On-sky performance of the QACITS pointing control technique with the Keck/NIRC2 vortex coronagraph

0.48 MB·

by  Elsa Huby

Astronomy&Astrophysicsmanuscriptno.Huby_2017_onsky_qacits_arxiv (cid:13)cESO2017
January24,2017
On-sky performance of the QACITS pointing control technique with
the Keck/NIRC2 vortex coronagraph
E.Huby1,⋆,M.Bottom2,B.Femenia3,H.Ngo4,D.Mawet2,5,E.Serabyn2,andO.Absil1,⋆⋆
1Spacesciences,Technologies,andAstrophysicsResearch(STAR)Institute,UniversitédeLiège,19cAlléeduSixAoût,4000Liège,
Belgium
2JetPropulsionLaboratory,CaliforniaInstituteofTechnology,4800OakGroveDrive,Pasadena,CA91109,USA
3W.M.KeckObservatory,65-1120MamalahoaHwy.,Kamuela,HI96743,USA
7 4 CaliforniaInstituteofTechnology,DivisionofGeologicalandPlanetarySciences,1200E.CaliforniaBlvd,Pasadena,CA91125,
1 USA
0 5CaliforniaInstituteofTechnology,DivisionofPhysics,MathematicsandAstronomy,1200E.CaliforniaBlvd,Pasadena,CA91125,
2 USA

n
January24,2017
a
J

3 ABSTRACT
2
Context.AvortexcoronagraphisnowavailableforhighcontrastobservationswiththeKeck/NIRC2instrumentatLband.Thevortex
] coronagraphusesavortexphasemaskinafocalplaneandaLyotstopinadownstreampupilplanetoprovidehighcontrastatsmall
M angularseparationsfromtheobservedhoststar.
Aims.Reaching the optimal performance of the coronagraph requires fine control of the wavefront incident on the phase mask.
I In particular, centering errors can lead to significant stellar light leakage that degrades the contrast performance and prevents the
.
h observationoffaintplanetarycompanionsaroundtheobservedstars.Itisthuscriticaltocorrectforthepossibleslowdriftofthestar
p imagefromthephasemaskcenter,generallyduetomechanicalflexuresinducedbytemperatureand/orgravityfieldvariation,orto
- misalignmentbetweentheopticsthatrotateinpupiltrackingmode.
o
Methods.A control loop based on the QACITS algorithm for the vortex coronagraph has been developed and deployed for the
r
t Keck/NIRC2instrument.Thisalgorithmexecutestheentireobservingsequence,includingthecalibrationsteps,initialcenteringof
s thestaronthevortexcenterandstabilisationduringtheacquisitionofscienceframes.
a
Results.On-skydatashowthattheQACITScontrolloopstabilizesthepositionofthestarimagedownto2.4masrmsatafrequency
[
ofabout0.02Hz.However,theaccuracyoftheestimatorisprobablylimitedbyasystematicerrorduetoamisalignmentoftheLyot

1 stopwithrespecttotheentrancepupil,estimatedtobeontheorderof4.5mas.Amethodtoreducetheamplitudeofthisbiasdownto
v 1masisproposed.
7 Conclusions.TheQACITScontrolloophasbeensuccessfullyimplementedandprovidesarobustmethodtocenterandstabilizethe
9 starimageonthevortexmask.Inaddition,QACITSensuresarepeatablepointingqualityandsignificantlyimprovestheobserving
3 efficiencycomparedtomanualoperations.ItisnowroutinelyusedforvortexcoronagraphobservationsatKeck/NIRC2,providing
6 contrastandangularresolutioncapabilitiessuitedforexoplanetanddiskimaging.
0
Keywords. Instrumentation:adaptiveoptics–Techniques:highangularresolution–Methods:observational
.
1
0
7
1. Introduction ingcommissioning(Absiletal. 2016).In addition,suchanim-
1
ager at L band benefits from advantageousatmospheric condi-
:In June 2015, a new coronagraphic mode available with the
v tions and generally more favourable planet-to-star contrast ra-
iKeck/NIRC2instrument(Serabynetal.2017)cameonline.The tio thanat shorterwavelengths,especially in the case of young
XL band imager is now equipped with a vector vortex coro-
self-luminous giant exoplanets. As such, the vortex mode at
rnagraph based on an annular groove phase mask (AGPM,
Keck/NIRC2 provides a promising tool to directly image exo-
a
Mawetetal. 2005). It consists of a circular subwavelength
planets on relatively compact orbits, thus complementing sec-
grating etched onto a diamond substrate (VargasCatalánetal.
ondgenerationhighcontrastinstrumentsworkinginthenearin-
2016), placed in an intermediate focal plane wheel and work-
frared,e.g.SPHERE(Beuzitetal.2008),GPI(Macintoshetal.
inginconjunctionwithadownstreamLyotstop.Combinedwith
2014)orSCExAO(Jovanovicetal.2015).
AdaptiveOptics(AO) correctionandadvancedpost-processing
Withtheimplementationofthisnewmode,thenecessityof
techniques (e.g. Soummeretal. 2012; Amara&Quanz 2012;
afinepointingcontroliscrucialfortworeasons:operationeffi-
GomezGonzalezetal. 2016), this mode allows high-contrast
ciencyandcontrastperformance.First,observingwithavortex
imagingofplanetarycompanionsandcircumstellardisksaround
coronagraphrequiresthestarimagetobeinitiallycenteredonto
stars at very small angular separations, with an effective inner
workingangle(IWA)of 0′.′12.A5-σsensitivitylimitof10mag thevortexmask,andifdonemanually,thisprocesscanproveto
at0′.′5forastarofmagnitudeK=5hasbeendemonstrateddur- bechallengingandtimeconsuming.AsillustratedinHubyetal.
(2015,2016)andrecalledinthenextsection,thisisparticularly
⋆ F.R.S.-FNRSPostdoctoralResearcher true in case of centrally obstructed telescopes, where counter-
⋆⋆ F.R.S.-FNRSResearchAssociate intuitivefluxasymmetrycanappearwhenthestarimageisvery
Articlenumber,page1of9
A&Aproofs:manuscriptno.Huby_2017_onsky_qacits_arxiv
closetothecenterofthemask.Thiseffectcanleadtothemisin-
terpretationoftheoffsetdirectionneededtoimprovethecenter-
ingofthestarimageandmakethecenteringprocesslongand/or
not optimal. Repeatability and rapidity of the alignment were
thus major incentives for the implementation of an automated
pointingcontrolsystem.
Additionally,reachingtheoptimalperformanceofthecoro-
nagraphrequiresahighqualitywavefront.Althoughaberrations
due to turbulence are largely removed by the highly efficient
Keck AO system, routinely delivering Strehl ratios on the or-
der of 85-90% at L band, noncommon path wavefront errors
inside the instrument are inevitable and the source of substan- Fig.1.PupilmasksusedinthesimulationsoftheKeck/NIRC2instru-
ment,asdefinedinTable1:entrancepupilontheleftandLyotstopon
tiallossofcontrastperformance.Indeed,reachingasmallIWA
theright.Thecircumscribedcircleinwhitedottedlinehasadiameter
with a focal-plane phase mask coronagraphcomes at the price
of10.93m.
of a high sensitivity to low order aberrations. The most detri-
mentalofthereare generallytip andtiltaberrationsduetome-
Table1.PupilconfigurationoftheKecktelescopeandLyotstopmask
chanicalflexuresand/oropticsrotatingforpupiltracking,which
usedforthevortexcoronagraphinNIRC2.
are commonon large telescopes. The controland correctionof
these aberrations are thus crucial and require the implementa-
parameter normalizedvalue
tionofadditionaldedicatedsensors.Here,wefocusonloworder
Entrancepupildiam. 1.00
aberrationsensors,whicharecriticalinparticularforsmallIWA
(circumscribedcircle)
coronagraphs. These sensors usually work in conjunction with
Centralobstructiondiam. 0.24
othertechniquessensitivetohigh-ordernoncommonpathaber-
Entrancepupilspiderwidth 0.0023
rations,suchasspecklenulling,electricfieldconjugation,phase
Inter-segmentgap 2.7×10−4
diversity,Zernikewavefrontsensing,orinterferometricmethods
Lyotstopexternaldiameter 0.80
(see Bottometal. 2017,andreferencestherein).Asa matterof
Lyotstopinternaldiameter 0.27
fact,specklenullingwasrecentlyimplementedonKeck/NIRC2
Lyotstopspiderwidth 0.0061
forusewiththevortexcoronagraph(Bottometal.2016,Bottom
etal.inprep).
Concerning low order aberration control, several solutions
instrument. This paper reports on the practical implementation
have been developedand implemented in currenthigh contrast
andthe on-skyperformanceofthis pointingsensor.In the next
instruments.Inordertocatch mostofthe noncommonpath er-
section,theprincipleofthistechniqueisrecalledandthediffer-
rors, the extra sensor must be placed as close as possible to
ent functions performed by the controller implemented for the
the coronagraphic mask. For instance, in the case of the Dif-
Keck/NIRC2 instrument are described. In Sect. 3, the experi-
ferentialTip-Tilt Sensor (DTTS) integratedto the coronograph
mentalcalibrationofthetip-tiltestimatorispresented,andpos-
of SPHERE (Baudozetal. 2010), a beamsplitter placed right
siblesourcesofbiasareinvestigated.InSect.4,theon-skyper-
before the focal plane phase mask sends a few percent of the
formanceofthecontrolloopisassessedandcomparedwithre-
light towards a separate detector in order to monitor the jitter
and drift of the star image. Another solution consists in using sultsobtainedwithouttheQACITScontroller.Lastly,inSect.5,
thestatusofthealgorithmisdiscussedandfurtherongoingde-
thelightthatisrejectedbythecoronagraph,i.e.thatisstopped
velopmentsaredescribed.
by the occulting focal plane mask (Coronagraphic Low-Order
Wave-Front Sensor, CLOWFS, Guyonetal. 2009), like in GPI
(Wallaceetal. 2010), or stopped by the diaphragm in the Lyot
plane, in case of a phase mask (Lyot-based Low-Order Wave- 2. ImplementationofQACITSatKeck/NIRC2
FrontSensor,LLOWFS,Singhetal.2014).OnlytheLLOWFS
system hasbeencharacterizedon-sky,reportingpointingresid- Throughoutthepaper,thetip-tiltamplitudeisgiveneitherinmil-
ualsof0.23masor5.8×10−3λ/D(Singhetal.2015).However, liarcseconds(mas)orinunitsoftheangularresolutionelement
it has to be noted that all these sensors are inherently notfully defined as λ/DL, with λ the wavelength and DL the equivalent
common path and require a specific optical layout, which can diameterof the Lyotstop, which definesthe angularresolution
makethemchallengingtointegratetoanexistingsystem. inthefinalimage.Inpractice,thisdiameterisequalto8.7mand
Anotherkindofsolutionisbasedonthesoleanalysisofthe correspondsto80%oftheentrancepupildiameter,definedhere
coronagraphicimages.Inthis case,the sensoris fullycommon as the diameter of the circumscribed circle, i.e. 10.93m. The
path,butthenumberofmodesthatcanbecorrectediscurrently Lyot stop almost matches the inscribed circle in the hexagonal
reducedto tip andtilt. Suchsensor hasbeenfirstproposedand entrancepupil,whichis9mindiameter.Thewavelengthistaken
tested in laboratory for the four quadrant phase mask (FQPM) asthecentralwavelengthoftheLbandfilter,i.e.λ=3.776µm,
coronagraph(Masetal.2012)incaseofanonobstructedpupil, which givesλ/DL=89.3mas. The entrancepupiland Lyotstop
leadingtoanaccuracyof6.5×10−2λ/Dmeasuredinlaboratory. shapesusedinthesimulationspresentedinthispaperareshown
Theprinciplewasthenextendedtothecaseofthevortexcoron- inFig.1.Thelattercorrespondstotheincirclepupilmaskavail-
agraphwithacentrallyobscuredpupil(Hubyetal.2015).Asa ableintheNIRC2instrument.Thepupilfeaturedimensionsare
completelynon-invasivemethodthatdoesnotrequireanysetup reportedinTable1.
modification, this technique called QACITS (Quadrant Analy- Inthissection,theprincipleoftheQACITSestimatorcorre-
sisofCoronagraphicImagesforTip-tiltSensing)thusappeared spondingto the Keck telescope pupilis described, and the dif-
as one of the logical and most accessible solutions for imple- ferent operations performed by the controller implemented on
mentation on the new vortex coronagraph of the Keck/NIRC2 Keck/NIRC2aredetailed.
Articlenumber,page2of9
E.Hubyetal.:On-skyperformanceoftheQACITSpointingcontroltechnique
−0.2 −0.1 0.0 0.1 0.2
Fig.2.Imagesofthetwoasymmetriccomponentsappearinginthefinal
coronagraphicimageshapeinpresenceofhorizontaltip-tiltaberration.
Positivetip-tiltamplitudeinducesashiftofthestarimagetowardsthe
right.Left:A cosψ,contributionofthecircularnon-obstructedpupil.
circ
Right: A cosψ, contribution of the central obstruction. The circles Fig. 3. Differential intensity measured on simulated images with tip-
obsc
delimiting the inner and outer regions have a radius of 1.6λ/D and tiltappliedalongthexaxis.Thebluesolidlinecorrespondstotheflux
L
2.7λ/D . A comparison of the horizontal profiles of these images is integratedintheinnerareaonly,whilethereddashedlinecorresponds
L
displayedinFig.6ofHubyetal.(2015). tothefluxintegratedintheouterareaoftheimage.Thelightbluedotted
and red dash-dotted lines are the linear approximations for the inner
andoutermeasurements,respectively,thatarevalidinthesmalltip-tilt
2.1.TheQACITSestimator regime(i.e.<0.15λ/D and<0.5λ/D ,respectively).
L L
The QACITS estimator aims to measure the pointing errors
affecting the beam incident on a coronagraphic phase mask outerregionoftheimage,andtheyhaveoppositesigns.This
directly from the analysis of the coronagraphic image shape contributionvarieslinearlywiththetip-tiltamplitude.
(Masetal.2012;Hubyetal.2015).Asaresult,itprobesaberra-
Tocompletethequantitativecomparison,thedifferentialin-
tionsthatare fullycommonpathwith the coronagraphicmask.
tensityamplitudeduetothecircularapertureintheinnerregion
Giventhefluxlevelremainingafterthecoronagraph,aberrations
(Fig.2,left)isabout4timeslowerthanthedifferentialintensity
induced downstream the mask have a negligible impact on the
amplitudeduetotheobstructioninthesameregion(right).
contrast performance. The QACITS estimator is based on the
measurementof differentialintensities, resulting from the inte- The analysis of the image in these two separate concentric
regionsallows the partial disentanglementof the two contribu-
grationandsubtractionofthefluxinthehalvesoftheimage,as
tions.Intheinnerarea,thetwotermsarepresentandhaveoppo-
inaquadrantpositionsensor,normalizedbythetotalfluxofthe
site signs,thuscompensatingeachotherandmakingthediffer-
noncoronagraphicpointspreadfunction(PSF).
entialintensitymeasurementambiguous,ascanbeseeninFig.3.
Hubyetal. (2015) have previously shown that the electric
Thisisalsothereasonwhytheobservercanbemisled,sincethe
fieldinthecaseofacentrallyobstructedpupilcanbedescribed
flux asymmetry in this region goes in the direction opposite to
asthesumoftwocontributionsrelatedrespectivelytothecircu-
the actual offset of the star, as illustrated in Fig. 4. Moreover,
lar unobstructedpupiland the central obstruction(addednega-
foratip-tiltamplitudeofabout0.4λ/D thecentralregionlooks
tivelyinamplitude),leadingtotwodistinctcontributionsinthe L
likeasymmetricdoughnut,whichcanbeadditionallyconfusing
flux asymmetry of the final image. These two components are
(seeSect.4.2).Intheouterareahowever,thelinearcontribution
expressed as combinationsof Bessel functionsmodulated by a
duetothecentralobstructionprevails,avoidingambiguityinthe
cosψtermwithψ theazimuthalangle,asitwasestablishedby
measurement.
Eq.9and19inHubyetal.(2015).Inshort,foratip-tiltampli-
In practice, the QACITS estimator used at Keck/NIRC2 is
tude T applied along the x axis, the final image I can be de-
based on the differential intensity measured in the outer area
scribed as the sum of a symmetric term and two asymmetric
only,approximatedbyalinearmodel,whichisvalidinthesmall
terms:
tip-tilt regime (i.e. < 0.5λ/D , see Fig. 3). The linear approx-
L
I ∝ Isym+T3×Acirccosψ+T ×Aobsccosψ, (1) imation of the inner estimator is also monitored, since it can
carryusefulinformation(seeSect.3.2.2).Hereafter,theestima-
where A and A arecombinationsofBessel functionscor-
circ obsc tors based on the inner and outer parts of the image using the
respondingtothecontributionsofthecircularpupilandcentral
linear approximation will be designated as the inner and outer
obstruction,respectively.Theshapeofthetwoasymmetricterms
estimators,respectively.Itshouldbenotedthattheshapesofthe
aredisplayedinFig.2.Theycanbedecomposedintwoconcen-
differentialintensity curvesdepend on the telescope configura-
tricregions,definedbythesigninversionoftheiramplitude.The
tion.Inparticular,forasmallercentralobstruction(asisthecase
boundariesbetweeninnerand outerregionsare locatedatradii
for VLT/NaCo or LBT/LMIRCam with 14% and 11% central
of1.6λ/D and2.7λ/D .Theweightofeachcomponentinthe
L L obstruction,respectively),theslopesofthelinearmodelapprox-
final image is a function of the tip-tilt amplitude, making each
imationswouldbelower.
termcontributedifferentlytotheasymmetryoftheimageinthe
tworegions:
2.2.Closed-loopimplementationatKeck/NIRC2
– thedifferentialintensityduetotheidealcircularpupilis40
timesstrongerintheinnerthantheouterregionoftheimage. TheQACITSalgorithmhasbeenimplementedinIDLandtakes
In the final image,this contributiongrowswith the cube of care of the initial centering optimization, the pointing correc-
thetip-tiltamplitude. tionandthescienceacquisitions,makingobservationswiththe
– the amplitude of the differentialintensity due to the central vortexmodeuser-friendlyandhighlytime-efficient.Beforecall-
obstruction is about 3 times stronger in the inner than the ing the QACITS sequence, the observer only has to make sure
Articlenumber,page3of9
A&Aproofs:manuscriptno.Huby_2017_onsky_qacits_arxiv
Fig. 5. Tip-tilt scanning sequences carried out for characterizing the
QACITSestimatorbehaviour.Left:Truetip-tiltvaluesasmonitoredby
Fig. 4. Top: Simulated images for different tip-tilt amplitudes applied
the secondary component of the binary system, for the scanning se-
to the right along the horizontal axis, from left to right: 0.1λ/D ,
L quences along the x(green crosses) and y (blue squares) axes. Right:
0.16λ/D and 0.38λ/D . Each displayed image is normalized by its
L L Comparisonof thesimulatedpeaktransmissioncurve(fluxintegrated
maximalvalue.Thedottedcirclesshowtheboundariesfortheinnerand
onadiskofdiameter 1λ/D )withtheexperimentalresultsmeasured
outerareas.Bottom:Color-codedrepresentationofthefluxasymmetry L
fromtheon-sky data. TheeffectiveIWAisreached at aseparationof
toemphasize thevisual comparison of theasymmetry inthedifferent
1.4λ/D =125mas(computedfromthesimulatedcurve).
images.Theamplitudeofthegradientisequaltothemeasureddiffer- L
entialintensityandallimagesareshownwiththesamecolorscale.
(2016)andinausermanualavailableonline1.Thepixelsizeof
that the star is roughly centered onto the vortex mask (within the coronagraphicimages is 10mas/pixel (Serviceetal. 2016),
1λ/DL), a pointing requirement that is routinely achieved by i.e.about9pixelsperresolutionelementλ/DL.SincethePSFis
the AO system once a reference position has been saved. The wellsampled,wehavenotinvestigatedindetailtheeffectoflow
QACITScontrollerthenoperatesinthreesteps: samplingon the QACITSestimator.Thiswill be studiedin the
future,inparticularforapplicationonotherinstruments.
– A calibrationstep,whichtakesanunsaturatedimageofthe
starfaroffthecenterofthevortexmask(off-axisPSF),and
skyimages.Thepositionofthecenterofthevortexmaskis
3. Vortexmodecharacterization
identified by fitting the vortex center glow that is visible in
sky images (Absiletal. 2016). The actual offset of the star 3.1.On-skycalibration
image is then estimated by fitting a Gaussian profile to the
off-axis PSF, and this offset is corrected by sending a tele- The calibration of the model for the QACITS estimators has
scope offset commandin units of pixels. At the end of this been performed on-sky. For that purpose, a wide binary sys-
tem was observed,with the brightest componentof the system
step,thestarisroughlycenteredontothevortexphasemask
centered on the vortex mask. The monitoring of the compan-
(typicallywithinafew0.1λ/D ).Thisstepusuallyrequires
L
∼2mintobecompleted. ionpositionprovidesameanstoestimatethetruepositionofthe
– Anoptimizationsequence,whichconsistsofafewiterations starimagebehindthemask.OnOctober29th,2015,acquisitions
werethustakenonthebinarysystemofHD46780(L =5.5,
oftheQACITSloopthatarerunfasterthanthescientificac- primary
L =7.2).The separationwas 737.2masat the dateof the
quisitions(usingshorterintegrationtime andsmallerframe secondary
size to minimize NIRC2 overheads). By default, the align- observation(orbitparametersfrom Heintz 1993). The long pe-
riod of 118.9 years ensures that this separation does not vary
ment is considered optimized when the measured residual
duringthetimeofobservation.
tip-tilt has an amplitude smaller than 0.1λ/D . This crite-
L
rion can be tuned in the QACITS parameters (the observer The relative position of the two components of the binary
canrequirethatseveralconsecutiveestimationsfallbelowa werefirstestimated inan unsaturatedimage,byfittinga Gaus-
chosenlimit). With thedefaultsettingsofT = 0.2s(inte- sian profile to each PSF. The vector connectingthe position of
int
grationtime)and N =10(numberofco-addedimages) the two star images is then used as a reference to estimate the
coadd
foraframewidthof512pixels(minimalvalueforthevortex true position of the primary star image with respect to the po-
centertobeincludedinthesub-image),oneiterationis20s sitionofthesecondaryinthecoronagraphicimages.Giventhat
long.Thissequencetypicallytakes1-2min. observationsarecarriedoutinpupiltrackingmode,therotation
– Thescienceacquisitionsequence,withtheQACITScorrec- ofthepositionvectorisalsotakenintoaccountbycorrectingfor
tionappliedaftereachacquisition.Withthetypicalsettings the parallactic angle. The positions that have been probed dur-
of T = 0.5s and N = 50 for a full frame width of ingthetip-tiltscanningsequenceareplottedinFig.5,showing
int coadd
1024pixels,onescienceacquisitionis46slong(for25sof thattheinitialpositionthatwasassumedtobealignedwiththe
actualintegrationtime). vortexcenterwasactuallyoffsetbyabout−0.25λ/DLintipand
tilt, dueto animperfectmanualpositioningat thebeginningof
The correction algorithm applied during the science se- thesequence.Basedonthesedata,theoff-axispeaktransmission
quence consists of a proportional-integralcontroller, with pro- ofthevortexcoronagraphhasbeencomputedbyintegratingthe
portionalandintegralgainsGP = 0.3andGI = 0.1,whichhave fluxinadiskofdiameter1λ/DLcenteredontheactualstarim-
been tuned experimentally to ensure the stability of the loop. ageposition.AsshowninFig.5,theseexperimentalresultsare
The loop is run at a frequency defined by the time needed for in excellent agreement with the simulated curve, leading to an
oneacquisition,i.eabout0.02Hz,hencecorrectingfortheslow effectiveIWAof125mas.
drift(seeSect.4.1).TheQACITScallingsequenceandparame-
tersforKeck/NIRC2aredescribedinmoredetailsinHubyetal. 1 https://www2.keck.hawaii.edu/inst/nirc2/observing
Articlenumber,page4of9
E.Hubyetal.:On-skyperformanceoftheQACITSpointingcontroltechnique
Fig.6.ExperimentalcharacterizationoftheQACITSmodelfromon-skydata.Leftandrightplotscorrespondtothedifferentialintensitymea-
surementsasafunctionoftip-tiltforthexandydirections,respectively.Inbothcases,thedifferentialintensitywascomputedintheinner(blue
crosses) and outer (red pluses) areas of the image. The best fit models in the least-squares sense are shown in solid lines (see Eq. 2 for their
definition),andtheresidualsareshownintheplotsbelow.
Fig. 7. Same as Fig. 6 with data sets simulated for the same values of tip-tilt as the experimental data (including the offset in the orthogonal
direction,asplottedinFig.5),insteadofactualmeasurements.
Table2.ParametervaluesforthebestfitmodelsasdefinedbyEq.2.Theresultsarereportedfortheon-skydatasets(seeFig.6)aswellasfor
thedatasetssimulatedwiththesametip-tiltsampling(seeFig.7).Theidealmodelcorrespondstotheresultofthefitperformedonthesimulated
modelshowninFig.3,includingamuchlargernumber ofsimulateddatapoints(notbiasedbythesamplingorbytheoffsetintheorthogonal
directionoftheappliedtip/tilt).Valuesbelow10−3areconsideredinsignificantandmarkedas0.
Outerarea(linearmodel) Innerarea(cubicmodel)
dataset a σ b σ a σ b σ c σ x σ
a b a b c 0 x0
on-skyx 0.109 ±0.008 0.004 ±0.002 0.790 ±0.167 -0.014 ±0.057 -0.082 ±0.020 0.003 ±0.031
simulatedx 0.111 ±0.006 0 ±0.002 0.986 ±0.090 -0.055 ±0.034 -0.095 ±0.012 0 ±0.015
on-skyy 0.113 ±0.013 0.010 ±0.003 0.767 ±0.045 0.128 ±0.015 -0.084 ±0.005 -0.039 ±0.007
simulatedy 0.104 ±0.004 0 ±0.001 0.976 ±0.109 0.014 ±0.032 -0.106 ±0.010 -0.002 ±0.016
idealmodel 0.104 ±0.007 0 ±0.002 1.013 ±0.051 0 ±0.019 -0.146 ±0.007 0 ±0.009
Articlenumber,page5of9
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No aberr. Astig. 1 Coma 1 Trefoil 1
Defocus Astig. 2 Coma 2 Trefoil 2
Fig.8.Simulatedimagesobtainedwiththevortexcoronagraphonthe
Kecktelescope,inpresenceofloworderaberrations:100nmrmsdefo-
cus,70nmrmsfortheothers.Allimagesaredisplayedwiththesame
grayscale.
Fig.9.Effectofcoma(70nmrms)ontheQACITSmodel.Ontheleft,
the image shows the shape obtained when the star image is perfectly
centered on the vortex mask, and the image below isthe color-coded
Figure 6 shows the differential intensity measured in these
representationofthemeasureddifferentialintensityininnerandouter
imagesinthe xandydirectionsasafunctionofthetruetipand regionsoftheimagetoemphasizethevisualcomparisonoftheasym-
tilt,respectively.Thedatapointscorrespondingtotiportiltam- metryinbothregions(thefluxgradientineachareahasanamplitude
plitudelowerthan0.5λ/DLwerefittedbypolynomialfunctions equaltothemeasureddifferentialintensity).
expressedas
ax+b,fortheouterarea,and aspossible.However,thissituationcorrespondstothebestcen-
(2)
a(x−x0)3+b(x−x0)2+c(x−x0),fortheinnerarea. teringofthestar imageonthemaskonlyiftheobservedtarget
is a point source, thus assuming that there is no bright asym-
All best fit parameters computed in the least-squares sense are metricstructurepresentin theveryclosevicinityofthe central
reportedinTable2. star(within2.7λ/D or240mas)andthattheopticalsetupisnot
L
Tovalidatethemodel,datasetsweresimulatedwiththesame affectedbyotheraberrations.Whiletheformersituationconsti-
valuesoftipandtilt(includingtheoffsetintherespectiveorthog- tutesanintrinsiclimitationoftheQACITSestimator,biasdueto
onal direction as shown in Fig. 5). These simulated data were opticalimperfectionscanbemitigatedtosomeextent,assuming
analyzedusingthesamefittingprocedure.Theresultsareshown thattheircauseisunderstood.
inFig.7andthebestfitparametervaluesarereportedinTable2.
Theidealmodelcomputedfromsimulationswithoutoffset(only
puretiportiltwasapplied,aspresentedinFig.3)andwithafiner 3.2.1. Effectofloworderaberrations
samplingisalsoanalyzedinthesamewayforcomparison.The
Low order aberrations have been investigated as sources of
correspondingbestfitparametersarealsoreportedinthebottom
asymmetry in the final image. The resulting image shapes are
lineofTable2.
shownin Fig. 8. While defocusandastigmatism havean effect
Asexpected,thedifferentialintensitymeasuredintheouter
ontheshapeoftheimage,theydonotaffectthecentralsymme-
area of the image is well approximated by a linear function,
try.Ontheotherhand,comaandtrefoildo.Atthesameaberra-
in particular for small tip-tilt amplitude (for tip-tilt amplitude
tion level, the effect of coma is almost one orderof magnitude
> 0.5λ/D ,thedatapointsdivergefromthelinearmodel).The
L strongerthan trefoil, and forthat reason, its effect hasbeen in-
slopesareinagreementwiththevaluespredictedbysimulations,
vestigatedinmoredetail.
butsmallglobaloffsetsareobserved(nonzerovalueforparam-
eterb).Thiseffectisstrongerforthedatacorrespondingtothey The impact of coma on the QACITS model is shown in
direction.Possiblecausesforthiseffectwillbediscussedinthe Fig.9:thedifferentialintensitymeasuredintheinnerareaissig-
nextsubsection. nificantlyaffected,whiletheouterdifferentialintensityismostly
The differential intensity measured in the inner area shows unchanged.Indeed,althoughitisnotvisuallyobvious,theasym-
the expectedsign inversionsaround±0.4λ/D . The modelpa- metryinducedbycomaismostlyconcentratedintheinnerarea
L
rametervaluesfittedontheon-skydataaregloballylowerthan oftheimage,andtheouterareaisbarelyaffected,asshownby
the values that were expected from the simulations (from 14% thesimulatedimageinFig.9.Asaresult,thisaberrationcannot
to21%forthefirstandthirdorderterms)butareconsistentwith explainthebiasobservedintheon-skydata.
the simulations within error bars, except for the second order
term in the y direction. Besides, the coefficients for the second
3.2.2. EffectofmisalignmentoftheLyotstop
ordertermswereexpectedtobenullaccordingtotheanalytical
modelderivedinHubyetal.(2015),asconfirmedbythesimu- IncaseofaLyotstopmisalignmentwithrespecttotheentrance
lations(seethebestfitparametervaluefortheidealmodel).As pupil,the coronagraphicimage becomesasymmetric too, as il-
discussed in the next subsection, possible explanation for this lustratedinFig.10. Inparticular,thedifferentialintensitymea-
behaviourincludesthepresenceofadditionalasymmetriccom- suredintheouterannulusissignificantlyaffected,resultinginan
ponentsintheimage. offsetoftheQACITSmodel,asobservedintheon-skydata.In
practice,itmeansthattheQACITScontrolloop,whichisbased
ontheouterestimatoronly,willconvergetowardsapositionthat
3.2.Sourcesofbias
doesnotcorrespondtothebestcenteredposition.Inthecaseof
In its current state, the QACITS closed loop control tends to a4%shiftrelativetotheentrancepupildiameter(asillustrated
make the outer part of the coronagraphic image as symmetric inFig.10),thisbiashasanamplitudeofabout0.12λ/D .
L
Articlenumber,page6of9
E.Hubyetal.:On-skyperformanceoftheQACITSpointingcontroltechnique
Fig.10.SameasFig.9inthecaseofamisalignmentoftheLyotstop
(shift of 4% of the entrance pupil diameter, along the horizontal di-
rection).Theimageobtainedwhentheasymmetryintheouterareais
compensatedbytip-tiltisshownintheinset.
Fig.11.BiasinducedbytheQACITSouterestimator,asafunctionof
theLyot stopshift,inunitsof percent of thetelescope entrance pupil
diameter (i.e. 1% corresponds to 11cm at the scale of the telescope
Thebiasinducedbytheouterestimatordependsontheam-
pupil). The red crosses show the bias induced by the outer estimator.
plitudeoftheLyotstopshiftwithrespecttotheentrancepupil.
The blue triangles are the inner estimator values as measured at the
Thisbiasisestimatedasthetip-tiltamplitudeforwhichthedif-
biasedposition.Thelightbluesquaresarethesameestimatesscaledby
ferential intensity measured in the outer annulus cancels out.
anadjustment factorof0.7.Thisfactoriscomputedasthesloperatio
Simulation results reported in Fig. 11 show that the bias in- ofthebest linearmodel fittedontheactual biasandinner estimateat
creases linearly with the amplitude of the Lyot stop misalign- smallLyotshiftamplitudes(<2%),shownasdashedlines.
mentuptoa2%shift.
Based on these simulation results, we propose a method to
estimate the amplitude of the bias affecting the outer estimator seeing was estimated to be 0′.′5 during the hour preceding the
thankstotheinnerestimator,whichislessaffectedbythemis-
acquisitionsequenceonJune9th(noseeingdataduringthese-
alignment(as highlightedin Fig.10): when the outer estimator quence) and 0′.′7 on average during the sequence on October
loop has convergedon the position where the outer differential
24th(CFHTDIMMseeingmeasuredat0.5µm).
fluxisminimal,theinnerestimatorappliedonthatbiasedposi-
DuringthenightofJune9th,theQACITScontrollerwasnot
tion leads to an estimate of the bias over-estimated by a factor
yet operational and the star image was initially centered man-
∼ 1.5 (see Fig. 11). Scaled by 70%, this estimate can thus be
ually onto the vortex mask and maintained as well as possi-
usedasasetpointtocorrectforthebiasoftheouterestimator.
ble by manually adjusting its position (every ∼ 10min) based
For Lyot stop shifts smaller than 2.5%, the residual bias when
on a visual assessment of the coronagraphicimage shape. The
applyingthismethodshouldbe lessthan0.01λ/D , i.e.1mas.
L data sequencetaken on HR8799is 15-minlong.The results of
Theimplementationofthisadditionalstepwillresultinalonger
the QACITS estimators applied in post-monitoring are shown
timededicatedtotheoptimizationofthecentering.Engineering
in Fig. 12 (left). For this particular data set, a clear drift is ob-
time will be needed to implement and test this upgrade of the
served,atarateof∼2.7masperminute.Theinnerestimatorisin
QACITScontroller.
completedisagreementwiththeouterestimatorbecausethetip-
tilt amplitude rapidly reaches values outside the validity range
of the linear approximation(typically> 0.15λ/D i.e. 13mas).
4. Performanceassessment L
Thewaythemodulusoftheinnerestimatordecreasesandthen
FirstlightofthevortexmodewiththeKeck/NIRC2instrument increasesduringthesequenceisconsistentwiththeexpectedbe-
wasachievedinJune2015(Serabynetal.2017).Duringthis3- haviouraroundthechangeofsignofthederivativefunction(see
nightrun,apreliminaryversionoftheQACITSloopwasclosed Fig.3), whiletheimplementedestimatorisbasedsolelyonthe
on the second night. Another vortex run took place in Octo- linearapproximation.
ber 2015, including one engineeringnight for the implementa- Forcomparison,thesameanalysishasbeenperformedona
tionofanimprovedversionoftheQACITSautomatedloop(as sequencetakenonthesametargetfourmonthslater,withanop-
described in Sect. 2) and for performing the calibration of the erationalQACITScontrolloop.Thesequencespansover90min
model(asreportedinSect.3).Inthissection,wepresentacom- intotal,includingagapwithoutdataduetotheinabilityoftrack-
parison of the results obtained on data sets taken before (June ing the star very close to zenith. The QACITS estimators dis-
9th,2015)andafter(October2015)thedeploymentoftheopti- playedinFig.12(right)showasignificantimprovementinsta-
mizedQACITScontroller,highlightingthe benefitsof the con- bility:thestandarddeviationofallouterestimatesis2.2masand
trolloop. 5.1masinthexandydirections,respectively.Thelargerdisper-
sionobservedalongtheyaxisindicatestheprobabledirectionof
thedriftthathadtobecorrectedbythecontroller.Theinneres-
4.1.Correctionfortheslowdrift
timatesaresomewhatoffset,withameanamplitudeof11.7mas.
Inthissubsection,wepresentacomparisonoftheQACITSes- Basedonthesimulationresultsassumingamisalignementofthe
timatorappliedinpost-monitoringontwodatasetstakenonthe Lyotmaskwithrespecttotheentrancepupil(Fig.11),thisam-
same target, HR8799, and under similar observing conditions: plitudecanbeinterpretedasashiftoftheLyotstopofupto2.7%
the target was observed close to transit (airmass ∼ 1), and the atthetimeoftheseobservations.
Articlenumber,page7of9
A&Aproofs:manuscriptno.Huby_2017_onsky_qacits_arxiv
Fig. 12. QACITS position post-monitoring of the image of the tar- Fig.13.On-skyresultsfromtheQACITSpost-monitoringofdatasets
get HR8799 onto the vortex mask in June (left) and October 2015 takenduringthenightoffirstlight(left)andonalaterruninOctober
(right). The inner and outer estimators are plotted with blue crosses withthecontrolloopclosed(right).Everypointrepresentsthemeanes-
and red pluses, respectively. The dashed circles have a radius of timateofasequence(comprisingbetween8and72acquisitionframes,
0.15λ/D =13mas.Thecolorshadeofthepointsbecomesdarkerwith 10 in average), with the error bars showing the standard deviation of
L
time.Everydatapointcorrespondsto20sand25sofintegrationtime theestimatesduringthesequence.Thedashedcircleshavearadiusof
inJuneandOctober,respectively. 0.1λ/DL=9mas.
5. Conclusionandprospects
4.2.Pointingstatistics
The QACITS controllerhas been successfully implemented on
Thesamepost-monitoringprocedurebasedontheQACITSesti-
the newly commissioned vortex mode of the Keck/NIRC2 in-
matorshasbeenappliedtoalldatasequencestakenonthenight
strument.Thebenefitsofthisautomatedcontrolaremultiple:
ofJune9th,2015,andduringthreeconsecutivenightsdedicated
to science targets in October 2015. The mean outer and inner – Theobservationsequenceisfullyautomated,includingtak-
estimates of every sequence are plotted in Fig. 13. The com- ingcalibrationframes,initialcentering,andstabilizationof
parison of the results with and without the QACITS controller thestarduringtheobservation,makingthevortexmodesuf-
is quite explicit: on the June night, the dispersion of the mean ficiently user friendly to be offered to the community (in
outerestimatorsis16masand32masinthe xandydirections, shared-riskmodesince2016B).
respectively,whileitisreducedto2.3masand2.6mas,respec- – Pointingqualityisnotobserver-dependent,andinparticular,
tively,duringthethreeobservingnightsinOctober.Theaverage thepitfallinducedbytheapparentsymmetryofthecorona-
standarddeviationwithineverysequenceisalsoindicativeofthe graphicshapewhenthestarisoffsetby∼30masisavoided.
– Pointingstabilityof2.4masrmsisachievedonaverage,with
improvedstability,asitisreducedfrom4.1masand7.9mason
thecontrollooprunningatafrequencyofabout0.02Hz,thus
June9th(inthexandydirections,respectively),downto2.0mas
correctingforlowfrequencydrifts.
and2.8masoverthethreenightsinOctober.
– Pointingaccuracyof4.5masisachievedonaverage.Thisac-
Additionally,forthe data taken withoutthe automatedcen-
curacyiscurrentlylimitedbysystematicerrorsinducedbya
tering of the star, the tip-tilt amplitude for the outer estimator
probable misalignment of the Lyot stop with respect to the
reaches 33mas (0.37λ/D ) on average, which roughly corre-
L entrancepupil. Still, the finalaverage accuracyprovidesan
spondstotheamplitudeforwhichthefluxasymmetryinthein-
improvementofafactor7overtheaccuracyachievedmanu-
ner disk changessign (see the model curve in Fig. 3). In other
ally,andamethodtoreducethisbiasdowntothe1maslevel
words, around this particular tip-tilt value, there is almost no
isproposed.
asymmetry visible in the inner region of the image, which ap-
pears as a symmetric brightdoughnut(see Fig. 4). As a result, GiventhesuccessandbenefitsoftheQACITScontroller,ef-
theobservercanbeeasilytrickedbythisapparentsymmetry,and fortsare currentlyongoingto developthesame kindofcontrol
canconsiderthatthestarimageiscenteredontothevortexmask, looponotheroperationalinfraredvortexcoronagraphs,namely
whileitisactuallyoffsetbyabout30masfromthevortexmask on the VLT/NACO, LBT/LMIRCam and VLT/VISIR instru-
center. ments. The QACITS algorithm is also under study for imple-
Incontrast,ontheOctobernights,themeanouterestimators mentation on the future mid-infrared ELT/METIS instrument
areallsignificantlyclosertozero(asexpectedsincethecontrol (Brandletal.2014),whichincludesavortexcoronagraphinits
loopwasbasedontheouterestimatoronly),butthemeaninner baselinedesign.Itcanbenotedthattheimplementationatother
estimators still show a systematic offset, of amplitude 6.4mas wavelengths is straightforward: since the tip-tilt amplitude is
on average. Assuming that this effect is due to a misalignment measuredinunitsofλ/D ,themodelisnotdependentonwave-
L
oftheLyotmask,andbasedonsimulationresults(Fig.11),this length.Asamatteroffact,theveryfirstlaboratorytestswitha
amplitudeindicatesthattheouterestimatorisaffectedbyabias nonobstructedpupilwereperformedinKbandwiththevortex
of4.4masonaverage(correspondingtoashiftoftheLyotstop coronagraphon PHARO at Palomar Observatory (Mawetetal.
of∼1.1%,whichlieswithinthe specificationsofthealignment 2010), and observations at M band were recently successfully
accuracy).It has to be noted, though,that this systematic error carried out with the vortex coronagraph at Keck/NIRC2. Even
is relatively constantover the three nights, with a preferreddi- moregenerally,thebasic principleofthe method,initiallypro-
rection. This means that both the science and reference targets posed by Masetal. (2012) for the Four Quadrant Phase Mask
areaffectedinthesamewayto someextent,andtheimpacton withanonobstructedpupil,maypotentiallybeadaptednotonly
differentialimagingtechniquesisthereforelimited(Mawetetal. to the vortex coronagraphbut also to other small IWA corona-
2017;Serabynetal.2017). graphsbased on a focal plane mask (e.g. the Dual-Zone Phase
Articlenumber,page8of9
E.Hubyetal.:On-skyperformanceoftheQACITSpointingcontroltechnique
Mask(DZPM,Soummeretal.2003)orthePhaseInducedAm-
plitude Apodizer (PIAA, Guyon 2003)). The major adjustment
concernsthecharacterizationoftheimagebehaviourinpresence
of tip-tiltand morespecifically the definitionofthe underlying
modelforthemeasureddifferentialflux,whichmaybederived
eitheranalyticallyand/orempirically,basedonexperimentalcal-
ibration.
Lastly, we intend to use the same kind of method combin-
ing the inner and outer estimators in the data processing.Post-
monitoring of the data using QACITS can indeed provide a
means to perform frame selection based on a centering quality
criterion,andnotonlyonafluxcriterionsubjecttoseeingcon-
ditions.Besides,theestimateofthetruestarimagepositionbe-
hind the coronagraphicmask allows a better registrationof the
framesandcanthuspotentiallyincreasethesignaltonoiseratio
ofplanetspresentintherotatingfield.
Acknowledgements. The research leading to these results has received fund-
ingfromtheEuropeanResearchCouncilundertheEuropeanUnion’sSeventh
FrameworkProgramme(ERCGrantAgreementn.337569)andfromtheFrench
CommunityofBelgiumthroughanARCgrantforConcertedResearchAction.
References
Absil,O.,Mawet,D.,Karlsson,M.,etal.2016,Proc.SPIE,9908,99080Q
Amara,A.&Quanz,S.P.2012,MNRAS,427,948
Baudoz,P.,Dorn,R.J.,Lizon,J.-L.,etal.2010,Proc.SPIE,7735,5
Beuzit,J.-L.,Feldt,M.,Dohlen,K.,etal.2008,Proc.SPIE,7014,701418
Bottom,M.,Femenia,B.,Huby,E.,etal.2016,Proc.SPIE,9909,990955
Bottom,M.,Wallace,J.K.,Bartos,R.D.,Shelton,J.C.,&Serabyn,E.2017,
MNRAS,464,2937
Brandl,B.R.,Feldt,M.,Glasse,A.,etal.2014,Proc.SPIE,9147,21
GomezGonzalez,C.A.,Absil,O.,Absil,P.-A.,etal.2016,A&A,589,A54
Guyon,O.2003,A&A,404,379
Guyon,O.,Matsuo,T.,&Angel,R.2009,ApJ,693,75
Heintz,W.D.1993,A&AS,98,209
Huby,E.,Absil,O.,Mawet,D.,etal.2016,Proc.SPIE,990920
Huby,E.,Baudoz,P.,Mawet,D.,&Absil,O.2015,A&A,584,A74
Jovanovic,N.,Martinache,F.,Guyon,O.,etal.2015,PASP,127,890
Macintosh,B.,Graham,J.R.,Ingraham,P.,etal.2014,PNAS,111,12661
Mas,M.,Baudoz,P.,Rousset,G.,&Galicher,R.2012,A&A,539,A126
Mawet,D.,Choquet,É.,Absil,O.,etal.2017,AJ,153,44
Mawet,D.,Riaud,P.,Absil,O.,&Surdej,J.2005,ApJ,633,1191
Mawet,D.,Serabyn,E.,Liewer,K.,etal.2010,ApJ,709,53
Serabyn,E.,Huby,E.,Matthews,K.,etal.2017,AJ,153,43
Service,M.,Lu,J.R.,Campbell,R.,etal.2016,PASP,128,095004
Singh,G.,Lozi,J.,Guyon,O.,etal.2015,PASP,127,857
Singh,G.,Martinache,F.,Baudoz,P.,etal.2014,PASP,126,586
Soummer,R.,Dohlen,K.,&Aime,C.2003,A&A,403,369
Soummer,R.,Pueyo,L.,&Larkin,J.2012,ApJ,755,L28
VargasCatalán,E.,Huby,E.,Forsberg,P.,etal.2016,A&A,595,A127
Wallace,J.K.,Burruss,R.S.,Bartos,R.D.,etal.2010,Proc.SPIE,7736,5
Articlenumber,page9of9