Consequences of spectrograph illumination for the accuracy of radial-velocimetry

0.31 MB·English

by  I. Boisse

Consequences of spectrograph illumination for the accuracy of
radial-velocimetry
Boisse,I.1,a,Bouchy,F.1,2,Chazelas,B.3,Perruchot,S.2,Pepe,F.3,Lovis,C.3,andHe´brard,G.1
1 Institut d’Astrophysiquede Paris, Universite´ Pierre et Marie Curie, UMR7095 CNRS, 98bis bd. Arago, 75014 Paris,
France
0 2 ObservatoiredeHauteProvence,CNRS/OAMP,04870StMichell’Observatoire,France
1
3 ObservatoiredeGene`ve,Universite´deGene`ve,51Ch.desMaillettes,1290Sauverny,Switzerland
0
2

n
a Abstract. Forfiber-fedspectrographswithastableexternalwavelengthsource,scramblingpropertiesofopti-
J calfibersand,homogeneityandstabilityoftheinstrumentilluminationareimportantfortheaccuracyofradial-
velocimetry. Opticalcylindricfibersareknowntohavegoodazimuthal scrambling. Incontrast,theradial one
6
is not perfect. Inorder to improve the scrambling abilityof the fiber and to stabilizethe illumination, optical
] doublescramblerareusuallycoupledtothefibers.Despitethat,ourexperienceonSOPHIEandHARPShaslead
M toidentifiedremainingradial-velocitylimitationsduetothenon-uniformilluminationofthespectrograph. We
conductedtestsonSOPHIEwithtelescopevignetting,seeingvariationandcenteringerrorsonthefiberentrance.
I
. We simulated the light path through the instrument in order to explain the radial velocity variation obtained
h
withourtests.Wethenidentifiedtheilluminationstabilityanduniformityhasacriticalpointfortheextremely
p
high-precisionradialvelocityinstruments(ESPRESSO@VLT,CODEX@E-ELT).Testsonsquareandoctagonal
-
o sectionfibersarenowunderdevelopment andSOPHIEwillbeusedasabenchtesttovalidatethesenewfeed
r optics.
t
s
a
[
1 Introduction the injection of light in the fiber-fed spectrograph. This

1 work is based on our experimenton two high-resolution,
v high-precision fiber-fed spectrographs, [email protected],
High-precisionradial-velocimetry(RV)isstillamaintech-
4 Observatoire de Haute-Provence(OHP), France (refs. [3]
nique in the search and characterizationof planetary sys-
9 and [11])[email protected],EuropeanSouthernObser-
tems.Uptodate,mostoftheknownextrasolarplanetshave
7
vatory(ESO)-LaSilla,Chile(ref.[7]).
beendetectedwiththismethod.Itwillremainatthefore-
0
. frontof exoplanetscience for the coming yearsthanksto
1
itsefficiencyinfindinglow-massplanetssinceHARPShas
0 reached an RV accuracy less than 1ms−1 (ref. [7]). In the Up to day, SOPHIE has a Doppler accuracy of about
0 aimto reachEarth-typeplanetsand,also measurethe ex- 5 ms−1, as most of high-accuracyspectrographsinvolved
1
: pansion of the universe, new spectrographsin the visible in exoplanetsurvey.In service since November2006,we
v arenowinstudytoreachaprecisionofonlyfewcms−1in identified thatinstrumentallimitationsmainly comefrom
i
X the near future(ESPRESSO@VLT [8], CODEX@E-ELT theoldCassegrainFiberAdaptater(developedforELODIE
[9]). spectrograph).InordertoreachthesofaruniqueHARPS
r
a To reach this level of accuracy, we must avoid a cer- levelofaccuracy,weconductedteststhatillustratethein-
tain number of limitations. Among these, one can quote complete fiber scrambling. Detected with SOPHIE, these
the stellar noise (activity, pulsation, surface granulation), effectswerealsocharacterizedonHARPSatalowerlevel.
thecontaminationbyexternalsources(moon,close-byob- Inthedevelopmentofthefutureinstruments,theoptimiza-
jects,backgroundcontinuum),andwhatlightmayencounter tionofSOPHIEisabenchtestfortheimprovementofthe
duringitspath(e.g.atmosphericdispersion).Aspartofin- spectrographilluminationuniformityandstability.
strumentalfactors,CCD cosmetics,chargetransferineffi-
ciency(ref.[2]),wavelengthcalibration(refs.[5]and[12])
shouldbementioned.Foranexhaustivediscussionofthese Theproceedingisstructuredasfollowed.Inathesec-
limitations,wereferto[10].Theauthorshighlighttheim- ondsection,weexposethepropertiesofthefiber-fedspec-
pact of the spectrograph illumination as one of the main trographsandthelightinjection.Thethirdsectionreviews
limiting factors, and the need to analyze and quantify it. the differenteffectsthatare detectedin theRV duetothe
Inthisproceeding,we thenfocusonthelimitationdueto incompleteandimperfectscramblingobtainedwiththestan-
dardfibers. In the lastsection, we describesolutionspro-
a e-mail:[email protected] posedtooptimizethefibersscrambling.
EPJWebofConferences
2 Fiber-fed spectrographs
HARPS and SOPHIE are fiber-fedspectrographswith si-
multaneousreferencecalibrations.Thestellarlightcollected
bythetelescopeareleadtotheinstrumentthroughastan-
dard step-index multi-mode cylindrical optical fiber. SO-
PHIE has two observingmode using two differentfibers,
the High-Resolution (HR) and the High-Efficiency mode
(HE). In HR mode, the spectrographis fed by a 40.5-µm
slitsuperimposedontheoutputofthe100-µmfiber,reach-
ing a spectral resolution of λ/∆λ = 75,000. In HE mode,
thespectrographisdirectlyfedbythe100-µmfiberwitha
resolutionpower of 40,000.Both SOPHIE fibers have an
skyacceptanceof3-arcsec.HARPShas70-µmfiberwith
a sky acceptance of 1-arcsec and a spectral resolution of
110,000(seeTable1).
Non-uniformilluminationofthe slitor outputfiberat
thespectrographentrancedecreasestheradial-velocitypre-
cision.Indeed,variationsinseeing,focusandimageshape
at the fiber entrance may induced non-uniform illumina-
tioninsidethepupilofthespectrograph.Theopticalaber-
rationsleadtovariationsinthecentroidsofthestellarlines
onthefocalplane.
Opticalfibers are used to lead the light fromthe tele-
scope to the entrance of the spectrograph. They have the
propertiestoscramblertheatmosphericeffectsandguiding
and centeringerrorsdiscussed previously.But the scram-
blingabilityofonemultimodefiberisnotperfectasshown Fig.1.Illuminationattheoutputofonecylindricalopticalfiber
inFig.1.Withaninputoffaxesimage,theazimuthalscram- inthenearandthefarfield.Sourcesareoffaxes.Ahighdegree
bling is good whereas it remains some effects in the ra- ofazimuthalscramblingisobserved.Incontrast,itremainssome
dial one. This pattern observed is bigger in far field than effectsintheradialone.
in near field. Near field and far field are respectively de-
finedas,thebrightnessdistributionacrosstheoutputface 3 RV effects of fiber imperfect scrambling
ofthefiber,and,astheangulardistributionoflightoffiber
outputbeam.Therefore,changesin the inputbeam cause
3.1 Centering/Guidingonthefiberentrance
onlycircularsymmetricchangesinthefiberoutputpattern.
Incontrast,imperfectradialscramblingallowssmallzonal The degreeof radial scramblingdescribes the stability of
errorstoremain. theoutputbeamastheinputimageismovedfromthecen-
ter to the edge of the fiber. It is possible to detect in RV
HARPSandtheSOPHIEHRmodeareequippedwith some variations due to the insufficiency of radial scram-
optical double scramblers. It is used to increase and im- bling.Thelightfromastablestarismovedwiththeguid-
prove the uniformity and stabilization of the illumination ing/centeringsystemfromanedgetotheotherofthefiber.
of the spectrographentrance (refs. [4] and [6]). The near We compute the RV with the data reduction pipeline for
field of a circular fiber is observed to be better scramble different positions of the star. The test was done for the
than the far field. In spite of only one fiber guiding the two modes of SOPHIE and for HARPS. The results are
light from the telescope to the instrument, double scram- computedinTable2.
bleriscomposedoftwofiberscoupledwithtwodoublets. Because thescramblingofthe fiberis notperfect,the
The system is designed to inverse near field and far field movementoftheinputimagecorrespondstoashiftonthe
in order to induce a better radial scrambling, although it
causesfluxlosses(ref.[1]).
Table 2. Variation observed in RV as the input image (star) is
movedfromthecentertotheedgeofthefiber.HARPSandSO-
Table 1. Comparison of parameters of fiber-fed spectrographs PHIEwithHigh-Resolutionfiberusedadoublescramblerinstead
SOPHIEandHARPS. oftheHigh-EfficiencymodeofSOPHIE.
Instrument Dopplerprec. 1pixel Resol.∆λ/λ Instrument fib.diam. RVeffect[ms−1]
HARPS 61ms−1 800ms−1 110,000 HARPS 1” ∼3
SOPHIE(HR) ∼4ms−1 1400ms−1 75,000 SOPHIE(HR) 3” ∼13
SOPHIE(HE) ∼10ms−1 1400ms−1 40,000 SOPHIE(HE) 3” ∼36
Newtechnologiesforprobingthediversityofbrowndwarfsandexoplanets
Fig. 3. Displacement of the position of a nine wavelength as a
Fig. 2. Schema illustrating how the vignetting of the telescope functionofainternal(green)oranexternal(red)occultationof
istranslatedattheoutput of thefiberHEinthefar-field,i.e.at theentranceofthespectrographontheCCDdetector.
theentranceofthespectrograph.Forexternaloccultation,theRV
decreasewhereas,forinternaloccultationtheRVincrease.
detector.Weobservedfirstthat,wideristhefiber,moreim-
portantistheRVvariations.Moreover,thevalueofmeter
per second per pixel vary as a function of the resolution
power.Movingtheinputimagefromthecentertotheedge
isthenexpectedtohavealowereffectforahigherspectral
resolution.Inaddition,weexpectthatthedoublescrambler
improvetheradialscrambling,i.e.decreasetheRVeffect.
3.2 Vignettingtelescope
Fig.4.Whentheseeingisgood,thetelescopeimageislesswide.
Thepupilofthetelescope,i.e.theentranceofthetelescope WiththedoublescramblerinHRmode,nearfieldbecomesfar-
orthefarfieldoftheinputlightisknowntobemorestable fieldandisprojectedonthecollimatorattheentranceofthespec-
thanthe telescope image.We wouldlike to test the effect trograph.ThepatterniscomparabletothatobservedinHEwhen
in RV of variations of the far field. For that, we vignette vignettingtelescope(Fig.2)
thetelescopeondayskywiththedome.Wecomputedthe
RV with the pipeline SOPHIE, estimating that the varia-
tionof BarycentricEarthRV isnotsignificantduringour 3.4 RVeffectsduetoseeingvariations
measurements.WemadethistestwiththeHEmodeinor-
der that the variation of the far-field of the telescope are Variations of the far-field of the input beam in HE mode
projected on the grating (i.e. without double scrambler). is equivalent to a variation of the telescope image in HR
WeremarkedthatthewayoftheRVvariationsdependson mode,duetothedoublescrambler.Variationsinseeingin-
exteriororinteriorvignetting(Fig.2). duce a variationof the inputimage as illustrate in Fig. 4.
Theinputbeamattheentranceofthespectrographiscenter-
illuminated(externaloccultation)whenatargetisobserved
with a good seeing in HR mode. The previous tests and
3.3 Opticalpathsimulations
simulationsexplainasystematiceffectobservedwithSO-
PHIE namedas”seeingeffect”. Fora sampleof starsob-
WesimulatedtheopticalpathintheSOPHIEspectrograph. servedathighsignaltonoiseratio(SNR)inHRmode,we
We observed a displacement of the spectrum in function observedadecreaseinRVcorrelatedwithanestimationof
of wavelengthon the focalplane when the inputbeam at seeing.ThevalueoftheseeingisnotmonitoredwithSO-
the entrance of the spectrograph is center-illuminated or PHIE.Weestimateditwithcalculatingtherelativefluxby
center-darked,correspondingrespectivelywhenvignetting unitofexposuretime:
external and internal part of the pupil in HE mode (cf.
Sect.3.2). Variation of the slit or the optical fiber illumi- SNR2
S = (1)
nationaredirectlytranslatedonthespectruminFig.3.We Texp10−MV/2.5
observed that the shift is more significant for an external
occultation.Because the effect is not symmetricalong an withSNR,thesignaltonoiseratioofthespectra,T the
exp
orderandnotmonitoredbythecalibrationlamp,thefinal time exposureofthe measurement,M the visualmagni-
V
computed RV vary. So, variations of the far-field pattern tude of the target. The seeing decrease when the seeing
projectedontothegratingcanintroduceradial-velocityshifts estimation S increase. With a lower seeing, the inputim-
atthedetectorloweringthefinalprecisioninRV. age is smaller than the fiber width, and as shown in the
EPJWebofConferences
symmetryof cylindricalfibers, octagonalandsquare sec-
tionopticalfibersaretheninstudyatGeneva,OHP-LAM,
andESOGarching.Asfirstresults,verygoodscrambling
propertiesare observed in the near field, whereas strange
patternsinfarfieldarenotwellunderstoodatthemoment.
Itneedsfutureteststoimprovethemeasurements,tostudy
its best used, on all the path or in a double scrambling
mode. SOPHIE is then going to be a bench test for the
futureinstruments.
We note that additional solution is being installed on
HARPStoreducetheguidingandcenteringlimitations.A
tip-tiltmirrorgeneratesmall-amplitudeandhighfrequen-
ciesvariationsinordertoinduceaspreadofthelight.
AnewguidingcameraisnowinoperationonSOPHIE
andallowsa betterguidingandcenteringandalso allows
tomonitortheexactseeing.
Seeing Estimation [arb.unit] References
Fig. 5. Radial-velocity measurements as a function of a seeing
index(cf.text).Theseeingdecreasewhentheseeingestimation 1. Bouchy&ConnesA&A,1999,Volume136,193
S increase.Astheseeingislower,theRVdecreases. 2. Bouchy,F.,Isambert,J.,Lovis,C.Boisse,I.etal.EAS
PublicationsSeries,Volume37,2009,pp.247-253
3. Bouchy,F.,He´brard,G.,Udry,S.,Delfosse,X.,Boisse,
simulations,inducedavariationofthefarfieldinputinthe I.etal.A&A,Volume505,2009,pp.853
spectrograph,duetothedoublescrambler.Weexpectand 4. Brown,T.M.1990,ASPC,8,335B
observed a decrease of RV when the value of the seeing 5. Lovis,C.&Pepe,F.2007,A&A,468,1115L
decreaseasshowninFig.5. 6. Hunter,T.D.&Ramsey,L.W.1992,PASP,104,1244
Moreover,whentheinputimageissmallerthanthedi- 7. Mayor,M.,Udry,S.,Lovis,C.etal.2009,A&A,493,
ameterofthefiber,wearemoresensibletoeffectofguid- 639
ing andcenteringsystem. The guidingandcenteringsys- 8. Pasquini, L., Manescau, A., Avila, G. et al. 2009,
temmayinducedvariationofthepositionoftheinputim- svlt.conf,411
ageontheslitortheentranceofthefiber,andsointroduce 9. Pasquini, L., Avila, G., Dekker, H. et al. 2008,
RV variations. These RV variation are quite random, and SPIE.7014E,51P
addsomenoise. 10. Pepe,F.&Lovis,C.,Phys.Scr.T130,(2008)014007
Whereastheseeingeffectisdirectlyrelatedtotheop- 11. Perruchot, S., Kohler, D., Bouchy, F. et al. 2008,
ticalpathinthespectrograph,itmaybequantified.Weare SPIE.7014E,17P
modelingthis variationswith ourdata in orderto remove 12. Wildi,F.,Pepe,F.,Lovis,C.,Chazelas,B.etal.2009,
thisnoisewithasoftwaretool. SPIE.7440E,19W
We note that the effect on HARPS is expected to be
lower. The 1” width of the fibers is smaller than the SO-
PHIEandroughlyequivalenttothetypicalseeingatLaSilla.
Furthemorethequalityimageprovidedbythe3.6-mtele-
scopeiscloseto0.7arcsec.Hence,HARPSislesssensible
tothevariationsofseeing.Moreover,theresolutionpower
of the instrumentis better and then the incomplete radial
scramblinghasalowerimpactinRVasshowninSect.3.1.
FurthermoretheopticalaberrationsonHARPSaresmaller
thaninSOPHIE.
4 Perspectivesand conclusions
The CODEX experimentcalls for a Doppler precision as
low as 1 cms−1. This work shows that the future instru-
ments need for better scrambling to reach this accuracy.
Newtypesoffibersforfeedingthetelescopeornewdou-
ble scrambler are neededto improvethe precision and to
minimizethenoiseduetovariationsoftheilluminationat
theentranceof thespectrograph.To removethe spherical