Physics
Fall2014Colloquium
Monday,September29th
Dr.ClairePoppett
LawrenceBerkeleyNationalLab
4:05PMinTH411
Refreshmentsservedat3:50PM
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Dark Energy Spectroscopic Instrument: Probing dark energy with
optical fibers and 5000 robots
Abstract:
TheDarkEnergySpectroscopicInstrument(DESI)isaStageIVground‐baseddarkenergyex‐
perimentthatwillbeinstalledattheMayallTelescopeinArizona.This iveyearsurveycover‐
ing14,000deg2toz=3.5willuse ivethousandroboticallypositionedoptical ibersthatcan
bequicklyrecon iguredwitha5μmpositioningaccuracy.Iwillpresentanoverviewoftheen‐
tireinstrumentwithafocusonthe iberperformanceinthenearandfar ieldoftwotypesof
roboticpositioners:tiltingspinemechanicalsimulatorsandeccentricaxis(orθ−φ)position‐
ers.Thefar ieldperformanceofthe iberisimportantsincetheinstrumentef iciencyisad‐
verselyaffectediflightfromthe ibersentersthespectrographatafasterfocalratiothanthe
spectrographcanaccept(f/3.57intheDESIdesign).Degradationofthefocalratiooflightis
causedbylightenteringthe iberoffaxis(tiltingpositioner)orbending,twisting,andstressof
the iber(eccentricaxis)positioner.Thestabilityofthenear ieldintensitydistributionofthe
iberdeterminesthespectrographpointspreadfunction(PSF).IfthePSFchangesfromthecal‐
ibrationtothescienceexposures,thiswillresultinanextractionbias.ForDESI,aparticular
concernisthedistortionsinthePSFduetomovementofthe ibersduringre‐pointing.