change makes visiting Gemini easier , and we hope that more users will consider visiting Gemini North or South via our Priority Visitor mode , as well as for Classical observing programs . Principal Investigators should also remember that our Bring-One , Get-One program for early-career astronomers is ideally suited to BFO .
Watch for a look back at the first six months of BFO at both Gemini sites in the next ( July ) issue of GeminiFocus .
First 20 Months of Hamamatsu CCDs in GMOS-S
In June 2014 we installed new red-sensitive Hamamatsu CCDs in GMOS-S . The main scientific driver behind this important upgrade was the improved quantum efficiency at the longest accessible wavelengths , combined with reduced fringing . We achieved both of these improvements , and initially everything looked positive . Since then , however , a number of problems have developed , some of which still defy explanation . A brief summary of the problems ’ history , and an update on the current status of the detectors , follows .
An Unfortunate Turn
After successful commissioning of the new CCDs , a couple of unexpected technical problems appeared that negatively affected the CCDs ’ expected performance : severe smearing of charge (“ bleeding ”) on CCD1 ; and a “ banding ” effect when binning , under which saturated pixels caused a depression with respect to the zero level across all the pixels from the same row within the affected amplifier . The bleeding problem was intermittent and made data on CCD1 essentially useless for all purposes ; the banding issue was manageable , although it made data reduction very complicated .
As science operations were already underway , we had to make the best of the situation while trying to find a technical solution for the problems . Eventually we found the charge bleeding problem in the controller backplane , which we replaced during the telescope shutdown in August 2014 . The fix brought the charge transfer efficiency ( CTE ) measurement back to ~ 0.999999 and therefore within specifications .
With the bleeding fixed , the “ banding ” issue remained , and by February 2015 , we saw more serious complications when it coupled to a column of hot pixels that accumulated spurious charge on amplifier # 5 ( on CCD2 ). With longer exposure times and binning , the saturation of this column became worse , causing the background level in the section of the CCD on the same amplifier to deviate from the normal level by up to 25 %. The best strategy was to avoid using the region of amplifier # 5 as much as possible . An investigation of the historic trend of this effect showed that the problem had become progressively worse , particularly for long exposures and full frame binned readout .
After lengthy investigations we identified a solution for the banding problem by implementing new ARC47 Rev . E video boards — which still had to be modified in order to offer the same good readout noise performance as the previous boards . Satisfactory results were finally reached in May of 2015 .
Mysterious Events
In May 2015 , another problem cropped up , namely a CTE issue affecting CCD1 in Nod & Shuffle ( N & S ) data . CCD2 and CCD3 were not affected at all . Again we formed an ad-hoc tiger team to work on a remediation plan . Complicating matters , GMOS-S , our most highly-demanded instrument , was in nearcontinuous use . Then what no one expected happened : by the end of July 2015 , the CCD1 CTE problem spontaneously disappeared , without any intervention , and N & S spectroscopy programs were resumed .
22 GeminiFocus April 2017