatories in towns and on campuses, often revolved around these forgotten devices.