Wavefront analysis from out-of-focus images ("donuts") is possible from F/A data of any DECam exposure, and from full science array on intentionally defocused exposures.


  • Donut analysis code commissioned - operating such that donuts are fit in real time between images, and results for focus, tip/tilt and decenter are returned.
  • Telescope tracking close to nominal open-loop performance; focus near best value.
  • Hexapod working, reprogrammed to have 500 arcsec tilt limits (unless installation delivered a best tilt that is less than 25 arcsec, each axis).


  • For science images, no special procedure is required, as Donut analysis is run by default
  • For images with donuts over entire focal plane, focus is moved +/- 1.5 mm. If the hexapod cannot move this far out of focus in both directions, +-1.0mm will be acceptable.
  • Take 30 to 100 second exposures on a field at moderate galactic latitude, to yield a good density of donut stars, without crowding.
  • Analyze images using Donut code.


  • Donut analyses run, create FITS images of cutouts.
  • Donuts from opposite sides of focus agree to within 0.1 waves accuracy, averaged over focal plane.
  • The Donut analysis produces a map of aberrations, (focus, astigmatism, coma) over the focal plane.
  • This map is compared against the optical model prediction -
  • Focus, tip/tilt and decenter hexapod corrections are output.


A failure means we will almost certain not have alignment close enough to obtain science quality images. Remedial action depends on the root cause. If it is software, revise Donut analysis code and/or run alternate Donut codes to clarify the problem.