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[ cols="^ " , ]         ' '' ''    frequency & 60hz ( tuning screws 59.5 to 60.5hz ) +    ' '' ''    moving mass & 2.26 lbs ( 1.03 kg ) +    ' '' ''    total mass & 2.26 lbs ( 1.03 kg ) +    ' '' ''    max stroke ( hard limit ) & @xmath1 0.135  ( 3.43 mm ) +    ' '' ''    estimated operational stroke & @xmath0 0.100  ( 2.54 mm ) +    ' '' ''
force reacted at 60hz and @xmath1 0.100  & 334
n +    ' '' ''    force reacted at 60hz and @xmath1 0.135  & 467 n +    fig .
[ fig : tva_combined ] shows one of the two gpi cryocoolers with a tva from csa moog mounted .
its specifications are listed in tab .
[ tab : tva ] .
the tva is specified for 60hz and can be finetuned by @xmath10.5hz using the depicted tuning screw . as stated in sect .
[ sect : theory ] the fine tuning is critical for absorbing efficiency . in the worst case amplification
can occur , particularly if a natural structural mode is located in the close neighborhood .
it is notable ( table  [ tab : tva ] ) that even though the tva mass is only about one kilogram and has a maximum stroke of a three and a half mm , a reacting force and a `` stroke '' @xmath2 : @xmath3 . ] up to 467n is created at 60hz ( corresponding to 48 kg if translated into weight ) .
after shipment and arrival of gpi to cerro pachon in chile , it appeared that the tvas lost their effectiveness which is currently investigated by csa ( and the tvas were dismounted from the cryocooler heads for inspection ) .
for the next commissioning run in september 2014 we expect these tvas to be mounted again with a corrected fine - tuning .          ' '' ''    mode & & & & & & +    ' '' ''    & & & & & & +    ' '' ''    1 & interface ring & 2,276 & 48.7 & 22.8 & 0.01 & gpi frame torsion about y +    ' '' ''    2 & interface rings & 2,172 & 52.2 & 21.7 & 0.01 & gpi frame rocking about y +    ' '' ''    3 & cal & 213 & 74.6 & 2.1 & 0.01 & cal plunging in z against rocking ifs +    ' '' ''    4 & cal & 450 & 80.4 & 4.5 & 0.01 & ifs bouncing against gpi frame +    ' '' ''    5 & cal & 690 & 93.8 & 6.9 & 0.01 & cal rocking about y +    ' '' ''    6 & cal & ?
& 114.9 & ? & & cal torsion +    ' '' ''    7 & cal & 422 & 126.5 & 4.2 & 0.01 & cal and ifs rocking about x , z +    csa moog was contracted to study gpi s modal frequencies@xcite .
they prepared a wire frame model for simulation .
test measurements were done using a modal hammer to excite the gpi structure and accelerometer sensors were attached to measure the response .
[ fig : csa - study - cominbed ] visualizes the wire frame model .
the excitation locations are marked with black arrows ; one arrow is hidden behind the purple cal unit .
the red dots indicate the locations of the accelerometer sensors . tab .
[ tab : gpi - modes ] shows the extracted modes from their data set in the frequency range from 40 to 150hz .
all recommended masses for the tmds by csa are based on a mass ratio of @xmath4 .
the study was done in december 2012 while gpi was in the a&t laboratory@xcite at ucsc and mounted to a carriage creeper ( dubbed the `` l - frame '' ) .
the first two modes with the lowest frequencies ( 49 and 52hz ) are likely to shift when gpi is attached to the telescope .
this is one of the reasons why no tmds have been installed yet .
the decision was taken to revisit this study once enough on - telescope experience is gained to evaluate if tmds are required . to this point
no tmds are installed and so far it seems that no tmds are required .
the main vibration line in our error budget ( 60hz ) is not on a structural mode assuming that the two low frequency structural modes of gpi have not shifted through the mounting at the telescope in an unfortunate way .
the topic of vibration in general is complex and is a topic prone to misunderstandings between mechanics , opticians , electrical engineers and scientists .
an awareness of context and language is critical in this interdisciplinary field . coordinating a sensible and successful vibration mitigation effort in an effective way
is a challenge and requires its resources and roles such as an interconnected project manager and a `` vibration officer '' .
there is a constant struggle with hard decisions : does it make sense to spend the extra dollar to finish a study that runs out of funding ? and avoiding the `` sunk cost '' effect in a `` prototype '' development environment is never easy .
gpi has the strongest constraints on its wavefront error budget of all gemini south instruments and certainly has risen the observatory s awareness of vibrations in the telescope environment .
the mitigation using an lqg controller for both pointing and focus has been implemented successfully .
this is an intriguing demonstration of how new technologies can be used to mitigate vibration effects from the controller side . in that sense , using the lqg controller , the impact of the cryocooler vibrations on gpi s performance is low if not negligible .
the only exception is the high - order wavefront front sensor of the cal unit .
it is a mach - zehnder type interferometer and the loss in fringe contrast due to the vibration prevents its commissioning .
fortunately , this is not critical , since the basic functionalities of the cal ( keeping the star bore - sight , and low - order wavefront sensing ) are not impacted .
furthermore , the gpi team has developed further alternatives to treat ncpa such as speckle - nulling . and so far , even though other instruments such as gems _ can _ see the vibration signal on their wavefront sensors , it seems that there is no impact on performance .
nevertheless , a `` root treatment '' is always preferable and it is in the observatory s natural interest to keep its `` vibration '' spectra as clean as possible .
recently , sunpower has developed an active cancellation system ( acs ) as add - on for its gt cryocooler series and we are investigating to upgrade our coolers with it .
there is widespread use of tmds for aircrafts , machinery , in the car industry , for trains and railways , buildings , power lines and in optical systems . and certainly there is still plenty of room to make a better use of these devices in the field of astronomical instrumentation and optical telescopes .    a good infrastructure of acceleration sensors is required to effectively understand and solve unwanted vibrations .
otherwise measurements are difficult to obtain and to confirm , and resources of the observatory are wasted in large overheads . in the case of the gemini observatory it seems recommendable to have at least 5 sensors around the cell of m1 ( and
one could dream about having each hydraulic m1 actuator equipped with a sensor ) .
elts will take this matter ( hopefully ) serious , plan for an integrated design and maybe feature a dedicated `` vibration control room '' .
as gpi and other ( xao ) instruments can always be seen as pathfinders for future telescopes and instrumentation , sharing our challenges is hopefully helpful to find suitable solutions in other complex environments .
gpi can serve as a good example for the successful implementation of a mitigation strategy and for how powerful mitigation strategies such as lgq filtering of the wavefront have become .
the authors thank paul langlois for suggestions and improvements on the figures .
the gemini observatory is operated by the association of universities for research in astronomy , inc .
, under a cooperative agreement with the nsf on behalf of the gemini partnership : the national science foundation ( united states ) , the national research council ( canada ) , conicyt ( chile ) , the australian research council ( australia ) , ministrio da cincia , tecnologia e inovao ( brazil ) and ministerio de ciencia , tecnologa e innovacin productiva ( argentina )
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the gemini planet imager ( gpi ) entered on - sky commissioning and had its first - light at the gemini south ( gs ) telescope in november 2013 . 
 gpi is an extreme adaptive optics ( xao ) , high - contrast imager and integral - field spectrograph dedicated to the direct detection of hot exo - planets down to a jupiter mass . 
 the performance of the apodized pupil lyot coronagraph depends critically upon the residual wavefront error ( design goal of 60nmrms with @xmath05mas rms tip / tilt ) , and therefore is most sensitive to vibration ( internal or external ) of gemini s instrument suite . 
 excess vibration can be mitigated by a variety of methods such as passive or active dampening at the instrument or telescope structure or kalman filtering of specific frequencies with the ao control loop . 
 understanding the sources , magnitudes and impact of vibration is key to mitigation . 
 this paper gives an overview of related investigations based on instrument data ( gpi ao module ) as well as external data from accelerometer sensors placed at different locations on the gs telescope structure . 
 we report the status of related mitigation efforts , and present corresponding results .    
 [ sec : intro ]