tremor-disk

Century Dictionary and Cyclopedia

1. n. The telescopic image of a star, as apparently enlarged by the tremors of the instrument and the atmosphere. These tremors spread the light, which would otherwise be condensed to a point, over a surface of some extent, thus rendering the image fainter and less effective on a photographic plate, or on the slit of a spectroscope. Athenæum, April 29, 1905, p. 534.