Testing and Data Reduction of the Chinese Small Telescope Array (CSTAR) for Dome A, Antarctica

0.63 MB·English

by  Xu Zhou

ResearchinAstron.Astrophys. 2009Vol.9No.XX,000–000 R
esearchin
http://www.raa-journal.org http://www.iop.org/journals/raa A
stronomyand
A
strophysics
Testing and Data Reduction of the Chinese Small Telescope Array
(CSTAR ) for Dome A, Antarctica
0
1
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2 XuZHOU1,4,ZhenyuWU1,ZhaojiJIANG1,4,XiangqunCUI2,4,Longlong FENG3,4,Xuefei
Gong2,4,JingyaoHU1,4,QishengLI1,GenrongLIU2,JunMA1,JialiWANG1,4,Lifan
n WANG3,4,Jianghua WU1,LirongXIA2,JunYAN1,4,XiangyanYUAN2,4,Fengxiang
a
J ZHAI2,RuZHANG2,ZhenxiZHU3,4

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1 NationalAstronomicalObservatories,ChineseAcademyofSciences,Beijing100012,China;
2
[email protected]
] 2 NationalAstronomicalObservatories/NanjingInstituteofAstronomicalOptics&Technology
M 3 PurpleMountainObservatory
I 4 ChineseCenterforAntarcticAstronomy
.
h Received[year][month][day];accepted[year][month][day]
p
-
o
r AbstractTheChineseSmallTelescopeARray (hereinafterCSTAR) isthefirstChinese
t
s astronomicalinstrumentontheAntarcticicecap.Thelowtemperatureandlowpressure
a testingofthedataacquisitionsystemwascarriedoutinalaboratoryrefrigeratorandon
[
the4500mPamirshighplateau,respectively.Theresultsfromthefinalfournightsoftest

1 observationsdemonstratedthatCSTAR wasreadyforoperationatDomeA,Antarctica.
v In this paperwe presenta descriptionof CSTAR andthe performancederivedfromthe
5 testobservations.
3
9
Keywords: instrumentation:detectors—techniques:photometric—stars:variables
4
.
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1 INTRODUCTION
0
1 Site testing at the South Pole (90◦south, 2835m elevation)and Dome C (123◦east, 75◦south, 3260m
:
v elevation) over the past decade has shown that the Antarctic plateau offers outstanding sites for as-
i tronomicalobservations.The extremely cold temperatureslead to very low infrared backgroundsand
X
atmosphericwater vaporcontent.The verylow wind speedsand stable middle and upperatmosphere
r
resultinfavorableseeingconditionsforhigh-resolutionimaging(Storeyetal.2007).Themedianfree-
a
atmosphere seeing in Dome C is 0.27 arcsec, and it is below 0.15 arcsec for 25 per cent of the time
(Lawrenceetal.2004).Inaddition,thelongdarkwinterontheAntarcticplateauallowscontinuousob-
servationsofvariableastronomicalobjects.
Dome A (77◦21′east, 80◦22′south, 4093m elevation), the highest point on the Antarctic plateau,
iswidelypredictedtobeanevenbetterastronomicalsiteeventhanDomeC,basedonthetopographic
similarityandDomeA’shigheraltitude.InJanuary2005,viaoverlandtraverse,DomeAwasfirstvisited
by the Polar Research Institute of China (hereinafter PRIC). This provides astronomers with a good
opportunitytoexplorethisspecialareaforastronomy.PRICplanstoestablisha permanentlymanned
stationatDomeAwithinthenextdecade,withastronomyasoneofthescientificgoalsofthestation.
Aspartofthisprogram,PRICconductedasecondexpeditiontoDomeA,arrivingviaoverlandtraverse
inJanuary2008.Onthisexpedition,thefirstChineseAntarcticastronomicalinstrument,CSTAR,was
deployedtoDomeA.Besidethetaskoftheastronomicalsitetesting,themainscientificgoalsofCSTAR
2 X.ZHOU,Z.Y.Wu,Z.J.Jiang,etal.
includevariablestar lightcurvesandstatistics, supernovaestudies,gamma-rayburstopticalafterglow
detectionandexoplanetdetection.
CSTAR was designed and constructed during 2006 – 2007 at the National Astronomical
ObservatoriesofChina(NAOC)andtheNanjingInstituteofAstronomicalOpticsTechnologyofChina
(NIAOT).AseriesoftestswereperformedonCSTARbeforethesecondexpeditiontoDomeAtoenure
thatitwasreadyfordeployment.Asaresultofthiscarefulpreparation,CSTARoperatedsuccessfully
during2008,aspartofthePlateauObservatory(PLATO)atDomeA (Yangetal.2009).InSection2,
wedescribethedesignandconstructionofCSTAR.TestobservationsattheXinglongstationofNAOC
andthedatareductionarepresentedinSection3.Finally,asummaryisgiveninSection4.
2 INSTRUMENTS
CSTARisasmall2×2Schmidt-Cassegraintelescopearray.EachtelescopeofCSTARhasanentrance
pupil diameter of 145mm (effective aperture of 100mm) and a focal ratio of f/1.2, giving a field of
view of ∼ 4.5◦ ×4.5◦ . Fig.1 showsthe opticaldesign ofthe CSTAR telescope, which consistsof a
catadioptricobjectivewithsphericalprimarymirror,deliveringlowchromaticaberration.Thefirstplano
lensservesbothasawindowandasafilter.Inordertokeepthefocusunchangedthrougha∼ 100◦C
temperature difference (from 20 to −80◦C), Zerodur and fused silica are used for the main optical
componentsand Invar36 is used forthe telescopetube. The tube is designedto be lightweight,well
sealed,andeasytoassemble.Theinsideofthetelescopetubewasfilledbypurenitrogentoavoidice
andfrostformationontheinternalopticalsurfaces.Eachtelescopetubeishermeticallysealed andan
ITO(Indium-Tin-Oxide)filmwascoatedontothefrontwindow.Anelectriccurrentispassedthrough
thisfilm,providing∼10Wofpowertokeepthesurfaceofthewindowwarmerthanambient.CSTAR
isspecificallydesignedforAntarcticoperation,havingnomovingpartsatall—includingtheopticsand
mechanicalsupportingsystem.Thefourtelescopesareinstalledinasteelenclosure,seeFig.2,andare
pointedattheSouthCelestialPole;ie.,eachtelescopeisinclined9◦38′ fromthezenith.Detailsofthe
CSTARtelescopestructurearedescribedinYuanetal.(2008).
The three telescopes CSTAR #2, #4, and #1 have fixed filters: g, r, and i, the fourthtelescope
CSTAR#3isfilter-less.ThemainparametersofthethreefiltersarelistedinTable1andthetransmis-
sion curves of those filters are presented in Fig.3. The filters are designed to be similar to the corre-
spondingfilters of the SDSS (Fukugitaetal.1996). Using these filters, CSTAR can obtain multicolor
photometricdataforeachobjectsimultaneously.
AnAndorDV4351K×1KframetransferCCDwithapixelsizeof13µmisusedforthedetector.
Frametransfertechnologyisidealforfastimagingasithastheadvantageofrequiringnomechanical
shutter. Avoidingthe need for movingparts is very desirable on the Antarctic plateau. The CCD was
enclosed in a control box, as shown in Fig.4. The cable at the back of the box connects to the PCI
controller card installed in the controlcomputer.The typical readoutnoise of the CCD is ∼ 3e with
maximumof ∼ 12e, andthe gainis set to 2.0 e per A/D. The peak quantumefficiencyof the Andor
CCD at −90◦C is ∼ 95%. During the typical exposure time of 30s and under the typical ambient
temperaturesoflessthan−50◦C ontheAntarcticplateau,thedarkcurrentoftheAndorCCD isonly
0.5e.ThedarkcurrentcanthusbenegligibleunderAntarcticconditions.
EachAndorCCD is controlledthroughthe CCI-010PCI controllercardinstalled in an industrial
controlcomputerforeachtelescope.Thecontrolcomputeriscomposedofa1TX-i7415VLmainboard,
IntelCentrino1.6GHzCPU,and1GBofmemory.Twokindsofstoragedisksareusedforthecontrol
computer. One is a 4 GB CompactFlash (CF) disk which can operate at low temperatures (down to
nearly−45◦C);theotherisanormal750GBIDEharddisk.Fig.5showsthefourcontrolcomputers.
Eachcomputerweighs8.3kg.TheWindowsoperatingsystemisinstalledontotheCFdiskbecauseofits
greaterreliabilityunderlowtemperatureconditions,whilethe750GBharddiskismainlyusedasdata
storage.TheCCDcontrolanddatacollectionsoftwareweredevelopedbasedontheAndor-SDK-CCD
softwaredevelopmentkitfortheWindows-XPoperationsystem.Thetimeofthecontrolercomputerof
CSTAR#3issynchronizedbyGPSandtheothercomputercorrectitsclockbyCSTAR#3.
CSTAR:testinganddatareduction 3
Therealtimedatareductionprocessstartautomaticallyafterthecontrollercomputerbooting.The
image coorected by bias and flat-field frames, and the catalogues objects is produced. The brightest
3000starsofthecataloguefrom1/3imagesismovedtoaspecialdirectoryfordatatransfereviairidium
satellitecommunication.
3 TESTINGANDDATAREDUCTION
3.1 Testing
InordertoassuretheperformanceofCSTARundertheextremelylowtemperatureconditionsofDome
A, the CCD system and several differentindustrial control computerswere tested. Finally, the whole
CSTAR system was tested at low temperature in the laboratory of NAOC. These tests indicated that
thefourtelescopesandtheCCD canworkatlowtemperaturesdowntonearly−80◦C,whilethefour
controlcomputerscanworkdownto−30◦C.In2007February6–9,theCCDandcontrolcomputers
were tested at Kalasu. Kalasu (see Fig4) is located in the Tajik Autonomous County of Taxkorgan,
on the Xinjiang Pamirs of China at an elevationof 4450m. We chose Kalasu as the test site because
of its low temperature and low atmospheric pressure conditions similar to the Antarctic plateau. The
atmosphericpressurewas ∼ 58.6kPa andthe temperaturesrangedfrom−5◦C to −18◦C duringthe
testingprocess.BoththeCCDandthecontrolcomputerswereshowntoworknormallyduringthetwo
daysoftesting,andthereare4750GBnormalharddiskswereselectedasdatastorageofCSTAR.
In 2007 September 3 – 7, test observations of CSTAR were performed at the Xinglong station
of NAOC. The four CCDs were cooled down to −40 – −50◦C by electronic cooling system of the
camera.Theweatherwasgoodinmostofthetimeduringfourobservationnights,andmorethan20,000
imageswereobtained.Thetypicalexposuretimewas20s.Fig6showsthe‘super’biasimagesforeach
telescope, which are the median of 100 bias frame images for each telescope. There is no obvious
variationandstructureinthe‘super’biasimages.These‘super’biasimagesareuniquebiasframesto
beusedforreductionofdatabothfromobservationsatXinglongandalsofromDomeA.
Variationsofnight-skybackgroundareobviousevenin thezenithdirection.If onetakesthe time
duringaphotometric,moonlessnighttoobtainalongseriesofsky-dominatedimagespointingdirectly
atthezenith,theeffectsofthenonuniformityofthenightskycanbeminimized.However,ourtelescope
observesthepolarskyareaatanairmassof1.54atXinglongstation.Themedianskybackgroundcan
only be used as an initial flat-field for image correction. Thus, we typically obtained ‘supersky’ flat-
fields bycombiningimagesof the sky (Zhouetal.2004). Duringthis combination,the brightstars in
the images were masked and rejected, and only the areas free from stars were used. By comparing
the images, the median level of each pixel could be selected to derive the final ‘supersky’ flat-field.
100 images of ‘supersky’ for each telescope of CSTAR were used to obtain the ‘supersky’ flat-field.
Theseflat-fieldsmostlyreflectthesmall, pixeltopixelvariationsintheimages.Fig.7showsthe final
‘supersky’flat-fieldimagesforeachtelescope.Someobviousstructurescanstillbeseen.
3.2 Datareduction
First,foreachfiltera‘super’biasframewassubtractedfromeachimage,thenthe‘supersky’flat-field
was divided by the bias-corrected images. The bias and flat-field corrected data of ∼ 20000 images
obtained by CSTAR during the four test-observation nights were processed with the automatic data
reduction software developed by Z. J. JIANG and X. ZHOU based on the DAOPHOT photometric
package(Stetson1987),whichwasusedinthedatareductionofBATC(Fanetal.1996;Wuetal.2007).
Because CSTAR has a large field and is undersampled, obtaining an accurate point-spread function
(PSF)forthesourcesdetectedacrossthewholeviewoffieldisverydifficult.TheDAOFINDprogram
wasusedtofindstarsineachimageandDAOPHOTwasusedtoperformsyntheticaperturephotometry
on the objects detected by DAOFIND. All instrumental magnitudes of the four telescopes were then
normalizedto the V bandmagnitudesof stars in the image 39530013.fit,which was observedby #3
telescopeon2007September5.
4 X.ZHOU,Z.Y.Wu,Z.J.Jiang,etal.
3.3 Erroranalysisandcorrection
Thereareobvioussystematicerrorsinthederivedaperture-photometrymagnitudes.Theerrorsmainly
comefromfollowingsources:
1. ThebiasstabilityofeachCCD
DuetothecontinuousobservationduringexposuresandtheframetransfermodeoftheCCD,there
is no opportunity to obtain real-time bias frames. The bias frames obtained at one time must be
used for observations from another day at Dome A. Because of variations in the environmental
parameters,suchastemperatureandinstrumentalstatus,thebiasofeachCCDcameramaychange.
Thisvariablebiaswillintroducelinearerrorsintheobservedmagnitudes.
2. Non-uniformityofthe‘supersky’flat-field.
The flat-field images were not obtained during ideal photometric nights, and not from the zenith
sky.Abrightnessgradientandasymmetrymayexistintheflat-fieldframes.Thevariationintem-
peraturefrom−40to−80◦C mayalsochangethecharacteristicsoftheflatfield.Duringthepolar
observationsbythefixedCSTARtelescopes,everystarwilltraceoutacircleontheCCD,andthe
residualflat-fielderrorwillgiveafalsevariationintheobservedmagnitudeofeachstar.
3. VariablePSFforstarsindifferentpositionsintheimagesofCSTAR.
ThetelescopesofCSTARhavealargefieldofview.TheopticaldesigncannotkeepthePSFexactly
uniformoverallpartsoftheimage.Whenweuseafixedaperturetomeasurethemagnitudesofthe
stars,thePSFdependsonthelocationontheimageandthiswillcauseavariationintheinstrumental
magnitudesofeachstarrelatetotheotherstars.
Becauseweareobservingasingleareaofthesky,andthesky’simageisrotatingontheCCD,we
have the opportunityto correctthe main residualsystem errorsmentionedabove.Using thousandsof
starswithverydifferentmagnitudes,wecaneasilydeterminethevariablecomponentofthebiasresid-
ualsbasedonthedifferentmagnitudesofthosestarsin twodifferentimages.Usingallofthe circular
tracesofthestars,thelarge-scaleresidualflat-fieldcorrectioncanbeobtained.Usingtheinstrumental
magnitudesfromseveraldifferentaperturesforeachstar,theaperturephotometrycurve-of-growthcan
beobtainedinallpartsoftheimagebutismainlycorrectedwithresidualflat-fieldcorrectionmentioned
above. The instrumental magnitude obtained from different aperture were calibrated to the standard
system. After all these corrections, the systematic errors in the derived photometric magnitudes can
be reduced to the level of 0.01 mag for the brightest stars in most of the images. Some sudden ab-
normalvariations,wheretheyexist,mostlycomefromthecirruscloudsinthesky.Fig.8presentsthe
magnitude-correctedflat-fieldimagesforeachtelescopeusingthousandsofstars.,andshowstheobvi-
ouscircularstructuresthatmatchthetracesofstarsontheCCD.
Two kinds of error estimates have been performed. One is theoretic statistical estimation based
on star’s magnitudeand its sky background.The other is obtained by real repeated observationof all
the objects in the images. By comparing the errors resulting from different images of the same field
withthesamefilter,wefindthatthemeasurementerrorsarenormally±0.01magforbrightstars.The
statistical errorscanbe regardedas thelowerlimitsofthe measurementerrors.Intheerrorestimates,
weignorepointswithabnormallylargedeviationstocalculatetherootmeansquare(rms)errors.The
abnormal variation may come from the true star brightness variation, or defect of the image (cosmic
ray, satellites, bad pixels, etc). Fig. 9 shows the photometric errors for each telescope of CSTAR at
different magnitudes. Because the errors estimated by this method include real statistical errors and
residualsystemerrorsfromthebiasandflat-fieldcorrection,theerrorsshowninFig.9shouldbelarger
thanthe actualobservationalerrors.Fig.9 alsoshowsthattheefficiencyof telescope#2ofCSTAR is
verylow andthatthe limitingmagnitudeofthistelescopeis about2maglower thanthatofthe other
threetelescopes.We knewthatthe CCD cameraforCSTAR#2was muchnoisierthantheothers,but
wewereunabletochangeitinthetimeavailable.
Asanexampleofthedataobtained,thelightcurveofoneofthebrightstarsfromthefourCSTAR
telescopesareshowninFig.11.ThemainscientificobjectivesofCSTARaretoassessthesite quality
ofDomeA andto studythevariableobjectsin the regionofthe SouthPole.Fig.10showstheimage
CSTAR:testinganddatareduction 5
Table1 PassbandparametersoffiltersusedbyCSTAR.
telescope CSTAR #2 CSTAR #4 CSTAR #1 CSTAR #3
filter g r i none
effectiveWavelength(nm) 470 630 780
FWHM(nm) 140 140 160
39530013.fitobtained by CSTAR during the test observationsat the Xinglong station of NAOC. The
variablestarsdetectedbyCSTARarelabeledbygreencircles.Thelightcurvesofthosevariablestars
arepresentedinFig.12.
4 CONCLUSIONS
CSTAR,China’sfirstAntarcticastronomicalinstrumentisdescribed.CSTARiscomposedoffoursmall
Schmidt-Cassegraintelescopes.Eachtelescopehasaneffectiveapertureof100mmandafieldofview
of∼ 4.5◦×4.5◦.Threeofthefourtelescopesareequippedwithg,r,ifilters,thefourthoneisfilter-
less.Aframe-transferAndorDV4351K×1KCCDisusedasthedetectoroneachtelescope.Aspecially
designedcontrolcomputerforeachtelescopeisusedfordataacquisitionanddatareduction.
Low-temperaturelaboratorytestingdemonstratesthatthetelescopesandtheCCDcanworkunder
extremelylow temperature(downto nearly−80◦C), while the controlcomputercan workat temper-
atures as low as −30◦C. Actual test observationsat Kalasu in the Xinjiang Pamirs indicated that the
CCD and control computer can work at these low temperatures and under low atmospheric pressure
conditions.
‘Super’ bias and ‘supersky’ flat-field images were obtained during the test observations at the
Xinglong station of NAOC. These test observations and the subsequent data reduction indicate that
CSTARcanworkstablyandobtainalargevolumeofscientificdata.Aspecialdatareductionmethod
wasusedtoreducetheobservationalerrorsforeachoftheobjectsdetectedbyCSTAR.Thedatareduc-
tionprocessisdoneautomaticallyin realtime, andcatalogueof brighteststar from1/3 ofthe images
obtainedarepreparedforfurtherdatatransferviairidiumsatellitecommunication.Finally,Eightvari-
ablestarsweredetectedbyCSTARduringthetestobservations.
Acknowledgements This work was supported by the Chinese National Natural Science Foundation
grandsNo.10873016,10633020,10603006,and10803007,andbyNationalBasicResearchProgram
ofChina(973Program),No.2007CB815403.WethankourcolleaguesattheUniversityofNewSouth
Wales,Australia,forassistanceineditingthispaper.
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6 X.ZHOU,Z.Y.Wu,Z.J.Jiang,etal.
Fig.1 OpticaldesignofCSTARtelescope.
CSTAR:testinganddatareduction 7
Fig.2 ApictureofCSTARenclosurewastakeninXingLongstationofNAOC.
8 X.ZHOU,Z.Y.Wu,Z.J.Jiang,etal.
Fig.3 Transmissionprofilesofthe3CSTARfilters.Thefiltercodes(seeTable1arelabeled
oneachfilter.NotethatCSTAR #3hasnofilter.
CSTAR:testinganddatareduction 9
Fig.4 TheAndorCCD enclosedinits controlbox.ThispicturewastakenatKalasu inthe
TajikAutonomousCountyofTaxkorgan,XinjiangPamirsofChina.
10 X.ZHOU,Z.Y.Wu,Z.J.Jiang,etal.
Fig.5 Computercontrolequipment.
Fig.6 Biasframeimagesforeachtelescope.