SPECKLE INTERFEROMETRY AT THE U.S. NAVAL OBSERVATORY. XXI.

The results of 2408 intensified CCD observations of double stars, made with the 26 inch refractor of the U.S. Naval Observatory, are presented. Each observation of a system represents a combination of over 2000 short-exposure images. These observations are averaged into 1602 mean relative positions and range in separation from 194 to 10741, with a median separation of 1196. This is the twenty-first in this series of papers and covers the period from 2015 January 13 through 2015 December 19. Significant instrumentation changes are reported in this paper.