Figure 20 . Program completion in 2015B at Gemini South . Note the significant numbers of unstarted programs ( driven by instrument unavailability ), and the excess of Band 3 programs in the 100 % -complete bin ( a signature of bad weather ).
Figure 21 . GPI ( center ) installed on the up-looking port of Gemini South .
FLAMINGOS-2 is at top , and GMOS-S is at bottom .
ance for that . Recently , we ’ ve made a change to the way we fill the queue for Gemini South . Figure 19 shows a repetitive five-year pattern of weather losses at Gemini South ; because we filled the queue each month as if the weather loss was uniform ( and it wasn ’ t ), we forced ourselves to battle the elements at the worst times of the year . In the 2016A TAC process , we adjusted the way we fill the queue : we no longer overload May-September , and allow more programs into the southern summer months . We ’ ll see how it goes now that we ’ re into the semester itself .
2015B : Major Challenges at Gemini South
As mentioned , Gemini South has had some challenging semesters of late . Semester 2015B , for instance , had plenty of adversity to go around : the Gemini Multi-conjugate adaptive optics System ( GeMS ) out of action due to a major earthquake that struck in
September 2015 ; the Gemini Planet Imager ( GPI ) still ramping up ; and many programs either lost completely to weather , or executed under marginal conditions . Based on the discussion above , we would therefore expect a significant hit on programs in all Bands , with Band 3 ( able to take the worst conditions , and therefore not containing any GPI or GeMS programs ) performing reasonably well . That ’ s borne out by the results shown in Figure 20 : a significant number of GPI programs were not attempted at all , hardly any GeMS programs started , and many programs ended up in the “ tail ” of completions below 100 %.
GPI and Telescope Vibration
In late 2013 , early commissioning tests of the Gemini Planet Imager ( GPI ) on the Gemini South telescope ( Figure 21 ) revealed a strong oscillation in the corrected wavefront , similar to defocus . The 60 Hz oscillation frequency pointed to the GPI Stirling cycle cryocoolers ( which run at 60 Hz ) as the cause . But we did not understand the mechanism that disturbed the optical wavefront . After fitting the telescope optics with accelerometers , a team of Gemini scientists and engineers detected the oscillations in the primary mirror ( M1 ). The center of M1 was vibrating relative to the outer edge with a peak-to-peak amplitude of 840 nanometers ( nm ) — sufficient to cause a focus-like shift of about 1 millimeter at the GPI focus . The vibration completely disappeared when we turned off the GPI cryocoolers .
To improve the delivered wavefront , the GPI team first developed a software filter to measure the 60 Hz focus oscillations . They then applied a correction signal to GPI ’ s adaptive optics . The filter improved GPI ’ s performance
42 GeminiFocus January 2017 | 2016 Year in Review