  Highly multiplexed, fiber-fed spectroscopy is enabling surveys of millions of
stars and galaxies. The performance of these surveys depends on accurately
positioning fibers in the focal plane to capture target light. We describe a
technique to measure the positioning accuracy of fibers by dithering fibers
slightly around their ideal locations. This approach also enables measurement
of the total system throughput and point spread function delivered to the focal
plane. We then apply this technique to observations from the Dark Energy Survey
Instrument (DESI), and demonstrate that DESI positions fibers to within 0.08"
of their targets (5% of a fiber diameter) and achieves a system throughput
within about 5% of expectations.
