The DESI survey requires excellent imaging quality across a 3-degree field of view on the 4m Mayall telescope at Kitt Peak.  To achieve this a new prime focus corrector was installed on the Mayall. The DESI Commissioning Instrument was designed and built by OSU to characterize the image quality of the new corrector and commission key components of the DESI metrology system prior to the installation of the 5000-fiber position DESI Focal Plane System.  

The Commissioning Instrument uses five (5) commercial CCD cameras; one at the center of the focal surface and four located around the periphery of the field at the cardinal directions. A set of 22 illuminated fiducials (the same type using on the DESI focal plane system) are distributed throughout the focal surface to test the fiber viewing camera system and software that is used to map between the DESI fiber positioners and celestial coordinates.

The Commissioning Instrument was installed on the 4m Mayall telescope in March 2019 and undertook a number of basic but important tasks required to bring DESI online: testing the new telescope control system (TCS), telescope guiding, the image optimization system, and DESI’s "Platemaker" algorithm that maps between sky and fiber-positioner coordinates.  It was removed in fall of 2019 to make way for installation of the 5000-fiber focal plane system.