Mem. S.A.It. Vol. 78, 697c© SAIt 2007 Memorie della

The Field Camera Unit project for the WSO-UVspace telescope

S. Scuderi1, I. Pagano1, M. Fiorini2, L. Gambicorti3, A. Gherardi3, F. Gianotti4,

D. Magrin5, M. Miccolis6, M. Munari1, E. Pace3, C. Pontoni1, M. Trifoglio4,

M. Uslenghi2, and B. Shustov7

1 Istituto Nazionale di Astrofisica – Osservatorio Astrofisico di Catania, Via S. Sofia 85,I-95123 Catania, Italy; e-mail: scuderi,ipa,cpo,mmu@oact.inaf.it

2 Istituto Nazionale di Astrofisica – Istituto di Astrofisica spaziale e Fisica Cosmica, ViaBassini, 15 I-20133 Milano, Italy

3 Universita di Firenze – Dipartimento di Astronomia e Scienza dello Spazio, Largo EnricoFermi, 2 I-50125 Firenze, Italy

4 Istituto Nazionale di Astrofisica – Istituto di Astrofisica spaziale e Fisica Cosmica, viaGobetti 101 I-40129 Bologna, Italy

5 Istituto Nazionale di Astrofisica – Osservatorio Astronomico di Padova, vicolodell’Osservatorio 5 35122 Padova Italy

6 Thales Alenia Space Italia SS. Padana Superiore 290, I-20090, Vimodrone (Milano), Italy7 Institute of Astronomy Russian Academy of Sciences 48 Pyatnitskaya st., 119017,

Moscow, Russia

Abstract. The Field Camera Unit (FCU) is one of the focal plane instruments aboard theWSO-UV telescope, a 1.7 m UV optimized instrument that will investigate numerous as-trophysical phenomena from planetary science to cosmology. The FCU will perform deepUV and diffraction limited optical imaging in both wide and narrow band filters using threechannels (FUV, NUV and UVO) optimized in different wavelength ranges and will havealso spectropolarimetric capabilities. The total wavelength range covered by the instrumentwill go from 115 nm to 700 nm. The FCU instrument will be developed and realized by theitalian scientific and industrial community. This paper will describe the scientific capabili-ties of the camera, its architecture and the expected performances.

1. Introduction

The World Space Observatory-UV is a in-ternational collaboration led by Russia tobuild a space telescope dedicated mainly toUV astrophysics. Table 1 summarize WSO-UV spacecraft characterisitcs while Figure1 shows the satellite in flight configuration.

Send offprint requests to: S. Scuderi

Telescope, launcher (Zenith 2SB) and platform(Navigator) will be developed in Russia. Thespacecraft will be in a geosynchronous orbit ata height of 35800 km and an inclination of 51.8degrees. The mission life time will be longerthan 5 years. The telescope will be a Ritchey-Chretien with a diameter of 1.7 m with a fo-cal ratio of F/10 and a corrected field of viewof 0.5 degrees. The primary wavelength range

698 S. Scuderi: FCU for WSO-UV

Table 1. WSO-UV spacecraft characteristics

Spacecraft mass with 2900 kgpropellantPayload mass 1600 kgInstruments compartment 750 Wpower consumptionData transmission rate, 2 Mb/s(S-Band)Service telemetry data up to 32kb/stransmission ratePlatform star trackers 30”pointing accuracyStabilization and 0.02”pointing accuracyAngular rate when 2x10−5 ◦/sstabilizedSlew rate 0.1 ◦/sMaximum duration of 30 hoursan observation session

is 100-350 nm with an extension into the vis-ible range. The telescope will host two spec-trographs and one imager. There will be a highresolution echelle spectrograph (HIRDES) de-veloped by Germany with a resolution of about50000 and two channels to cover more effi-ciently the entire wavelength range. The sec-ond spectrograph, developed by China, will bea long slit low resolution (R ∼ 1000-2500)spectrograph (LSS). This instrument will havealso two channels. The imager, Field CameraUnit, will be devoloped in Italy and will allowto obtain diffraction limited and deep UV andoptical images. More detailed informationonon the WSO-UV project and its focal plane in-struments can be found in Sachkov et al. (2007)and Pagano et al, (2007a).

2. The Field Camera Unit project

The Field Camera Unit (FCU) will provide theWSO-UV satellite with imaging capabilities.The FCU instrument will be developed and re-alized by the italian scientific and industrialcommunity. The Italian Space Agency (ASI)has financed the phase A/B1 study of the FCU

Fig. 1. WSO-UV spacecraft in flight configuration.The solar panels and the on board radio complex aredeployed. The sun shade and the telescope cover arein the nominal position for observations.

project. The main objectives of this study willbe the following:

– Definition of scientific objectives– Definition of performances requirements– Opto-mechanical design– Definition of the detectors architecture.– Definition of electronics architecture– Preliminary definition of operational

modes of the cameras– Preliminary definition of calibration proce-

dures– Definition of interfaces with the spacecraft– Definition of cameras mass, power and

thermal budget.

A technical team and a science team have beenset up and will deliver the results of the studyto ASI by the end of 2007.

The scientific interest of the italian as-trophysical community toward the project isstrong and several scientific projects are be-ing developed to be carried out with the FCUinstrument ranging from planetary science toextra-galactic astronomy (Pagano et al. 2007b).

Being a space telescope imaging camerathe highest priorities of the FCU instrumentwill be to guarantee high spatial resolution andhigh UV sensitivity while trying to maximizethe wavelength coverage and the size of thefield of view. To meet this challenge we aredesigning an instrument that will have threedifferent channels each of them specialized in

S. Scuderi: FCU for WSO-UV 699

Table 2. FCU channels main characteristics

ChannelFar-UV Near-UV UV-Optical

Spectral Range 115-190 nm 150-280 nm 200-700 nmField of View 6.6’x6.6’ 1’x1’ 4.7’x4.7’Scale 0.2”/pixel 0.03”/pixel 0.07”/pixelPixel Size 20 µm 20 µm 15 µmArray Size 2kx2k 2kx2k 4kx4kDetectors MCP (CsI) MCP (Cs2Te) CCD (UV-optimized)Filters, Polarizers, Disperser Up to 10 Up to 23 Up to 24

a specific wavelength range. Table 2 summa-rizes the characteristics of the three channels.The Far-UV channel will cover the 115-190 nmrange. To reduce losses in the throughput, thischannel will not have any optics but the mir-ror to feed it. The scale of the telescope will beunchanged yielding a large field of view at ex-penses of spatial resolution. So for this channelwe favoured sensistivity over revolution. TheFUV channel filters wheels will accommodatenarrow, wide band filters and a prism whichwill allow low resolution (R ∼ 100-300) slitlessspectroscopy. The NUV channel will cover the150-280 nm range overlapping the FUV rangeon the shorter wavelengths side and the UVOrange on the longer one. To exploit the diffrac-tion limited optical quality of the telescope inthis wavelength range, the NUV channel hasthe highest spatial resolution. Its filters wheelswill accommodate wide and narrow band fil-ters but also prism and polarizers. The possibil-ity to have a spectropolarimetric mode is beinginvestigated. Finally the UVO channel will ex-tend to visual wavelengths to exploit the widespectral sensitivity of its CCD detector. Thepixel scale of this channel is a compromise be-tween the need of a large field of view and of ahigh spatial resolution. Filters, grisms and po-larizers will allow to have narrow and broadband imaging, low resolution (R ∼ 100-300)slitless spectroscopy and imaging polarimetry.

3. Opto-Mechanical design

Figure 2 shows the layout of the focal surfaceof the telescope with superimposed the fields

of view of the three channels of the FCU. Thisarea has a diameter of 150 mm and as it canbe deduced from the figure is rather crowded.Figure 3 shows the space where the FCU willbe located in the instrumental compartment,that is between the primary mirror unit and theoptical bench that supports the spectrographs.This space is a cylinder of about 1 meter in di-ameter and about 20 cm in height. Due to thiskind of geometry it is necessary to use a mir-ror to fold the optical beam coming from thetelescope in a direction parallel to the opticalbench where the three channels of the FCU willbe deployed.

Fig. 2. Sketch of the WSO-UV telescope focal sur-face with the positions of the slits of the spectro-graphs and of the FGS. The FOV of the FCU chan-nel as projected onto the sky are also shown.

700 S. Scuderi: FCU for WSO-UV

Fig. 3. A 3D model of the scientific instrumenta-tion compartment showing the area where the FCUinstrument will be located, that is on the opticalbench between the primary mirror unit and the spec-trographs.

After a preliminary analysis we decided todevelop an all reflective design to maximizethroughput and to stay as much as possibleclose to, if not on, the optical axis to simplifythe design. We are considering three optical de-signs which differ for the central pick up mirrorlayout.

1. Rotating pick up mirror: in this case thefolding mirror will be flat and using amechanism will rotate feeding one of thethree channels at a time.

2. Pyramid pick mirror: this layout foreseeto use three fixed mirrors (possibly hav-ing optical power and a specialized coat-ing) which will be oriented to feed all threechannels at the same time.

3. Fixed pick up mirror coupled withdichroic(s): the mirror will fold the opticalbeam, a first dichroics will extract fromit the FUV beam and a second one willseparate the NUV beam from the UVObeam.

The last layout has interesting advantages be-cause will not use any mechanism and wouldallow to image the same field simultaneouslywith the three channels. Unfortunately it hasbeen discarded because, in the focal surface ofthe telescope, there is not enough room to ac-commodate the resulting opto-mechanical lay-

out. Furthermore it requires R&D activities, re-lated to the design and the manufacture of thedichroics, which are not compatible with theschedule of the project. The surviving designshave both advantages and disadvantages. Themost evident drawback of the first layout isthe possibility of a mechanism failure whichwill reduce the FCU capabilities. For the sec-ond design the main problem is the mechani-cal accommodation of the mirror in the centerof the focal surface. A trade-off process, takinginto account the problems related to the opto-mechanical design or to the FCU or telescopeoperations, is well under way.

Fig. 4. Spot diagrams for the UVO channel and therotating mirror optical layout. The three circles arethe Airy’s disks at 200, 500, and 700 nm (from thesmaller to the larger). The spot diagrams are shownfor different positions on the field of view.

Figure 4 shows the spot diagrams for one ofthe optical design using a rotating mirror. Thespot diagrams refers to the UVO channel andhave been calculated for different positions onthe field of view. The Airy’s disk at three dif-ferent wavelengths (200, 500, 700 nm) are alsoplotted showing that at 500 nm and 700 nm thedesign is always diffraction limited while atshorter wavelengths this is not true on borderof the field.

S. Scuderi: FCU for WSO-UV 701

Fig. 5. Opto-mechanical for the three channels ofFCU in case of a rotating mirror layout. NUV chan-nel is the one on the bottom part of the drawing, theUVO channel is the one that expands on the wholebench and the FUV channel is the one closest to thecenter of the optical bench.

Figure 5 shows a top view of one of theopto-mechanical designs of the FCU for a ro-tating mirror layout. The three channels can beeasily distinguished (see the caption of Figure5 for explanation. Apart the positions of themirrors, filter wheels and detectors are alsoshown. The optical bench, in this case, has adiameter of 1.3 meters.

4. Detectors

The FCU will use two different kind of detec-tors. For the FUV and NUV channels, MCPbased detectors in sealed configuration will beused. The sealed configuration will eliminatethe need for pumping and reduce contamina-tion problems. Each of the MCP detectors willbe specialized for the wavelength range whereit will be used. So the FUV detector will havea CsI photocathode directly deposited on theMCP while in the case of the NUV detectorCs2Te will be used as photocathode depositedon the detector’s window. In both cases theMCP will be coupled to a fluorescent screenand then to a fiber optic tapered bundle that,in turn, will be coupled to a CCD (see Fig.6). The MCPs will have a diameter of 40 mmand will be resampled in a 2kx2k format. TheUVO channel will instead use a CCD detectorwith 15 µm pixel size and 4kx4k format. TheCCD will be back side illuminated. Its cool-

ing system will either be an active one, usinga Thermo Electric Cooler or a passive one, inwhich case the CCD will be connected to anexternal radiator.

5. Operating modes

We are planning to implement the followingoperating modes for the FCU instrument:

– optical mode - All the three channels willallow to have narrow, medium and wideband imaging and also low resolution slit-less spectroscopy. Polarimetric modes areforeseen for the NUV and UVO channelsand a spectropolarimetric mode in understudy for the NUV channel.

– parallel mode - In case the pyramid pickup mirror will be selected solution for theopto-mechanical layout, two channels ofthe FCU could be used at the same timelooking at different parts of the sky. Theimplementation of this mode will be sub-ject to thermal and power bugdet analysis.Furthermore one channel of the camera canbe used when one of the spectrographs isworking as primary instrument. In this casethe only restriction could come from limi-tation of data transfer rate.

– high temporal resolution mode - TheFUV, NUV and UVO channels can all beoperated in a mode where subarrays of thedetector can be read-out, thereby reduc-ing the readout time and permitting a more

Fig. 6. Skematic view of an intensified CCD.

702 S. Scuderi: FCU for WSO-UV

Fig. 7. System throughputs of FCU instrumentchannels as functions of wavelength compared tothat of HST imaging instruments.

Fig. 8. Discovery efficiencies of of FCU vs. wave-length compared to that of the HST imaging instru-ments. Discovery efficiency is defined as the sys-tem throughput multiplied by the area of the fieldof view. Note that the y-axis is logarithmic.

rapid observing sequence. The FUV andNUV detectors can also operate in a timetag mode whereby the image location andthe time of arrival of the detected photonsare recorded permitting a few ms temporalresolution at the expense of increased datavolume.

– calibration mode - The FCU optical de-sign will include a calibration unit with aset of continuum source lamps covering thefull wavelength ranges of the three detec-tors. These lamps will be used periodicallyto perform flat field calibrations.

6. Performances

Figure 7 plots the expected system through-puts of the three FCU channels comparedto the ones of cameras that have flownor will flew on board the Hubble SpaceTelescope i.e. WFPC2, ACS/WFC, ACS/HRCand WFC3/UVIS (Bond et al 2006). The plot-ted quantities are the total throughputs, includ-ing the telescope, all of the optical elementsof the instruments themselves, the sensitivi-ties of the detectors and filter transmissions.Throughputs for FCU are estimated based onthe best information currently available andare subject to change. The throughput of theUVO channel is much better than that ofWFPC2 which, by the way, is the oldest ofthe HST cameras, and is comparable to that ofACS/HRC and WFC3/UVIS being better in theUV range. ACS/WFC, being optimized in thevisual range, has a much better throughput thanthe UVO channel.

Another quantity that is useful when com-paring different instruments, especially in thecontext of wide-angle surveys, is the discoveryefficiency, defined as system throughput timesthe area of the FOV as projected onto the sky.In Figure 8 we plot the discovery efficienciesof FCU vs. wavelength again compared to thatof the HST imaging instruments (Bond et al2006). Due to its large field of view, the UVOchannel of the FCU has a discovery efficiencyequal or greater than that of ACS/WFC. Theperformances of the FUV channel are even bet-ter when compared to HST because, in thiscase (by the way ACS/SBC is not present in theplot), no camera working in this wavelengthrange has a large field of view.

Acknowledgements. The present work, a PhaseA/B1 study of the Italian participation to WSO-UV,is funded by Italian Space Agency (ASI), under con-tract ASI/INAF No. I/085/06/0.

References

Sachkov, M., et al., 2007, Astrophysicsand Space Science Proceedings series, M.Chavez, E. Bertone, D. Rosa-Gonzalez &

S. Scuderi: FCU for WSO-UV 703

L. H. Rodriguez-Merino(eds.), Springer, inpress

Pagano, I., et al., 2007, Astrophysics and SpaceScience Proceedings series, M. Chavez,E. Bertone, D. Rosa-Gonzalez & L. H.Rodriguez-Merino(eds.), Springer, in press

Pagano I., Shustov B., Kappelmann, N., deMartino, D., Piotto, G., Scuderi, S., and

Turatto M., WSO-UV: The World SpaceObservatory Project for the Ultraviolet,2007, Proceedings Series of the ItalianPhysical Society, F. Giovannelli & G.Mannocchi (eds.), in press (invited)

Bond, H. E., et al. 2006, ”Wide Field Camera3 Instrument Mini-Handbook, Version 3.0”(Baltimore: STScI)