Imaging spectroscopy of Saturn and its satellites : vims-v onboard Cassini

Planet[ Space Sci[\ Vol[ 35\ No[ 8:09\ pp[ 0152Ð0165\ 0887Þ 0887 Elsevier Science Ltd\ Pergamon

All rights reserved9921Ð9522:87:,*see front matter

PII ] S9921Ð9522"87#99926Ð2

Imaging spectroscopy of Saturn and its satellites ] VIMS!V onboard

Cassini

F[ Capaccioni\ A[ Coradini\ P[ Cerroni and S[ Amici

Istituto di Astro_sica Spaziale CNR\ Viale dell|Universita|\ 00\ 99074 Rome\ Italy

Received 03 April 0886 ^ revised 4 February 0887 ^ accepted 03 February 0887

Abstract[ The instrument VIMS "Visible and InfraredMapping Spectrometer# to be ~own on Cassini is amultispectral imager covering the spectral range from9[29Ð4[0 mm[

Some major areas for the scienti_c objectives of theCassini!VIMS instrument will be

, study of the atmospheres of Saturn and Titan ^, study of the surface composition and spatial dis!

tribution of di}erent species for Saturn icy satellites ^, study of the structure and composition of Saturn

rings[

VIMS instrument is made up of two independentchannels\ one covering the visible range from 9[29Ð0[94 mm and named VIMS!V\ the second covering thespectral range from 9[74Ð4[0mm[

In this paper we will describe the scienti_c objectiveswhich will be addressed speci_cally by VIMS!V\ andhow the observations in the visible range will comp!lement those obtained by the infrared channel ^derivedobservational constraints for VIMS!V will be discussedwherever appropriate[ A general description of theinstrument will follow with a detailed examination ofthe peculiar features of the instrument[ Finally\ thecalibration activity will be analysed and the resultsdiscussed[ Þ 0887 Elsevier Science Ltd[ All rightsreserved

Introduction

The instrument we are going to describe falls into thecategory of the multispectral imagers which represent ane}ective tool for the systematic analysis of the surfacesand atmospheres of the planets and their satellites[ Theirspectral capability\ in fact\ allows us to identify the target

Correspondence to ] F[ Capaccioni\ Fax ] 9928 5 19559077 ^E!mail ] capaccioÝsaturn[ias[rm[cnr[it

surface mineralogic composition ^ on the other hand\ theirimaging capability allows us to determine the spatial dis!tribution of minerals and compounds\ thus enabling us torelate the composition of a soil to its geologic history[ Thisapproach is e}ectively applied also to the atmospheres ofplanets and satellites\ with the additional advantage ofproviding a simple means to follow the spatial!temporalvariations of atmospheric structures[

The scienti_c objectives of the VIMS instrument willinclude ]

, Study of the icy satellites[ This is probably one of themain objectives of VIMS research ^ it will be possible todetermine the composition of the dark:organic materialson satellites and of volatiles\ to study the physical stateof volatiles and to determine the spatial distribution ofall of these constituents[

, Study of the atmospheres of Saturn and Titan[ In this_eld VIMS will allow us to study the temporal behaviourof winds and eddies in the atmospheres of these twoplanetary bodies\ to derive 2!D wind _elds\ to determinesolar energy deposition pro_les\ to study the com!position and distribution of atmospheric and cloud spec!ies "including aerosols#\ to search for lightning\ to deter!mine Titan surface properties at the wavelengths of Visand near!IR atmospheric windows[

, Study of the rings[ It will be possible with VIMS also tostudy the composition and spatial distribution of speciesfor Saturn rings[

We will describe the general scienti_c objectives ofVIMS instrument as a whole and discuss the particularcontribution of VIMS!V to each of them ^ where appli!cable\ we will describe how these scienti_c objectives havebeen translated into observational strategies and instru!ment technical solutions[

Cassini!VIMS is made up of two independent channels ]one covers the 9[29Ð0[94 mm spectral range\ while theother covers the spectral range 9[74Ð4[0 mm[ While JPLhad the responsibility for the overall instrument\ the vis!ible and near!infrared channel\ named VIMS!V\ has been

0153 F[ Capaccioni et al[ ] Imaging spectroscopy of Saturn

Table 0[ VIMS!V system performances "After Reininger et al[\ 0883#

Spectral Coverage 249Ð0949 nmSpectral Sampling "VIMS!V capability# 0[35 nmSpectral Sampling "Cassini requirement# 6[2 nm "4 pixel summing#IFOV 9[06×9[06 mradE}ective IFOV 9[4×9[4 mrad "2×2 pixel summing#FOV "Cassini requirement# 0[72>×0[72>FOV "VIMS!V capability# 1[3>×1[3>

built in Italy by O.cine Galileo\ under a contract by theItalian Space Agency "ASI#[

VIMS!V is equipped with a frame transfer of 145×401pixels CCD matrix detector on which spatial and spectralinformations are simultaneously stored[ Radiation col!lected from the VIMS!V telescope is focused on the spec!trometer slit[ The slit image is spectrally dispersed by adi}raction grating and then imaged on the CCD[ Thus onthe CCD rows are recorded the slit images of the di}erentwavelength bands[ The CCD is passively cooled tobetween −39 and −19>C[ On!chip summing of pixelsmakes the instrument extremely versatile\ allowing it toimplement a large number of operating modes to copewith di}erent observing conditions[

The maximum capabilities of VIMS!V are a spectralresolution of 0[35 nm and a spatial resolution of 056 mrad[While in the Nominal mode of operation\ due to on!chip summing of 4 spectral × 2 spatial pixels\ VIMS!Vachieves a spectral resolution of 6[2 nm and a spatialresolution of 499 mrad[ The SNR is kept always above099\ which is the requirement imposed by the scienti_cobjectives[ The total Field of View of the instrument is1[3>×1[3>[ The main performances of the instrument arereported in Table 0[

It should be noted that\ due to budget restrictions lead!ing to descoping of the project during its completion\ thepossibility of operating VIMS!V at its maximum capa!bilities "High Resolution mode# at present is restricted byon!board software[ The High Resolution mode\ however\has been fully tested and it will be possible during thecruise phase to Saturn to upload the necessary software[

Icy satellites of Saturn ] scienti_c objectives

The icy satellites of Saturn show complex patterns ofsurface morphologies\ re~ecting a history of resurfacingevents\ i[e[ Dione and Thetys\ and in some cases of exten!sive tectonic activities\ i[e[ Enceladus\ as well as a col!lisional evolutionary history with perhaps successive epi!sodes of destruction and reaccumulation\ i[e[ Mimas andThetys "McKinnon\ 0874 ^ Squyres and Croft\ 0875#[

The impact craters densities vary consistently over thesurface of almost all of the icy satellites of Saturn\ pointingto the fact that a solid crust\ formed during the earlyphases of the intense meteoritic bombardment\ has beenendogenically modi_ed by successive resurfacing events\due probably to the decay of radioactive elements embed!ded in the silicatic components "Coradini et al[\ 0884 ^Schubert et al[\ 0875 ^ Ellsworth and Schubert\ 0872 ^ Fed!erico and Lanciano\ 0872 ^ Consolmagno and Lewis\

0867#[ Moreover\ the presence of volatiles\ like ammonia\methane and carbon monoxide\ trapped in the water icematrix as either clathrates or hydrates are essential toexplain the complex geophysical history of the satellites"Coradini et al[\0884 ^ Ellsworth and Schubert\ 0872 ^ Stev!enson\ 0871 ^ Consolmagno and Lewis\ 0867#[ Some formof icy volcanism\ possibly involving ammonia!rich ~uidsis implied to account for the abundant tectonic landformsfound on Enceladus surface "Coradini et al[\ 0884 ^ Kargel\0881 ^ Forni et al[\ 0880\ Stevenson\ 0871 ^ Lewis\ 0861#[

Water ice is known to be a major contributor to thesurface composition of most of the saturnian satellitesbut\ at least for three of them\ Iapetus\ Hyperion andPhoebe\ varying amounts of dark materials are known tobe present[ The hemispheric albedo asymmetry of Iapetusis well known ^ one of its hemispheres is _ve times brighterthan the other "Buratti and Moshear\ 0884 ^ Morrison etal[\ 0864#[ Iapetus composition\ however\ points to anexogenic origin for the dark materials on its leading hemi!sphere\ probably originated from the outermost satellitePhoebe "Cruickshank et al[\ 0872 ^ Bell et al[\ 0874#[

A number of detailed scienti_c objectives for VIMS inthis _eld can be identi_ed as follows ]

, Determination of the surface composition of the sat!ellites of Saturn with the maximum spatial resolutionpossible[ The spatial resolution which can be achievedby VIMS during a satellite ~y!by will depend criticallyon the geometry and velocity of the ~y!bys[ The OrbiterTour is currently being optimised\ however the presentreference tour "T07!2# allows for several ~y!bys\ bothtargeted and non!targeted\ of the icy satellites[ In Table1 are reported the nominal and high!spatial resolutioncapabilities of VIMS!V[ Even in the nominal mode theresolution will range from 2 to more than 09 times betterthan Voyager images[ Under optimal conditions it willbe possible to achieve spectral imaging of the wholeilluminated hemisphere of the satellite for di}erentobservation angles\ thus allowing us to characterize indetail constituent ices\ minerals and:or solid organicphase\ their relative concentrations and\ possibly\ dis!tribution of grain sizes[

, Determination of the composition of any dark materialfound on Saturn|s satellites\ with particular attention toIapetus[ This material shows chemical similarities tosome compounds in primitive carbonaceous chondrites[Relate the composition of this material to compositionalstudies of other Solar System bodies on the surface ofwhich primitive:organic materials are expected to befound "comets\ asteroids#[ This will provide links andconstraints on the Solar System origin and evolution[

0154F[ Capaccioni et al[ ] Imaging spectroscopy of Saturn

Table 1[ The nominal and high spatial resolution of VIMS!V for the icy satellites ~y!bys of tourT07Ð2

Nominal HighDistance Phase angle range Resolution mode Resolution mode

Satellite "km# "degrees# "km# "km#

Mimas 37826 06Ð63 13 7Enceladus 649 17Ð89 9[264 9[014Tethys 03772 04Ð099 6 1[2Dione 0459 42Ð74 9[679 9[159Rhea 0153 57Ð65 9[521 9[109Iapetus 0184 41 9[537 9[105

, Determination of the composition\ distribution andphysical state of volatiles on icy satellite surfaces[

, Comparison data obtained by VIMS for di}erent sat!ellites and di}erent ~y!bys ^ it will thus be possible toobtain mineralogical maps of main geologic units on thesurface of the satellites[

Icy satellites science with VIMS!V ] compositional

analysis and scienti_c requirements

VIMS as a whole instrument is particularly well suited tostudy the composition of icy satellites of Saturn\ as theices and di}erent components expected to be found ontheir surfaces all have diagnostic absorption features inVIMS spectral range\ i[e[ in the range between 9[29 and4[0 mm[

As far as VIMS!V is concerned\ the mineralogic map!ping of the surfaces of the icy satellites will be one ofits primary tasks[ Its relatively high spatial resolutiontogether with its spectral resolution will allow to derivedetailed mineralogical maps for Saturn|s satellites[ Itshould be recalled again that of particular interest to thisaim would be the possibility to upload the High resolutionoperation mode for VIMS!V[ The spatial resolution in thenear and infrared range will be increased 2 times "Table1#[

Particular points which could be studied speci_cally byVIMS!V are described below[

Study of water ice and of its contamination by particulatematerials

The mean optical path length of a photon in water icedepends on its wavelength "Clark and Lucey\ 0873 ^ Irvineand Pollack\ 0857# ^ the greater the optical path length"and thus the weaker the absorption feature#\ the greaterthe possibility of detecting an impurity "particulatematerial mixed with the icy matrix#[ Thus\ in general\ thespectrum of an ice!particulate mixture might be used moree}ectively to determine the particulate concentrationwhen many absorption features are analysed simul!taneously[

The absorption of water ice at 9[70\ 9[89 and 0[93 mm\all in the spectral range of VIMS!V\ are particularlyimportant for the identi_cation of the composition of

minerals mixed with ice[ In this range in fact the iceabsorption coe.cient is very low\ thus allowing embeddedgrains of a mineral with an absorption in this region to bedetected at very low levels of concentrations "Clark andLucey\ 0873 ^ Clark\ 0871#[

Compositional studies from dia`nostic spectral featuresin the visible!ion and UV surface irradiation

The surfaces of the icy satellites are unshielded from exter!nal radiation and energetic particles which will cause pho!tolysis\ due to the interaction with solar UV radiation\and radiolysis of water ice\ due to cosmic rays and to theambient plasma "Johnson\ 0885#[ In general a water ice_lm exposed to energetic ion bombardment will showa considerable reddening of the UV:Vis region of thespectrum as outlined in laboratory experiments by Sacket al[ "0880#[ The authors have achieved a considerableUV absorption enhancement below the 349 nm probablyattributed to the modi_cation in the microstructurecaused by ion bombardment\ as well as in the increase ofthe surface roughness[ Moreover\ speci_c features couldincrease the UV absorption[ For instance the 179 nmabsorption feature of Europa has been attributed to SOproduced by sulfur ions implantation from Jupiter|s mag!netosphere "Sack et al[\ 0881#[ However\ a recent summaryof laboratory data "Johnson and Quickenden\ 0886#\points to the fact that trapped OH "a photodissociationproduct of water ice due to solar UV photons# also hasan absorption feature at 179 nm[ However\ although OHhas been observed in the gas phase of the icy saturniansatellites "Shemansky et al[\ 0882# it has not yet beenidenti_ed in satellite re~ectance spectra[ The 179 nmabsorption feature is relatively broad causing a reddeningof the spectrum short of 249 nm\ and should be clearlyidenti_ed by VIMS!V "Fig[ 0#[

Finally\ exposition of H1O\ NH2\ H1S and NH3HS toUV irradiation has evidentiated a general decrease of thespectral re~ectivity of the samples below 9[4 mm\ with theaddition of new spectral features around 9[5mm "Lebofskyand Fegley\ 0865#[

Non volatile or`anic solids

Experimental simulations of the e}ects of the solar wind\planetary magnetospheric particles and cosmic rays on


Fig[ 0[ Comparison of Europa trailing emisphere re~ectance toleading emisphere re~ectance ratio "solid line# and laboratoryspectra for trapped OH after irradiation of crystalline icesamples with two di}erent g rays doses "after Johnson andQuickenden\ 0886#

Fig[ 1[ Re~ection spectra of materials produced by laboratoryirradiation of CH3 !containing mixtures\ after Thompson et al[\"0876# Identi_cation from top to bottom is the following ] "0#non irradiated H1O ] CH3 clathrate frost ^ "1# same mixture afterirradiation to 098 erg:cm1 "about 4×093 s# ^ "2# same mixtureafter irradiation to 0909 erg:cm1 "about 4×094 s# ^ "3# organicresidue of irradiation of pure CH3 with Ar ions to 098 erg:cm1 ^"4# organic residue "tholin# produced upon 199V discharge of aN1 ] CH3 gas mixture at 199 mbar

surfaces containing methane\ methane clathrate and otherhydrocarbons ices indicate that\ while they are initiallywhite ices\ they all produce yellowish to brownish organicproducts upon charged particle irradiation "Fig[ 1 ^Thompson et al[\ 0876#[ Progressive darkening is observed

upon continued irradiation[ Re~ection spectra ofmaterials produced by laboratory irradiation of CH3 con!taining mixtures are diagnostic in the region between 9[3and 9[7 mm "Thompson et al[\ 0876#[

VIMS!V spectral range is also useful to study the pres!ence of macromolecular organic!bearing materials\ suchas kerogen!like compounds\ and carbon polymorphs\although their very low re~ectance poses a serious limi!tation to their detectability "Cloutis et al[\ 0883#[

Minerals

The region of 9[7Ð0[94 mm shows the bands of silicateminerals[ Although these bands are rather broad\ whendi}erent silicate minerals are mixed together or with otherminerals\ the band positions and shapes will change "Pie!ters and Englert\ 0882 ^ Ga}ey et al[\ 0878#[ Moreover\hydrothermal alteration products\ phyllosilicates andclays\ all show charge transfer features due to Fe1¦ andFe2¦ in the short wavelength region of VIMS!V range"Sherman\ 0889#[ It must be recalled that Fe2¦ absorptionfeatures are sharpened at low temperatures[

Saturn science with VIMS and VIMS!V

As already mentioned in the introduction\ the study ofthe atmosphere of Saturn will be one of the main scienti_cobjectives for the Cassini mission in general and VIMS inparticular[ Previous ~y!bys of Saturn during the Voyagermission allowed only minor atmospheric changes to beobserved\ due to the very long metereological time scaleof the planet[ Now with the Cassini mission there is agood chance to obtain a global coverage\ i[e[ VIMS willbe able to use the extended coverage in time of the missionand its geometry\ which will allow it to obtain data bothfrom polar and equatorial orbits\ to study latitudinal\longitudinal and temporal variations for many atmo!spheric features[ Multispectral images of Saturn obtainedwith VIMS will provide a database of images which willallow us to carry out long term studies of the metereologyof Saturn[ Some of the main scienti_c objectives for Saturnscience with VIMS are listed here ]

, Determination of vertical distributions of variable gasspecies including condensibles such as ammonia andpossibly water ^ and disequilibrium species "phosphineand ortho:para hydrogen#\ over latitude and longitude\and their variation with time[

, Determination of vertical aerosol distributions andassociated optical and microphysical properties\ andtheir variation with longitude\ latitude and time[

, Determination of constraints on chemical:dynamicalmechanisms from spatial:temporal variability in atmo!spheric phenomena ^ study of the relationship betweengas variations and variations in cloud opacities and ver!tical distributions[

, Determination of the 2!D solar deposition rates, Morphology of the ammonia cloudtops and distribution

of the hazes, Determination of temporal variations of wind_elds


, Determination of stratospheric temperature pro_les\from multispettral occultation measurements

, Study of lightning ^ determination of rates and ~uxes ^determination of its spectral signature and of its 2!Ddistribution in the atmosphere[

Some of these scienti_c objectives will be speci_callyaddressed by VIMS!V[

In the visible part of Saturn spectrum three gaseousspecies have been observed ] H1\ NH2 and CH3[ Of thesespecies\ H1 is by far the most abundant but\ unfortunately\the spectral resolution of VIMS!V does not allow obser!vation of its band[ NH2 is di.cult to observe due to theweakness of its band in the visible resulting from thestrong depletion by condensation[ On the other hand\VIMS!V is particularly appropriate for the study of CH3[Methane shows strong absorption bands between 9[3 and9[5 mm[ They can be monitored by means of VIMS!Vand\ as these bands are not saturated\ as opposed to themethane bands in the IR\ from the study of their depthsthe methane content of the upper atmosphere can bederived and\ possibly\ the pro_le of CH3 in the upperatmosphere[ The evaluation of the spectral albedo of theplanet can be given as well as the optical depth of thehaze layers\ in the region having homogeneous spectralre~ectances[ In fact\ haze layers can be studied by meansof the slope of the spectra in the regions not contaminatedby absorption bands[

Finally\ the imaging capability of VIMS!V will allowthe production of volatile abundance maps\ and will givean important contribution to the study of the Saturnianclouds structure[ The relatively high spectral resolution ofVIMS!V allows a detailed processing of the images to becarried out\ in order to highlight cloud structures\ henceproviding a better description of their spatial distribution\composition and time evolution\ which will be used tomodel atmosphere dynamics[ Furthermore\ using themaximum allowable spatial resolution "14 km in the high!resolution mode and 63 km in the nominal resolutionmode at the closest periapsis distance# it will be possibleto study the strati_cation of the higher level of the atmo!sphere\ and the optical depth at di}erent heights ^ windvelocities at various latitudes can be estimated throughoutthe Saturnian tour[

Titan science with VIMS and VIMS!V

CASSINI!VIMS represents a powerful means to studythe atmospheric structure of Titan atmosphere and thevariations in longitude\ latitude\ altitude\ and with time ^the combined study through the two channels\ the visiblewith a maximum spectral resolution of 0[35 nm and theinfrared with a spectral resolution of 05[5 nm\ will allow usto determine composition and distribution of atmosphericcomponents\ clouds and hazes[

The existence of several spectral windows in the spectralrange between 9[4 and 1mm will allow us to obtain near!infrared mapping of the surface of Titan[ The spatialresolution of 9[06 mrad in the visible and 9[49 mrad in theinfrared will allow us to obtain\ at close ~y!by distancesto Titan\ spatial resolutions lower than 0 km[

Once again\ the unique feature of the CASSINI missionis to allow extended observations covering a long timespan\ during which observations will be carried out formany di}erent viewing geometries[ In particular\ in thecase of the observation of Titan through VIMS\ this willimply a full coverage of both emission and phase anglesfor day time observations and a full coverage of emissionangles for night time observations[ Thus it will be possibleto recover the dependence of the atmospheric features ongeometry "latitude\ longitude and altitude# and to studytheir temporal variation[

The scienti_c objectives for VIMS at Titan have beenpresented and discussed in some detail in Baines et al["0880#[

VIMS investigation at Titan will address in particular ]

, Observations of atmospheric properties\ compositionand processes[ Atmospheric composition ] measurementof the vertical distribution of condensible and chemicallyactive gas species\ including methane\ ethane and acety!lene\ and study of their spatial distribution and temporalvariability[

, Study of the formation and composition of aerosols ]aerosol content and optical properties\ and their dis!tributions[

, Observations and compositional mapping of thesurface[ Surface properties can be studied by VIMS inthe atmospheric windows located at approximately 0[2\0[5 and 1 mm[ More atmospheric windows can be foundin the visible at 9[72\ 9[83 and 0[97 mm\ but it is doubtfulwhether these could be used due to the high aerosolopacity in this spectral region[

, Study of lightning and associated phenomena ] deter!mination of rates and ~uxes\ and observation of verticaland spatial distribution[

, Study of atmospheric dynamics[ Determination of wind!_elds as revealed by observed movements of spatiallyvarying clouds and hazes[ Stratospheric temperatures\maps and pro_les[

Speci_cal objectives for VIMS!V can be identi_edamong those outlined above as follows ]

, CH3 abundance[ For methane\ the use of di}erentabsorption bands will allow us to probe at di}erentlevels into the atmosphere of Titan\ thus allowing us todetermine the vertical distribution of this species[ In therange between 9[5 and 9[8 mm\ methane produces a seriesof absorption bands "Fig[ 2#\ the most prominent ofwhich "around 9[78 mm# is sensitive to the amount ofmethane in the top layer of the atmosphere[ This leadsto the determination of the methane column density\assuming an appropriate model atmosphere[

, Aerosol content and optical properties[ The study ofthe variation of the geometric albedo with wavelengthrepresents a main source of information on the aerosols[Observations have shown that aerosols a}ect the spectraparticularly in the range between 9[2 and 9[5 mm "seeRages and Pollack\ 0879#[ This poses several constraintson their size distribution[ Moreover\ the study of thevisible spectra obtained at di}erent phase angles willprovide insight in the scattering properties of the aero!sols[ Finally\ high spatial resolution limb scans can be


Fig[ 2[ Geometric albedo as a function of wavelength for Titannormalized to the visual geometric albedo of 9[10 at 9[44 mm\after Nelson and Hapke "0867# Methane bands are evident inthe region 9[4Ð9[8 microns

used to derive the vertical distribution of the aerosols"McKay and Toon\ 0880 ^ Samuelson\ 0880#[

, The surface of Titan\ notwithstanding its thick atmo!sphere\ will be mapped by VIMS!V in selected spectralwindows\ such as the 437 nm and the 898 nm bands\thus allowing albedo maps to be produced "Smith et al[\0885 ^ Coustenis et al[\ 0884 ^ Gri.th\ 0882#[

Saturn rings science with VIMS!V

The Saturnian rings represent a secondary target for theVIMS!V instrument[ When observing the rings we getonly indirect information on the single particles\ as themeasurements are relative to the collective behaviour ofthe particles in the rings ^ nonetheless re~ectance obser!vations by the visible channel will o}er the opportunityto study the size\ shape\ composition and scatteringproperties of the ring particles[ The particle phase functiondepends on the particle size\ refractive index\ surfacemicrostructure "Hapke\ 0875# ^ thus\ the inversion of thering phase curve\ measured over a large range of phaseangles\ will provide information on those quantities[ Thering albedo shows a high degree of dishomogeneitydepending on location\ e[g[ the C ring is considerablydarker than the A and B rings[ VIMS!V will contributeto discerning how much of these di}erences are due tocomposition and:or to scattering properties of the singleparticles[

Instrument con_guration

VIMS!V is composed of two modules\ the optical headand the electronic assemblies\ housed in separate boxes"F[ Reininger et al[\ 0883#[ VIMS!V optical head\ as shown

in Fig[ 3\ consists of two units ] a scanning telescope anda grating spectrometer\ ideally joined at the telescope focalplane where the spectrometer entrance slit is located[ Thetelescope mirrors are mounted on an optical bench whichalso holds the spectrometer unit[ In fact\ the optical benchis the reference plane for the whole instrument[

The telescope is an o}!axis Shafer design consisting oftwo pairs of concentric spherical mirrors\ which providesa real image on a ~at distortion!free focal plane[ Thesolution\ with spherical mirrors\ guarantees ease of manu!facture\ alignment and test[ Additionally\ it has been poss!ible to use rectangular cut!outs of the spherical mirrors\to match the slit geometry "a 5 mm ×13 mm rectangularshape#\ with a considerable saving in terms of volume andmass[

The telescope primary mirror is mounted on a scan unitwhich accomplishes two speci_c tasks ] a# pointing and b#scanning[ The scanning capability allows us to achieve the499 mrad IFOV of the nominal mode of operation bya two step motion of the primary mirror during eachintegration time ^ moreover\ bidimensional images areproduced by scanning the object target in the down trackdirection "pushbroom technique#[ The pointing capabilityis needed to image selected target regions within a rangeof about 0[1> of the optical axis\ and to observe the Sun\through the solar port\ during in!~ight radiometric cali!bration[

The in!~ight calibration unit is placed at the entranceof the telescope[ The unit consists of ] a# two LEDs for atwo!points calibration of the spectral dispersion and b# asolar port for direct solar imaging and hence\ radiometriccalibration[ To image the Sun\ as the remote sensing palletis body!mounted to the spacecraft\ the spacecraft itselfhas to be reoriented ^ the instrument scan mirror needs tobe rotated to an optical angle of 3[7> from boresight[ Inthese conditions\ light from the Sun is allowed to passthrough a cut!out in the instrument ba/e and thenthrough a prism which attenuates the solar radiation andredirects it towards the telescope primary mirror[

The optical layout of the spectrometer consists of anO}ner relay con_guration\ the secondary mirror of therelay being the concave di}raction grating[ The adoptedsolution does not require additional optical components"collimators and:or refractive elements# to achieve highresolution performances[ The spot diagrams show that allthe rays fall inside a 13 mm dimension "the width of asingle CCD pixel# over the full spectral and spatial ranges[

The grating

The grating represents the core of the spectrometer ^ itis a holographically recorded convex di}raction gratingmanufactured by Carl Zeiss[ It is made of NG4 black glassto optimize the groove fabrication during the holographicrecording and the successive ion beam etching processes[

The groove pro_le is rectangular and their spacing con!stant[ A variable groove depth has been used in adjacentsections of the grating to improve its e.ciency spectrumand to optimize the e.ciency to the mission needs[ Inparticular\ at the two extremes of VIMS!V spectral rangethe grating e.ciency has to be improved to o}set the


Fig[ 3[ Scheme of the lay!out of VIMS!V "After Reininger et al[\ 0883#

decrease of the solar irradiance and of the CCD QuantumE.ciency[ We thus obtain a ~atter instrument responsewhich limits the danger of saturation and improves theSNR of weak signals[ All this has been achieved by etchingfour concentric regions on the grating with alternategroove depths of 299 nm\ covering 56[4) of the gratingarea\ and 339 nm\ covering the remaining 21[4) of thegrating area[ This pattern has been optimized by Zeissto minimize interferences and stray light[ The resultinge.ciency is greater than 14) in the ranges 249Ð349 nmand 749Ð0949 nm\ while it drops to about 09) in theregion 499Ð699 nm[

VIMS!V calibration

The VIMS!V instrument has been calibrated in two steps ]a# at the O.cine Galileo premises\ prior to the integrationwith the infrared channel ^ b# at JPL after the integrationon the remote sensing pallet[ The activity carried out atJPL was mainly devoted to geometric measurements\ e[g[the coalignment of the two channels\ and to the measure!ment of the relative radiometric response[ Furthermore\the instrument spatial response\ i[e[ measurement of theimage quality through the instrument Modulation Trans!fer Function and Point Spread Function\ were evaluatedas part of the Full Functional Tests performed at O.cineGalileo prior to the calibration activity[

In what follows we will deal only with the calibrationsperformed at O.cine Galileo\ which were the radiometriccalibration\ the spectral calibration and the calibration ofthe solar port[

The absolute radiometric calibration was aimed atdetermining the radiometric calibration coe.cients which

convert detector output\ in digital numbers\ to input radi!ance over the full spectral range and the full FOV[ Theresults can be represented as a Unit Response curve which\for each CCD row\ the conversion factor is given as afunction of the wavelength[

The spectral calibration was aimed at determining thespectral width and central wavelength of each spectralelement\ the spectral out!of!band response of the instru!ment\ and the dark current versus the integration time[

The purpose of the solar port calibration was twofold ]a# to measure the angle between the optical axis of theinstrument and the solar port optical axis\ b# to determinethe conversion factor between the instrument responsewhen illuminated at boresight and at the solar port angle[

Calibration setup

Figure 4 shows a scheme of the calibration set!up[ VIMS!V has been placed inside a vacuum chamber\ equippedwith a thermally stabilized radiator connected to the CCDand capable of keeping the CCD at a temperature ofbetween −39 and −19>C under a residual pressure ofbetter than 09−3 mbar[ The chamber has a window "TVCwindow# with transparency better than 9[89 throughoutthe entire spectral range[ VIMS!V has been positioned ontwo computer controlled rotating tables for _ne pos!itioning around azimuthal and elevation angles[

Two lamps have been used to cover the full spectralrange ] a Xenon lamp for the range 9[2Ð9[3 mm and aTungsten lamp between 9[3 mm and 0[924 mm[ The lamp\with its housing\ which includes a condenser and a di}us!ing screen to improve light uniformity\ has been placed atthe input slit of a Jobin!Yvon HR539 monochromator


Fig[ 4[ Scheme of the calibration set up used for calibration measurements carried out at the O.cineGalileo

capable of a resolution "band width at half height# ofbetter than 9[94 nm over the VIMS!V spectral range[ Themonochromator output has then been used to illuminatea slit "or pinhole or test targets according to the needs#placed at the focus of an o}!axis collimator[ The col!limated beam has then been fed to the instrument placedinside the vacuum chamber[ Unfortunately\ the col!limated beam has an unknown spectral irradiance ^ thuswe had to devise a method to measure it[ This was achievedby means of a beamsplitter\ of known optical properties\placed on the optical path at 34> in front of the chamberwindow[ The re~ected portion of the beam was collectedby a calibrated photodiode to monitor the irradiance out!put of the light source setup[ An additional calibratedphotodiode was placed every 49 nm "or 49 mono!chromator steps# directly in front of the collimator to havea direct calibration at the collimator aperture[

It must also be mentioned that\ with the available col!limator\ only 0:5th of the full VIMS!V FOV could beinstantaneously illuminated\ thus a time consuming pro!cedure has been implemented to repeat a full spectralsweep "9[2Ð0[94 mm in 0 nm steps# to cover the _eld ofview of the instrument[

Calibration results

We will report here results obtained from the radiometriccalibration and from spectral calibration\ relative to spec!tral resolution and central wavelength[ Data analysisregarding the out of band response of the instrument andsolar port calibration have not yet been completed andwill not be reported on here[

An example of a typical CCD acquisition is shown inFig[ 5\ after removal of dark current and backgroundsignals[ As illustrated in the previous section\ the _gureshows that not all of the FOV is illuminated ^ furthermore\the illumination along the slit is not uniform[ The totalCCD coverage is shown in black in Fig[ 6[ CCD coveragewas not completed due to an error in the positioning ofthe rotating table which was detected only at the end of

the calibration campaign[ Thus\ not all of the 85 NominalPixels were individually calibrated[

Radiometric calibration

The radiometric calibration of VIMS!V has been carriedout as follows ] the Unit Response of the instrument isde_ned as the output in digital numbers when the instru!ment entrance pupil is fed with a light beam of 0W:cm1\and this beam is collected entirely into a single spectel andinto a unit solid angle\ for an integration time of 0 s[This quantity was directly measured ^ its dependence onwavelength "or spectel è# is shown in Fig[ 7[ Throughthe relation shown in the _gure it is possible to convertintensity as measured in digital numbers to radiance\ fora given wavelength[

Spectral calibration

Central wavelengths can be determined for each calibratedspectel as follows ] Measured intensity values "in digitalnumbers# are plotted vs sampling step ^ typical plots areshown in Fig[ 8"a# for pixel 0\ 1 and 2 "Xenon lamp#\ whilein 8"b# are shown typical plots for 2 central pixels "pixels18\ 29\ 20# in the spectral range analysed using the Tung!sten lamp[ Both _gures refer to measurements at bore!sight[

The spectral bands corresponding to the Full WidthHalf Maximum for each plotted curve are taken to rep!resent the _rst "l on# and the last wavelength "l o}# ofthe spectel ^ the central wavelength is then derived byaveraging these two values[ For the examples shown inFigs 8"a# and "b# these values are reported in Table 2[

Central wavelengths of pixels\ where individual cali!bration measurements are missing\ have been assumed tobe the average of the values obtained for the calibratedpixels lying in the same spectral column[ This is guaran!teed by the fact that during full performance tests pre!


Fig[ 5[ A typical CCD acquisition\ after removal of dark current and background signals


Fig[ 6[ CCD calibration coverage\ shown in black\ along the spatial "arrel# and spectral "spectel#directions\ lighter regions were not covered

Fig[ 7[ Spectral dependence of VOMS!V Unit Response[ The output intensity value is in digitalnumbers for 0 s integration time and for an input beam of 0 W:cm1 collected entirely into a singlespectel and into a unit solid angle


Fig[ 8[ "a# Spectral calibration of pixels 0Ð2 "Xenon lamp#[ "b# Spectral calibration of pixels 18Ð20"Tungsten lamp#


Fig[ 09[ Spatial dependence of dark current mean values ^ for each spatial pixels dark current valueshave been averaged over the spectels

Fig[ 00[ Mean values of the background signal


Table 2[ Spectral band widths for the selected spectels shown in Fig[ 8

Spectel l on2Dl on "nm# l o}2Dl o} "nm# l cen2Dl cen "nm#

0 291[2629[140 200[98429[091 295[6682 9[0291 200[98429[091 207[98729[026 203[53729[0372 207[98729[026 224[35329[956 210[57329[973

18 442[17529[003 448[66229[937 445[03929[91929 448[66229[937 456[11829[959 452[43329[91520 456[11829[959 464[93729[096 460[26729[948

viously carried out at O.cine Galileo the alignmentbetween slit direction and grating grooves has been foundto lie within 0 CCD pixel full resolution pixel "i[e[ 0:4 of0 nominal pixel#[

It should be noted that the shape of the curve is notexpected to be a Gaussian\ but rather a step function[ Thisis due to the fact that\ while the spatial resolution of themonochromatic beam is of 0 nm\ the spectral width ofeach spectel is 4×0[35 nm�6[22 nm[ Thus\ inside eachspectel _veÐsix wavelengths are well sampled "~atresponse#[

As shown in Fig[ 8"a#\ results from the Xenon lamp areless reliable than those obtained in the spectral regionwhere the Tungsten lamp was used for the investigation[This is due to a combination of factors[

, The lower intensity of the signal in this region[ As shownin Figs 8"a# and "b#\ the signal in the region of the shorterwavelengths is almost a factor of 09 lower[

, In this region\ the output from the Xenon lamp is a linespectrum\ which reduces the ~atness of the response[

, Intrinsic lower stability of the lamp[

Dark current

For each measurement\ a column of the CCD was alwaysdevoted to the acquisition of the dark current[ In Fig[ 09is shown a typical plot\ where dark current mean values\averaged for each matrix row "spectel#\ are plotted versusthe spatial pixel[ The slight upward slope evident in theplot is a typical feature of CCDs\ and it depends on theacquisition modality[ In fact\ the CCD readout is per!formed by rows ^ a _nite time will thus separate the _rstfrom the last acquisition\ yielding an increase in darkcurrent read out values due to the charge generated duringthe delay of the reading time\ in agreement with thatshown in our plot[ It should be noted however\ that inour case\ this e}ect is only a ~uctuation around the aver!age values[

Finally\ Fig[ 00 shows mean values of background noisemeasurements carried out during calibration[ Backgroundsignal measurements have been acquired each 14 nm forthe Xenon lamp and each 49 nm for the Tungsten lamp[It can be noted that mean values of background signalare comparable to dark current mean values almost every!where\ with the notable exception of values at 214 and349 nm\ which are signi_cantly higher[ This again impliesthat measurements at lower wavelengths "³349# are less

reliable\ due also to the lower intensity value of the signalin this spectral region[

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Imaging spectroscopy of Saturn and its satellites : vims-v onboard Cassini

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