Gemini South Multi-Object Spectrograph (GMOS-S) detector Video boards upgrade: improved performance for the Hamamatsu CCDs.

GMOS-S was upgraded with new Hamamatsu CCDs on June 2014, featuring an improved red sensitivity with respect to the previous detectors and significantly less fringing. Early after the commissioning, an issue was identified when observing in any of the binned readout modes, namely that saturated pixels produced a decrease of counts with respect to the bias level in neighboring pixels. This effect, also known as 'banding', spanned the entire width of the amplifier, and while it did not destroy information, it rendered data reduction very cumbersome. Making matters worse, due to the saturation of a bad column on amplifier number 5 (on CCD2, near the middle of the focal plane), it ended up affecting the entire amplifier for almost all exposures longer than a minute. A team of Gemini instrument scientists and engineers investigated the issue and identified the root cause of the problem as originated in the ARC controller video boards. After significant lab testing, it was verified that a newly available revision of the video boards would solve the problem, though modification of the software was required in order to be compatible with them. This work was performed during the last semester of 2014 and the first semester of 2015. The new video boards were installed and commissioned during August 2015. As of September 1st, the new boards are fully installed and integrated, and the 'banding' effect has been completely eliminated. A short period of time was devoted to the recharacterization of the detector system and the new values for the gains, read noise and full well capacity have been derived. As an added benefit, the full well was increased by ~ 10 percent with respect to the previous value. The GMOS-S new detectors are now operating normally in the Gemini observing queue, and performing at full capacity.