Tearing patterns affecting flat-field frames in charge-coupled devices are a visually striking obstacle to performing pixel response nonuniformity (PRNU) corrections. These patterns can be explained by lateral field distortions, caused by the nonuniform distribution of holes in the channel stops between sensor columns. Over the course of Large Synoptic Survey Telescope camera development, a number of practical fixes have been suggested to get rid of tearing. However, applying these fixes to our 16-channel Teledyne-e2v sensors leaves at best a distortion pattern at the vertical edges of every segment. Our working hypothesis is that the origin of the tearing is the parallel clocking itself, which moves the holes that are present in the channel stops regions. The efficiency of these transfers depends strongly on the details of the clocking operations, resulting in the observed variety of distortion patterns. Removal of most of the distortion patterns can, therefore, be achieved by executing a purge operation, which flattens back the hole distribution in the channel stops, immediately before acquiring a frame. A more effective solution is to switch all clocking operations to use a bipolar voltage set.
The Dark Energy Spectroscopic Instrument (DESI) is under construction to measure the expansion history of the Universe using the Baryon Acoustic Oscillation technique. The spectra of 35 million galaxies and quasars over 14000 deg² will be measured during the life of the experiment. A new prime focus corrector for the KPNO Mayall telescope will deliver light to 5000 fiber optic positioners. The fibers in turn feed ten broad-band spectrographs. A consortium of Aix-Marseille University (AMU) and CNRS laboratories (LAM, OHP and CPPM) together with LPNHE (CNRS, Universities Pierre et Marie Curie and Paris-Diderot) and the WINLIGHT Systems company based in Pertuis (France), are in charge of integrating and validating the performance requirements of the full spectrographs. This includes the cryostats, shutters and other mechanisms. The first spectrograph of the series of ten has been fully tested and the performance requirements verified for the following items: focus, image quality, straylight, stability, detector properties and throughput. We present the experimental setup, the test procedures and the results.