iss nocturnal images as a scientific tool against light pollution

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ISSnocturnalimagesasascientifictoolagainstLightPollution

UniversidadComplutensedeMadrid-LICAreportapril2011 1

>

Abstract

ThepotentialofthenightpicturestakenfromtheInternationalSpaceStationwithaNikonD3s

digitalcameratofightagainstlightpollutionisshown.AscientificanalysisofISS026-E-26493RAWimageofMadridatnightwithtechniquesusedbyastronomersandcartographersisperformed.

We suggest an observational setup to obtain useful scientific information from the pictures

includingseriesofexposuresandcalibrationframes.

1.Introduction

Light pollution (the introduction by humans, directly or indirectly, of artificial light into the

environment) isamajorissueworldwide,especiallyinurbanareas.Itincreasestheskyglowand

preventsusfromobservingadarkstarrysky.As‘Starlight,ACommonHeritage’,promotedbythe

International Astronomical Union (IAU) and the UNESCO, which is a international campaign in

defenseofthevaluesassociatedwiththenightskyandthegeneralrighttoobservethestarssaid:"Anunpollutednightskythatallowstheenjoymentandcontemplationofthefirmamentshould

beconsideredaninalienablerightofhumankindequivalenttoallotherenvironmental,social,and

culturalrights,duetoitsimpactonthedevelopmentofallpeoplesandontheconservationof

biodiversity."StarlightDeclaration.LaPalma,Spain2008.

Astronauts aboard the International Space Station (ISS)1 are publishing (Twitter for instance)

picturesoftheEarthtakenfromthespace.Thesebeautifulpicturesarefreelyavailableandcanbe

obtained froma repositorymaintained byNASAon Internet. A portion of the images isbeing

takenatnightandsomeofthemshowanetworkoflightofthebigcities.Thisilluminationcomes

mainlyfrompubliclightingofthestreetsandbuildings.Theintensityinthepictureisrelatedtothelightbeingsenttothespaceandbrightlightrevealsanexcessorbaduseoflighting.

Seethevideo“CitiesatNight:anorbitaltouraroundtheworld”http://www.ngdc.noaa.gov/dmsp/movie/CitiesAtNightWorldTour720X480Edit7.wmv

1 The International Space Station (ISS) is a co-operative programbetween space agencies:

NationalAeronauticsandSpaceAgency(NASA)fromUnitedStates,theRussianFederalSpace

Agency(Roscosmos),CanadianSpaceAgency(CSA),EuropeanSpaceAgency(ESA)andJapan

AerospaceExplorationAgency(JAXA)forthejointdevelopment,operationandutilizationofapermanentlyinhabitedSpaceStationinlowEarthorbit.

DepartamentodeAstrofísicayCienciasdelaAtmósfera

GrupodeAstrofísicaExtragalácticaeInstrumentaciónAstronómica

UniversidadComplutensedeMadrid


Authors:JaimeZamorano,AlejandroSánchezdeMiguel,SergioPascual,

JoséGómezCastaño,PabloRamírez&PeterChallupner

LICAreport,April2011Version1.82011/05/04





Amongotherscientificstudiesthatcouldbeperformedwiththeseimagesweareinterestedin

thoseconnectedwithlightpollutionanditseffectonthenightskybrightnessandonthevisibility

ofthestars(seeforinstanceCinzanoandElvidge2004).Detectinglightpollutionisstraightforward

byvisualinspectionoftheJPEGpictures.Theseimagesspeakbythemselvesandareveryusefulto

drawpublic attention of the problem. Unfortunately the JPEG isa lossycompressionmethod,

meaning that someoriginalimage information islostand cannot berestored.Toobtain useful

scientificvaluesfromthesepictures,theoriginalRAWfilesareneeded.BeingtheCMOSdetector

of the digital camera employed (Nikon D3s) a linear device, the intensities of each pixel are

proportional to theemittedlight andonecandirectlycomparebetweendifferent zonesof the

image.Besides,thecolorofthelightsourcescanbeobtainedbycomparingthevalueoftheimage

indifferentchannelsorbands.Fromthesecolorsthenatureofthelightbulbemployedcanbe

inferred.

ThisiswhywehaverequestedandobtainedfromNASAiss026e026493.nef,whichisaRAWimage

(withaBayermatrix)withtheformatofthedigitalimagesofNikon.Informationofthispicturecan

beobtainedattheGatewaytoAstronautPhotographyofEarth:

http://eol.jsc.nasa.gov/scripts/sseop/photo.pl?mission=ISS026&roll=E&frame=26493 Exifdata:NikonD3Sf=200mmf/41/15s12800ISO4256x2832pixels2011:02:1123:11:50

Thisisapreliminaryreport.Themainaimsofthisstudyare,amongothers:

1)ToobtainusefulandscientificinformationofthelightpollutionatMadridcityarea

2)ToemphasizetheimportanceoftheseISSnocturnalimagesforscienceandpublicoutreach.

3)TodesignacalibrationsequencetobeusedbyastronautsonboardISSforthesekindofnight

pictureswhen theyaretakenforscientificstudies.

Figure1.PublishedJPEGimageofMadridintruecolor.ThispicturewastakenbyScottKelly.





Figure2.DetailofimageISS026e026493inMadriddowntown.AdeepzoomoftheRAWimageto

showtheBayermatrixisalsodisplayed.

Duetothelightdirectlyemittedtothespaceorreflectedintheground,theimageshowsclearly

recognizablefeaturesofMadridatnight.Theseincludestreets,parks,airport,a soccerstadium,

roads,etc.

Figure3.BayerGBRGcolorfilterarray forNikonD100.NotethatthecamerausedwasaNikon

D3s.Takenfrom“ResolutioninColorFilterArrayImages”byJonPetersonandCobusHeukelman

(http://scien.stanford.edu/pages/labsite/2010/psych221/projects/2010/PetersonHeukelman/Web

site/index.html)

2.Imageprocessing

ThedetectorusedaBayermosaic:acolorfilterarray(CFA)whichconsistsofonered,twogreen

andonebluefilterinasquare2x2arrangement.Thefirststepconsistsinseparateorsplitthe

threechannels(R,G&B)ofthedigitalimageinordertoobtainusefulscientificimages.Weused

IRISanastronomicalimageprocessingfreesoftware(http://www.astrosurf.com/buil/iris)developed

byamateurastronomers.

The command SPLIT_CFA (http://www.astrosurf.com/buil/iris/tutorial5/doc17_us.htm ) splits the CFA





(Color FilterArray) structure into fourdistinct files (one for eachof the colors/positions in the

periodic Bayer matrix). The final FITS files (Flexible Transport Images System, a standard for

astronomicaluse)correspondtoimages(2100x1400pixels)withtheintensitiesforthered,green

andbluepixels.ForthegreenpixelstwofilesarecreatedduetothestructureoftheBayermatrix

(seeFigure2).

Eachpixelhas14bitsdepth,i.e.1E14=16384quantizationlevels.Whenapixelhasreceivedlight

inexcessofthishighestvalue,thepixelappearssaturatedandtheonlyinformationthatonecould

extractisthattheintensityishigherthanthisvalue.Somepixelsintheimagearesaturated.Itis

easytopreventtheimagefromsaturationusingashorterexposureorbyadjustingthesensitivity

(lowerISOvalue)orreducingtheapertureofthelens.Theresultingimagewouldbedimmerand

thefainterregionspoorlymeasured.Itwouldbedesirabletoobtainaseriesofexposurestogetall

theregionsproperlyregistered.Readlateronbracketingseriesofexposures.

Figure4.ThefourchannelsoftheColorFilterArraystructureoftheRAWimageofMadrid.





Figure5.ThesameregionofMadridpictureinbothgreenchannels.Thisisnotatruecolorimage.

Theintensityiscolorcodedaccordingtothecolorbaratthebottombeinghigherintensitypixels

(andthosesaturated)inyellowcolor.Somebadpixelsaremarkedinbothimages.

Thegreenchannelshavebeenselectedforthisstudybecausetheyyieldmoreintensitydatawith

better spatial resolution; the spectralrange isalso similar tothe astronomical JohnsonVband

(read lateron absolute calibration). A region of these green channel FITS files iss_G_1.fit and

iss_G_2.fitisshowninfigure5.Theimagesarerathersimilar,asexpected,exceptforsomebad

pixels.Theseartifactsofthecamerashouldberemovedpriortoanymeasuresincetheyarenot

relatedtothelightingbuttothecameradetector.Badpixelsshouldappearinthesameposition

ondifferentframesforthesamecamera.Readlaterondarkcalibrationandmaskingofbadpixels.

Fortunately we can search and clean these pixels comparing both images using themake up

proceduresofastronomyimageprocessingpackages.InthisworkwehaveusedREDUCEME,an

astronomical data reduction package to get rid of these cosmetic defects by careful visual

inspection(http://www.ucm.es/info/Astrof/software/reduceme/reduceme.html ).

SCIPY(www.scipy.org),alibraryofPythonroutinesandCextensionsdevelopedasanopen-source

softwareformathematics,science,andengineering,hasbeenusedtorebuilttherawimagewith

thepixelsofeachgreenchannelinitsoriginalpositions.TheFITSfileswerereadandwrittenwith

PYFITS (http://www.stsci.edu/resources/software_hardware/pyfits), a development project of the

ScienceSoftwareBranchattheSpaceTelescopeScienceInstitute.Azerovaluehasbeenassigned

tothepixelscorrespondingtotheblueandredchannels.





Figure6.Intermediateimagewithpixelsbelongingtothetwogreenchannelsandthefinalimage

inGchannelwithfullresolution.(Note:notthefinalversion.Badpixelsshouldberemoved)

Theemptypixelshavebeenfilledwithalinearinterpolationusingtheneighborpixels.Attheend

ofthisprocess,animagewiththesameresolutionastheoriginalwithinformationselectedforthe

greenchannelisobtained.

Figure7.Zoomovertheimageandagraphwiththepixelsvaluesalongthelinemarkedonthe

image.Thevaluesofthesaturatedpixelsofthebrightspot(yellowcoded)arelost.

Bright spots (those with yellow color on the figures) present saturated pixels. No useful

informationcan bederived fromthesevalues. For the examplepictured in figure 7,maximum

valuecanbeestimatedfittingaGaussiantotheunsaturatedpixelsatthewingsofasinglelinecut.

Inthiscasethepeakvalueisaround90,000counts,althoughthemethodisuncertain.Toobtain

unsaturatedpixelsweneedtoreducetheexposuretimebyafactorof∼6(∼90,000/15,000),i.e.

1/100sinstead1/15s(seebelow).

Theplatescalefora200mmfocallensis17.19arcmin/mmatthefocalplaneofthedigitalcamera.

Thistranslatesto8.72arcsec/pixelusing thesizeof thepixels (8.45microns/pixel).Assuminga

distance of 350 km between ISS andMadrid, the final plate scale of the images is around 15m/pixel.Thisisa‘backoftheenvelope’calculationthatdidnottakeintoaccounttheinclination





andthefinalvalueaftergeoreference(seebellow)is16m/pixel.

Platescaleshouldnotbeconfusedwithresolution.Thisparametercanbederivedanalyzingthe

imagesofpointsources.ThePointSpreadFunction(PSF)ofbrightandunsaturatedspotsisaround

5pixels,i.e.theresolutionisapproximately80m.ThePSFdependsontrackingwhichisverygood

forthisimage since thepoint sourcesappearonlyslightlyelongated in theEast-West direction

(ellipticity≈0.7). During theexposure thesatellitemovesonthesky1.12deg/s x1/15s=269

arcsec,i.e.around500m.Assuminganinclinationof43degreestheangularscaleoftheISSimage

isaround6.5 arcsec/pixeland thus the targetmovedaround42pixels.Sothe imagewould be

smearedorblurry anduselesswithout thetrackingsystem.Readmoreabout theastronomical

“barn-doortracker”builtbyastronautDonPettitatCitiesatNight:TheViewfromSpace”(2008)

(http://earthobservatory.nasa.gov/Features/CitiesAtNight/ )byCindyEvans&WillStefanov.

3.Imagegeoreferenceandspatialdataanalisys

Georeferenceisapreviousnecessarystepbeforeperformingacorrelationbetweentheimages

andlightsourcesfromthefield.TheimageshavebeengeoreferencedbyusingsoftwareGVSIG

(http://www.gvsig.org ). Orthophotos have been used as a cartographic base provided by the

spanish PNOA project (National OrthophotographicAerial Plan), and IGN (the Spanish Instituto

GeográficoNacional )basemap,tohelpinidentifyinggeographicreferences.Wealsohaveused

GLOBALMAPPER(http://www.globalmapper.com)andUDIG(http://udig.refractions.net),tocorroborate

theresults.

Georeference has been done using the reference system UTM30 EPSG 25830, to obtain the

positionsoftheobjectsphotographedincoordinateswithmeterscale.Totransformintolatitude

andlongitudepositionsinthesystemETRS89EPSG2458,acoordinatetransformationhasbeen

appliedwith an included utility in GVSIG. The result is a geotiff image for each band, giving

correspondencepixelmeter,withaspatialresolutionof16metersperpixel.Thisprocedureletusto obtain correlation between the position of the detected light source on the image and its

counterpartinageographicelement,andhenceitsinfluence.

Figure8.Calibrationpoints(left)andresultingGeoTIFFimageofchannelGoverPostGISvectorial





layerwithbiodiversityinformation(right).

ThegeotiffimageisnowanewlayerthatcanbeusedwithGIS(GeographicInformationSystems)

tools.ToperformspatialanalysisIDEE(ThespanishSDI,SpatialDataInfrastructure),hasbeenused

(www.idee.es ). These data sources provide several layers where geographical elements, as

buildingsorcities,canbefound.Asanexample,weshowNaturalParksaroundMadridonfigure

8,tostudypossiblelightinfluenceonprotectedareas.

Figure9.WFSlayerintegrationshowingISSimageontopofamapofMadridcity(left)andwith

themainroads(right).

WMS (WebMap Service) and WFS (Web Feature Service)OGC (OpenGeospatial Consortium)

services have been used as data source protocol. These provide raster and vectorial data to

performthespatialanalysis.AlsoaPostGISdatabasehasbeenusedtostorenospatialdatalike

populationorelectricalpowerconsumptionbycities.Thesedatahaveauniqueidentifierthatcan

beused to link themwith spatial elementsand they provide additional layers to the analysis.

Spatial database has been used to perform SFSQL (Simple Feature StandardQuery Language)

analysis,likedistancecomputations,delimitperimeterlightsourcesorselectthemfromaspecificarea.Sextante(www.sextante.org),agvSIGextension,hasbeenusedtocomputerastercropsand

interactionwithvectorlayers.

Vectoriallayersareusedtocropselectedregionsfromtheimage.Intheexample,aselectionof

municipal boundaries has been done to delimit urban nucleus. Automatic processing can be

achievedusingmatplotlibpythonlibrary(http://matplotlib.sourceforge.net).Thislibraryisalsouseful

to get gradientmaps distribution. To perform several studies, the following layers have been

selected:(a)IDEEbasemap,(b)Catastro.Buildinginformation,and(c)Biodiversity,fromSpanish

MinisteriodeMedioAmbienteyMedioRuralyMarino.





Figure10.IntegrationoftheISSimageontopofageographicallayershowingperfectmatchatthe

terminalbuildingsofMadridBarajasinternationalairport.

Oncewehave identified geographical coordinates fromthe image, correlationovertheground

objectshasbeenmade.Todothis,abufferaroundeachlightsourceisdefinedandtheresulting

geometryisusedtogetgeographicelementsfromeachlayer.Thedataprovidedbythismethod

returninformationabouttypeofelement(building,naturalzone,localroad,highway,stadium…),

typeofuse,owner,etc

Wehavedevelopedaneasyinteractivewebtoolthatallowusto identifybrightgroundsources.

ThegeoTIFFimagehasbeenpublishedusingaGeoserver(www.geoserver.org)cartographicserver.

An OpenLayers (www.openlayers.org) based application is used to get latitude and longitude

positions selected by the user using this interface. Then, Google Street View API (Application

ProgrammingInterface)isusedtogeta360ºviewaroundthosepoints.Thistoolcanbereachat

www.astroide.es/ucm/lightsources.Theseimagesprovidethefirstlookofthezoneand,inmostcases,

animmediateidentificationoftheluminaires.

NOAA DMSP satellites (Defense Meteorological

Satellite Program)providedaily imageduringnight

periodsfromallaroundtheearth.Thisinformation

isaccessibleviaWCSandWMSserversanditcanbe

used with a gvSIG client. Although these images

couldbeusedtocalibratetheISS,theirplatescales

are2.7km/pixel,i.e.farfromtheresolutionoftheISSimages.TheNOAAserviceprovides24bitspixel

information, and the WCS service served it on

severalrasterformats.

Figure11NOAADMSPimagecorrespondingtoApril

1st2011.(http://www.ngdc.noaa.gov/dmsp/)

One of the objectives of the study is, as was mentioned above, determining which are any sources

of illumination and to produce quantitative maps of the illuminated zones. From the image we havegenerated a three-dimensional and dynamic field, where the heights represent different levels of





illumination. First we determine isophotes in vector format from the georeferenced ISS026E026493image (Fig. 12 shows a zoom of this image) and then generate a raster image pixels filling by a

near neighbor interpolation, a Digital Elevation Model (MDE). d) Generation of a 3D image (Fig.13),and an overlay for the MDE, orthophotos from PNOA are used (Fig. 14).

Figure 12 Isophotes of the processed image at Barajas International Airport zone

Figure13DigitalElevationModelusingvaluesfromgeoreferencedimage(Fuenlabrada)





Figure. 14 Ortophotographic interactive image overlay on Digital Elevation Model. (Fuenlabrada)

As a result we have obtained a 3D ortophotographic image that keeps the georeferencing (X, Y).Elevation information (Z) has replaced by the value of the illumination counts from the original

image. This digital model represents more attention to the areas that produce higher lighting. Themodel allows us some interactive navigation though the resulting image. So we can pan, change

the perspective or zoom to identify each item. Fuenlabrada, a city at south of Madrid, has beenselected for this test.

4.Crosscalibrationwithgrounddata

Ausefulandimmediateyieldofthisstudyconsistsinobtainingthelistoftheworstsourcesoflight

pollutionthatisusefultodrawtheattentionofthetechniciansinlightingorbettertothepeople

in charge of political decisions. To show the solutions and not only the problem, it would be

interesting to obtain a relationship between public lighting and impact on satellite images.

Detectingplaceswherethevaluesarehigherthanexpectedwouldallowustoshowtheuseofbad

equipmentortheexistenceofpoorlydesignedinstallations.

Thispartoftheworkisbeingmadecollectinginformationanddataonearth.Digitalphotometers

(alsoknownasluxmeters) tomeasure the lightbrightnessof the street illumination are being

employed.Detaileddataonselectedplacesaretakenonagridofpositionsonthestreetandusing

aphotographictripod.Tospeedthegatheringofdata,thephotometerisplacedontopofacar

andlinkedtoalaptopcomputer.ThepositionaldatacaptureisperformedwithaGPSatthesame

time.Thespeedduringthecoursesneverexceeds50km/hondowntownstreets(<90km/hon

ringroadsofthebeltway)anddataiscollectedatapaceofonepersecond.Moreinformationcan

befoundinthereportsofthetraineeprojectsofundergradstudentsCepero(2009),Rodríguez

(2010),andRuiz(2010)underthesupervisionofprof.Zamorano.

ToobtainrecentgrounddatatocomparetotheISSimage,wemadeacourseatthebeginningof

thenightcorrespondingto2011/03/22.Thetotalpath(41.358kmlong)wasgeorreferencedusing

agpstracklogduringluxometermeasurements(seeFig.15).Wehaveusedthegeometryofthis

pathtogetthevaluesofeachpointfromtheISSgeorreferencedimage(SeeFig.16).





Figure15.Wholecourseof22ndmarch2011wherelightintensitymeasuresweretakenwitha

photometerontopofthecar.ThepathisdisplayedovertheISS026E026493image.Besides

Madriddowntown,amainroadandpartofMajadahonda(acitynearMadrid)weresurveyed.

Figure16.Illuminationvalues(inunitsofcandles)fromISS026E026493alongthepath(inmeters).

It isnot straightforward touse thisdatabecause somecaveatswere found. The idealscenario

wouldbethatwherethegroundandsatellitedataweretakenatthesametime(oratleastthe

samenightoraclosenightatthesamehour).





In themeantime between the date were the ISS image was taken and the ground data was

collectedsomechangestookplaceinpubliclighting. Forusitisobviousthatthemainstreetof

UniversidadComplutensecampusatCiudadUniversitariawasswitchoff(seethebluelineartrack

inthemiddleofFigure17)andwemadearemarkinourlogbook.

Changesatstreetsnotsofamiliarfor

us could be unnoticed. Thus one

shouldbecautiouswhencomparing

space and ground data. Conversely

the Picasso tower, one of the

brightest sources, was switch off

duringtheISSpass.Seebelowsome

comments on street geometry as

anothersourceoferror.

Figure17.Firstpartofthecourseof

22nd march 2011 corresponding to

Universidad ComplutenseofMadrid

campus (Ciudad Universitaria). The

track is coded with a color scale

wherebright yellow corresponds to

brightestmeasures.

Totacklethisproblemwecouldcomparethewholesetofdatainablindfoldorunbiasedwayanddiscard data not adjusting to the expected relationship, which introduces a bias.Our referred

method istouse onlydataofselectedplaces according to the notesin our logbook.Wehave

discardeddatacorrespondingtostreetswhereafractionorthetotalnumberofluminaireswas

switchedoff.Thispartofthestudyisunderwayatthetimeofwritingthereport.

WearealsotryingtolinkthenightskybrightnessmeasuredatplacesoutsideMadridcityarea

with the number and intensitiesof the lightpollution sources around. This study isout of the

scopeofthisreport.InterestedreaderscouldbrowsethetraineeprojectofPilaDíez(2010).

5.Otherinterestingissues

1)Imageprojection.

It should be taken into account that the imagewas not taken from the perpendicular to the

ground (i.e. the ISS was not at the zenith ofMadrid). According to the Gateway toastronaut

PhotographyoftheEarththecameratilt(lookangleawayfromastraightnadir,i.e.straightdown

belowthespacecraft)was43degrees.Toobtainamapprojectiontoanappropriatecoordinate

systemtheimageshouldbecorrectedforthisinclinationorgeoreferencedaswehaveshown.

2)Absolutecalibration.

The image of Madrid was taken from ISS that was at an altitude of around 350 km from theground.ThespacebetweenMadridandISSwasnotemptyandpartofthelightislostinitstravel





through theatmosphere. Thisatmospheric extinction dependsonwavelength (higher forblue,

lowerforredphotons)anditvariesfromnighttonight(andevenduringthenight).Astronomers

observestandardstarsalongthenightand,usingthemethodofabsolutephotometry,determine

atmosphericextinction.

Sometestsshouldbeperformedtoobtainanabsolutecalibrationoftheimagestakenwiththe

camera on board ISS (see bellow our recommendation on calibration tests) and besides the

extinctioncoefficient(andtheinclinationangle)ineachchannelatthemomentoftheshotmust

beknow.

TheastronomicalobservatoryofUniversidadComplutensedeMadrid(ObservatorioUCM)hasan

all-skycamera(AstMon-UCM)tomapthenightskybrightness(whichisrelatedtolightpollution)

and monitor the extinction in the astronomical photometric bands Johnson B, V and R.

Unfortunately,AstMon-UCMwasonmaintenanceduringthenightof11thFebruaryof2011.The

meanextinctioninJohnsonVbandfor8nightsofFebruarywas0.32(precision0.18).

The green channel is close to

this V band (compare

transmission curves in figure 3

and figure 18), thus we can

estimate (taken into account

the light path on the

atmosphere,whichdependson

inclination)thataround30%of

thelightwaslost.

Figure 18. Transmission curvesof the Astrodon Johnson filter

set(http://www.astrodon.com/)of

AstMon-UCM. Green line

correspondstoVbandanditis

similar to G filter of Nikon

cameras(seeFigure3).

3)Colorofthelightsources

Streetlightinguseslampsofdifferenttypes.Someofthemarehighpressuresodium(HPS)whose

spectrum is plenty of emission lines and the resulting color is white. The preferred type forastronomers is Low Pressure Sodium (LPS) lampswhich ismonochromatic (orange) and easily

filtered.Otherlampsaremadewithmercuryasthemetalhalidelamps.Thespectrumofthenight

skyatapollutedsiteasMadridisacombinationofthespectrumofthelampsandmercury(HgI)

andsodium(NaI)emissionlinesarepresent.ThestrongestlinesareHgI436nmandNaI589nm

doublet.

Tocompletetheworkoutlinedinthisreport,weshouldstudylightpollutionusingthe3channels

orbandsprovidedby theRAW image. The colorof the lightsourcescouldprovide information

aboutthenatureofthelampsemployedinlightingsincemercurylampsarebluerthantheyellow

sodiumvaporlamps.WeintendtoobtaininthenearfutureaspectrumofMadridnightskysimilartothatoffigure14andcompareitwiththecolor,intensityandnumberoflightsourcesdetected





inthesatelliteimages.

Relatedwiththistopic,wehavestudiedthechangeinintensityandcoloroftheskyoverMadrid

duringtheEarthHour(http://www.earthhour.org/)of26March2011.ISSimagesofMadridduring

thehourwhensomebuildingsswitchedofftheirlightsarenotavailable.ProgrammedISSpictures

ofbigcitiesduringthenextyearsEarthHourswouldbedesirabletomeasuretherealimpacton

theintensityoflightemittedtothesky.Asimpleapproachtothiscalculationcouldbemadeby

comparing the total intensity of the ISS nocturnal imageobtainedany night (adding upall the

pixels) and the intensity obtained after discarding those corresponding to areas which were

switchedoff.Unfortunatelytheavailableimageissaturatedandonlyahintoftheintensityofthe

brightestspotscouldbeobtained.Thisisanotherargumentfortheseriesofexposuresproposed

(seebellow).

Figure 19. Tracingsofnightsky spectra atapolluted(red)and atadark site (blue).Main line

identificationsforartificialfeaturesaremarked.Thecoloredcurvesonthetopofthefigureare

thetransmissionfunctionsofUBVRstandardastronomicalphotometricbands.Althoughthelight

pollutionappearstobemoreimportantintheVband,thestrongNaI589nmdoubletisthemain

featureoftheredfilter.Figure3of“Theeffectsofimproperlightingonprofessionalastronomicalobservations”,FerdinandoPatat,(astro-ph1011.6175,2010)EuropeanSouthernObservatory.





TheAstMon-UCMofObservatorioUCMwasprogrammedtoobtainaseriesofimagesinB,VandR

Johnsonphotometricbandsduring26March2011.Thenightskybrightnessatzenithderivedfrom

these images were compared with those of previous and following Saturdays to show up

differencesinlightingevolutionatthetimearoundthatoftheEarthHour(local8:30-9:30pm).

Ourpreliminaryanalysisshowsthat,fortypicalclearnights,thevalueofthenightskybrightness

atzenithisveryconstantinthethreebandsanditisverydifficulttodetectvariationevenatthe

timewhensomebuildingsareroutinelyswitchedoffalongthenight.Forinstance,themedian

values for 19March (the previousSaturday, between 20:55and 22:03)are SB(B)=17.98+-0.02,

SB(V)=17.22+-0.03andSB(R)=16.61+-0.04inastronomicalunitsofmag/arcsec2.

Thenightof 26March 2011was cloudyatMadrid.Thecloudspreventedusfrommeasurethe

nightsky brightness.However, the dataobtainedfromAstMon-UCMcouldcontain very useful

informationsincethecloudcoveractedasascreentoreflecttheescapinglight.Usingthevalues

ofthisfalseskybrightness(cloudbrightnessshouldbesaid),wenoticedavariationthatcouldbe

realifthealbedoandtotalreflectivityofthecloudsdonotchangeduringthesehours.

AstMon-UCMisacamerathat

delivers images and, as

expected due to the high

variationregistered,thecloud

coverdidvaryduringthistime

as figure 20 shows clearly.

Thus themeasuredchange is

mainly due to this unwanted

effect.

Figure20.Cloudevolutionfor

partofthenightof26March

2011 as recorded with

AstMon-UCMfortheJohnson

Rband.

4)Streetgeometry,groundalbedoandweatherstation.

Theimpactofpubliclightingonlightpollutiondependsstronglyonseveralparameters.Thefirst

oneisthetypeofstreetlamp,beingtheoldestonesveryinefficientsincetheylostanonegligiblepartofthe lightdirectly tothe space. For similar street lamps, the observedeffectonsatellite

images depends also onpropertiesof the street as its widthor the height of the surrounding

buildingsoreventhegroundalbedo.Imagestakenindifferentweatherstationoftheyearshould

differduetothetreecanopy(leavesthatcoverorshelterlight)andthepresenceofsnow(with

highalbedo)ontheground.

5)ComparisonwithNOAADMSdata

OurgrouphasgainedavaluableexperienceonanalysisofNOAAimages(Sánchez2007;Sánchez&

Zamoranoetal.2008,2010).UsingDMPSsatellitenightimagestakenin2000wehaveestimatedthe

saturated surface in many European countries. The objective of this study was to compare theilluminationconditionsanditseffectsinlightpollution.Withimagesintheinterval1992-2007,we





derivedtherealpowerconsumptioninpubliclightinginSpainanditsevolution.

6.Recommendedobservationalsetupandcalibration

Weintendinthissectiontodefineanobservationalsetupandproceduretoobtainmoreuseful

scientificdatafromdigitalpicturestakenfromISS.Itisworthnotingthattheactualsetupofthe

NikonD3scameraincludessomeofourrecommendations:torecordRAWdataimagesandtouse

14bitsinsteadof12bitsdata.

To measure spatial variation of intensities along a frame it is necessary to correct of lens

vignetting.This effect isnoticeable inmostwidefield lenseswhose images showa decrease in

illuminationneartheedgesofthefieldofview.TheNikonD3sonboardISSisusingalenswith

focal length of 200mmand the effect should benegligible. The recommended laboratory test

consists in picturing a white cardwhose surface is homogeneously illuminated. Toobtain this

illumination imageor FlatField inside ISS wepropose topicture a flat illumination theme. The

targetshouldbedeterminedaccordingtoavailableitemsas,forinstance,agraywallpanel.

Bad pixels, which develop during the detector aging, should bemapped and discarded. These

pixelsalsoappearwhenaDarkframe,i.e.apicturewithoutincominglight,istaken.Darkframes

with different exposure times are routinely obtained as calibration fileswhen long exposures

observationsareperformedtocorrectfromthermalsignal.Sincetheexposuretimesproposedto

thisstudyareshortenough,thereisnoneedofsuchframes.

Linearity tests should be made to obtain the response curve. Differences from linearity are

expectednearsaturationlevelduetotheanti-bloomingsystemofsomecameras(NikonD3shas

anti-blooming?).Severalpicturesofthesamesubjectwithsamesettingsexcepttheexposuretime

willallowustobuildtheresponsecurveassumingthattheintensitiesofthepixelsintheimagedonotvaryduringtheseriesofexposures.

Thereisnotasingleoptimumexposuretimeorsettingofthedigitalcamerawhichrecordswith

goodsignal tonoisethelarge rangeofvaluesofanocturnal imageofabigcityasMadrid.The

imageanalyzedhassomebrightspotwithsaturatedpixelsevenusing14bitsperpixel.Theeasy

andsafesolutiontosaturationwouldbetoreducetheexposuretime(keepinguntouchedtherest

ofparameters).Unfortunately,sincesignalisdirectlyproportionaltoexposuretime,thismethod

wouldresultinadimmerimage.

Togetenoughsignalatthedarkestregionsandusefulunsaturatedvaluesatbrightspotaseriesofexposuresshouldbetaken.ThedatafromtheseimagescanbecombinedtobuildaHighDynamic

Range(HDR)imageorusedseparatelytomeasureregionsofdifferentbrightness.Thisprocedure

iseasilyprogrammedwiththeNikonD3sdigitalcamerausingthebracketingexposuresystem.The

analysisoftheimageswouldallowtodeterminetheresponsecurveofthecameraandtodetect

therangeofthedetectorlinearity.

Absolutecalibrationof the ISS images wouldbe a greatbonus.We could read pixel values in

physicsunits insteadof counts and determine the sensitivityof the digital camera.A standard

source is needed to perform a photometric calibration. Calibrated photodiodes are used at

laboratorytests.AssumingthatthedigitalcameraisonboardISS,itisnecessarytouseacalibratorofknownintensityoutsidethesatellite.Althoughmanypeoplewouldthinkonearthbasedlight





sourcesthebestchoiceistouseastronomicalsources.Aseriesofshortexposuresofabrightstar

(Sirius,Vega,Capella,etc,nottheSun),withthesamesetupforthenocturnalimagesoftheearth,

will be enough. Since the satellite is at high altitude, there is no atmospheric extinction. The

inclinationofthedigitalcamerawhenshootingtothestarisnotrelevantinthiscasesincethe

lightpathalongtheatmosphereisnullwhenpointingfarfromearth.

Table1.Recommendedframesforcalibration

SetUp NikonD3Sf=200mmf/4RAW14bits

Dark 10x 1/60sBracketing+-3 12800ISO Lenscoveron

Calibration 10x 1/60sBracketing+-3 12800ISO Selectaknownbrightstar

FlatField 10x AutoBracketing+-1 400ISO Flatilluminatedsurface(1)

(1)Grayorwhitewallpanelforinstance.Thecamerashouldberotatedbetweenshotstoremove

differencesinillumination.

Table2.Recommendedscientificframes

SetUp NikonD3Sf=200mmf/4RAW14bits

Frames 1/60sBracketing+-3 12800ISO Keepthecamerasteady

Note:Bracketingin1EVsteps.

7.Conclusions

Observationoftheearthatnight,showingtheartificial lightscausedbyhumansisofextraordinary

interestformanypurposes,includingscientificfields.Oneofthemisthelocationandmeasure(and

studyoftheevolutionintime)oftheirbrightnessasatoolforfightingagainstlightpollution.While

waitingforasatellitesystemforglobalandcontinuousobservationandfullydedicatedtothistaskasNightsat (http://www.ngdc.noaa.gov/dmsp/nightsat.html, Elvidge et al. 2007), ISS images taken with a

digitalcameraprovideveryusefulscientificdata.

ThespatialresolutionoftheISSpictures,withaplatescaleofaround16m/pixel,isbetterthanthat

ofNOAA (2.7 km/pixel).A resolutionofaround 80m isenough for our purposes, althoughaerial

photographyfromanairplaneoraballoonwouldbeagreatcomplementforfinerdetaildata.

We have presented in this report an ongoing analysis of a nocturnal image of Madrid taken by

astronautsaboardtheInternationalSpaceStation.Thepreliminaryresultshaveshownthepotential

oftheseimagesassourcedataforscientificstudies.Althoughwefocusedinlightpollution,therearemoreinterestingtopics.Theimportanceoftheseimagesforpublicoutreachisbeyondanydoubt.

Therawimagehasbeenprocessedusingfreewaresoftwaretosplittheircolorchannels,togetridof

bad pixels, to build a green channel frame with full resolution. This channel is similar to the

photometric Johnson V band used in astronomy and easily calibrated with standard stars

observations.Georeferenceof the images isa firstandnecessary step inordertounambiguously

locatethebrightspotsandtocomparewithgroundbasedmeasurements.

Acalibrationsequencetobeusedbythephotographer-astronautsonboardISSforthiskindofnight

pictureshasbeendesigned.Onlyonecalibrationsequenceshouldbeenoughprovidedthebrightstarimage is not saturated. The procedure of photo series with bracketing should be performed for





imagestakenforscientificstudies.WeareawarethatthescheduleofastronautsaboardISSisvery

tight,buttheprocedureisnottimeconsuming:oneshotwillproduce7pictures(bracketing+-3,1EV

steps).

References

- “Contaminación lumínicaen laUCM (2010). I. Evolución de la iluminaciónen laUCM” Isabel

RodríguezHerranz(2010)(http://eprints.ucm.es/11368/)

-“ContaminaciónLumínicaenlaUCM(2010).II.MapadelailuminaciónenlaUCM”RoqueRuiz

Carmona,Roque(2010)(http://eprints.ucm.es/11367/)

- “Mapa de contaminación lumínica de la UCM” Pablo Cepero Rodríguez (2009)

(http://eprints.ucm.es/12288/)

- “Mapa de brillo de fondo de cielo de la Comunidad de Madrid” Berenice Pila Díez (2010)

(http://eprints.ucm.es/11364/)

-“TheNightsatmissionconcept”.Elvidge,C.D.,Cinzano,P.,Pettit,D.R.,Arvesen,J.,Sutton,P.,Small,C.,Nemani,R.,Longcore,T.,Rich,C.,Saran,J.,Weeks,J.&Ebener,S.inInternational

JournalofRemoteSensingVol.28,No.12,20June2007,2645–2670

-“NightskybrightnessatsitesfromDMSP-OLSsatellitemeasurements”Cinzano,P.;Elvidge,C.D.

(2004)MonthlyNoticesoftheRoyalAstronomicalSociety,Vol.353,p.1107.

-" DifferentialPhotometryStudyoftheEuropeanLightEmissiontotheSpace",SánchezdeMiguel,

A.(2007)inStarlight:ACommonHeritage,ProceedingsoftheStarlight2007Conference,LaPalma

(PDF)

- "Contaminación lumínicaenEspaña 2010" (http://eprints.ucm.es/12284/) Alejandro Sánchez de

Miguel, JaimeZamorano, BerenicePila-Díez, Jesús Rubio Jiménez, RoqueRuiz Carmona, IsabelRodríguez Herranz & Alicia González Pérez (2010) in "Highlights of Spanish Astrophysics VI"

ProceedingsoftheIXScientificMeetingoftheSpanishAstronomicalSociety(SEA)heldinMadrid,

September,2010

- "Light pollution in Spain. An european perspective” (http://eprints.ucm.es/12281/) Sánchez de

Miguel,A.&Zamorano,J.(2008)In"HighlightsofSpanishAstrophysicsV"ProceedingsoftheVIII

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Acknowledgements

WewishtothanktotheNASAJSCImagingLabandmorespecificallytotheHeadoftheCrewEarth

Observation,MsSueRunco,whohasbeensokind toprovideuswiththeRAW image.Wealso

acknowledgethe timespendbyastronautsaboard ISS totakethebeautiful picturesandmade

themavailable tothepublic during theseyears. Inparticularwearegratefulto SoichiNoguchi

(JAXA)andCol.DouglasWheelockandbyScottKelly(NASA)whoappearasauthorsofnightshots

ofSpain.

WethankusefulcommentsfromNicolásCardielandJesúsGallego.“LaboratoriodeInvestigación

CientíficaAvanzada” (LICA) isa facilityofUniversidadComplutensedeMadrid and ithas beenfunded by the Spanish MICINN AYA2009-06330-E and AstroMadrid Program CAM S2009/ESP-




Universidad Complutense de Madrid - LICA report april 2011 20

1496:“AstrofísicayDesarrollosTecnológicosenlaComunidaddeMadrid''(ComunidaddeMadrid

and the European Union). We acknowledge support from the Spanish Programa Nacional de

Astronomía y Astrofísica under grant AYA2009-10368. Partially funded by the SpanishMICINN

underConsolider-Ingenio2010ProgramgrantCSD2006-00070:''FirstSciencewiththeGTC''.

iss nocturnal images as a scientific tool against light pollution

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