The World Space Observatory (WSO-UV) - Current status
Michela Uslenghi, Isabella Pagano, Cristian Pontoni, Salvatore Scuderi, Boris Shustov
aa r X i v : . [ a s t r o - ph ] J a n Chin. J. Astron. Astrophys. Vol.0 (200x) No.0, 000–000( ) Chinese Journal ofAstronomy andAstrophysics
The World Space Observatory (WSO-UV)Current status
Michela Uslenghi ⋆ , Isabella Pagano , Cristian Pontoni , Salvatore Scuderi and Boris Shustov INAF – IASF - Milano, Via E.Bassini 15, I-20133 Milano, Italy INAF – Catania Astrophysical Observatory, Via S.Sofia 85, I-95123 Catania, Italy INASAN, Moscow, Russia
Abstract
This paper reports on the current status of the World Space Observatory WSO-UV,a space mission for UV astronomy, planned for launch at the beginning of next decade. It isbased on a 1.7 m telescope, with focal plane instruments including high resolution spectro-graphs, long slit low resolution spectrographs and imaging cameras.
Key words: space vehicles: instruments — telescopes — instrumentation: spectrographs —instrumentation: high angular resolution — ultraviolet: general
UV spectroscopic and imaging capabilities are fundamental for astrophysics since thermal phenomena attemperatures T > , CO, OH, CS,CO +2 , CO ) are in the UV range. This results in UV providing the most sensitive tools to trace the dis-tribution of (baryonic) matter in the Universe, other than to diagnose the chemical composition, physicalproperties and kinematics of astronomical objects of all types.In recent years, there have been three major instruments working in the UV: Hubble Space telescope (HST),
Far Ultraviolet Spectroscopic Explorer (FUSE) and
Galaxy Evolution Explorer (GALEX). Thefirst two are observatory–like missions, whereas GALEX is dedicated to an all–sky survey and is currentlyproviding wide field and low resolution spectra of a large number of astronomical objects which will requiredetailed UV follow-ups. However, after the failure of the HST STIS spectrograph in 2004, no facilities toget medium to high resolution spectra in the classical UV domain (1000–3000 ˚A) have been available tothe community.Access to UV is becoming problematic. Even with the planned upgrade of HST with the
Cosmic OriginSpectrograph (COS), during the service mission SM4, HST is planned to work till 2013, thus posing theproblem of developing new facilities for UV astrophysics in the post-HST era, before the advent of futurelarge (8m class) UV telescopes, currently under discussion but not yet included in the plans of any spaceagency (and then to be scheduled unlikely before 2020–2025).The World Space Observatory Ultraviolet – WSO-UV – is a multi-national project grown out of theneeds of the astronomical community to have future access to the ultraviolet range. Planned to operate for5(+5) years starting from 2011, it will fill in the gap between HST and the future large UV telescopes,complementing the wavelength coverage of the IR
James Webb Space Telescope (JWST), which will beoperative in the same period.
M. Uslenghi et al.
1) Primary Mirror Unit (PMU)2) Mechanical Frame3) PMU basic frame4) Radiator with heating elements5) Tube6) Secondary Mirror Unit (SMU)7) External Baffle8) Telescope Light Protective Cover (LPC)9) Primary Mirror Baffle10) Secondary Mirror Baffle11) Knife edge rings12) Optical Bench (OB)13) Field Camera Unit (FCU)14) Spectrographs15) External Electronics Box16) Scientific Instruments Protective Cover
Fig. 1
General view of the T–170M telescope.
Table 1
Main characteristics of WSO-UV.
Spacecraft
Spacecraft mass with propellant 2900 kgPayload mass 1600 kgInstrumentation Compartment power consumption 750 WData transmission rate (S-band) 2 Mb/sService telemetry data transmission rate 32 kb/sPlatform star tracker pointing accuracy 30 arcsecStabilization and pointing accuracy 0.03 arcsecSpacecraft angular rate in stabilization mode 2x10-5 degree/secSpacecraft slew rate 0.1 degree/secMaximum duration of scientific observation in continuous mode 30 hr
Telescope
Optical System Ritchey-Chr´etienTelescope entrance pupil diameter 1.7 mEffective focal length 17 mParameter Value F/ratio 10FoV diameter 30 arcmin (148,48 mm)Scale 12,13 arcsec/mmWavelength range 100–310 nm (with extension to the visible)Primary wavelength 200 nm
The World Space Observatory-UV is an international collaboration led by Russia to build a space telescopeoptimized in the UV range and devoted to investigate numerous astrophysical phenomena from plane-tary science to cosmology (Barstow et al. 2003, Pagano et al. 2007). The satellite will be based on the”Navigator” platform, a service module used also for other Russian projects (e.g. Elektro and RadioAstron). ⋆ E-mail: [email protected] he World Space Observatory 3
Fig. 2
Instrumental Compartment, showing the location of the focal plane instruments.Table 1 summarizes WSO-UV spacecraft characteristics. Telescope, launcher (Zenith 2SB) and platform(Navigator) will be developed in Russia, whereas focal plane instruments will be provided by Germany,China and Italy, with contributions from UK, and Spain. Ground Segment is under design mainly in Spainand Russia, with possible contributions from other countries. The spacecraft will be put in a geosynchronousorbit at a height of 35 800 km and an inclination of 51.4 degrees.
The WSO-UV telescope T–170M is a new version of the T–170 telescope designed by LavochkinAssociation (Russia) for the Spectrum-UV mission. It is a Ritchey-Chr´etien with a 1.7m hyperbolic pri-mary mirror, focal ratio F/10 (platescale 12.13 arcsec/mm) and a corrected field of view of 0.5 degrees. Theoptical quality of the two mirrors is λ/ rms at 633 nm. The primary wavelength range is 100-350 nmwith extension into the visible range, but the optics are optimized at 200 nm.The telescope general view is given in Fig. 1: the main structural elements are the Primary MirrorUnit (PMU), the Secondary Mirror Unit (SMU) and the Instrumental Compartment (IC). There are threeattachment points of the telescope to the spacecraft service module in the bottom frames part. The OpticalBench (OB), carrying the scientific instruments and the Fine Guidance Sensors, is mounted on the PMUframe. The SMU is attached to the telescope with a spider. WSO-UV will have spectroscopic and imaging capabilities. The telescope will host two spectrographsand one imager. There will be a high resolution echelle spectrograph (
High Resolution Double EchelleSpectrograph – HIRDES), and a
Long Slit (low resolution)
Spectrograph (LSS). The imager, Field CameraUnit (FCU), will allow diffraction limited, deep UV and optical images. Fig. 2 shows the location of theinstruments in the Instrumental Compartment.
M. Uslenghi et al.
Fig. 3
Schematic overview of HIRDES (Kappelmann et al. 2006).
HIRDES (Kappelmann et al. 2006, see Fig. 3) design is based on the heritage of the ORFEUS (
Orbitingand Retrievable Far and Extreme Ultraviolet Spectrometer ) missions (Barnstedt et al. 1999). It comprisestwo echelle instruments, UVES (178-320nm) and VUVES (103-180nm), with high spectral resolution(R ∼ The Long Slit Spectrograph will provide low resolution (R ∼ ×
75” slit, with spatial he World Space Observatory 5
Fig. 4
Layout of the Field Camera Unit.resolution 0.5-1 arcsec. LSS study, ongoing in China, is currently in phase A, with a Phase B expected tobe completed by January 2008.
The Field Camera Unit (FCU) will include three channels (Scuderi et al. 2007):
FUV channel
It covers the far UV providing medium resolution images. To satisfy the high sensitivityrequirement below 200 nm, this channel is optimized in the range 115-190 nm, with 0.2 arcsec/pixelscale. Since it uses the focal ratio of the telescope, this design minimizes the numbers of optical ele-ments (only a pick-up mirror is required, to deviate the optical beam towards the detector), maximizingthe throughput. The FUV channel will be equipped with a k × k pixels, photon counting, MCPdetector (with CsI photocathode) and will have a FoV= . × . arcmin . This channel shall beequipped with a filter wheel hosting broad and narrow band filters, neutral filters, and a R ∼
100 lightdisperser, allowing also low resolution slitless spectroscopy
NUV channel
It operates in the range 150-280 nm, providing ”close to diffraction limit” images at 200nm, with a 0.03 arcsec/pixel scale. The NUV channel will be equipped by a k × k pixel MCPdetector (with CsTe photocathode) and will have a FoV= . × . arcmin . This channel shall beequipped with two filter wheels hosting broad and narrow band filters, neutral filters, polarizers and aR ∼
100 grism, providing slitless spectroscopic and polarimetric capabilities
UVO channel
Near ultraviolet-visual diffraction limit imager, operating in the interval 200-1000 nm, witha 0.07 arcsec/pixel scale, equipped with a k × k pixels CCD, providing a field of view of . × . arcmin . This channel shall be equipped with a set of broad band and narrow band filters, a ramp filter,polarizers and a R ∼
250 grism.FUV and NUV channels, using photon counting detectors, will allow also high time resolution obser-vations, down to few ms time scale. Fig. 5a shows a preliminary estimate of the system throughput of thethree FCU channels compared to the cameras which have flown or will flew on board of HST (WFPC2,ACS/WFC, ACS/HRC and WFC3/UVIS, Bond H.E. et al. 2006). As it can be seen, the performance of
M. Uslenghi et al.
Wavelength (nm) Wavelength (nm) T h r o u g h p u t Total System Throughput T h r o u g h p u t x a r e a ( a r c s ec ) Discovery Efficiency
ACS/WFCWFC3/UVISACS/HRCWFPC2WSO-UV/FCU WFPC2ACS/HRCWFC3/UVISWSO-UV/FCU ACS/WFC
Fig. 5
System throughput (a) and discovery efficiency (b) of the FCU cameras as function of thewavelength, compared with HST imagers.the FCU cameras compares well with the HST instrument ones. Another useful quantity when comparingdifferent instruments is the discovery efficiency, plotted in Fig. 5b, defined as the product of the systemthroughput and the area of the Field of View (FOV) as projected on the sky. Due to its large FOV, UVO hasa discovery efficiency equal or greater than ACS/WFC. In the case of the FUV, the performance are evenbetter when compared to HST because no camera working in this range has a large FOV.The Italian Space Agency founded a Phase A/B1 for the FCU. Phase A has been completed in July2007. Phase B1 will last four months.
Acknowledgements
The participation in the WSO-UV project in Italy is funded by Italian Space Agencyunder contract ASI/INAF No. I/085/06/0.
References
Barnstedt J. et al., 1999, A&ASS, v.134, p.561-567Barstow M. A., Binette L., Brosch N. et al., 2003, Proc. SPIE, Vol. 4854, pp. 364-374Bond H.E. et al., 2006, ”Wide Field Camera 3 Instrument Mini-Handbook, Version 3.0”, Baltimore STScIKappelmann N., Barnstedt J., Gringel W. et al., 2006, Proc. SPIE, Volume 6266, p.25Pagano I., Shustov B., Kappelmann, N., et al., 2007, Proceedings Series of the Italian Physical Society, F. Giovannelli& G. Mannocchi (eds.), Vol 93, p. 691Scuderi S., Pagano I., Fiorini M., Gambicorti L., Gherardi A., Gianotti F., Magrin D., Miccolis M., Munari M., Pace E.,Pontoni C., Trifoglio M., Uslenghi M., and Shustov B., 2007, Mem.S.A.It, in press
DISCUSSIONJOERN WILMS:
What is the time resolution of the UV detectors?
MICHELA USLENGHI:
Time resolution will be ∼
10 ms
JIM BEALL:
The geosynchronous orbit presents some challenges with respect to satellite electronics. Canyou comment on this? How did you pick the orbit?
MICHELA USLENGHI:
We are aware of the problem and industry is now working on the electronicsdesign on that base. The first choice for the orbit was an L2 one, the baseline (recently) changed to thegeosynchronous one due to cost issues.