A small lens placed within a photometer to provide an image of the telescope's objective lens on the photocathode. It is placed behind a mask in the focal plane of the objective, which isolates the object chosen for measurement. By imaging the objective lens rather than the object, small movements of the image caused by air currents do not cause it to move across the photocathode, which may vary in sensitivity across its surface. The use of such a lens was first suggested by the French physicist (Marie Paul Auguste) Charles Fabry (1867–1945).
Subjects: Astronomy and Astrophysics.