Overview

The considerable scientific success of the International Ultraviolet Explorer satellite (IUE) observatory and successor instruments such as the Goddard High Resolution Spectrograph (GHRS) and Space Telescope Imaging Spectrograph (STIS) spectrographs on-board Hubble Space Telescope (HST) amply demonstrate the importance of the far ultraviolet wavelength range, from ~100 to 300nm, to modern astronomy. Of particular importance has been access to high resolution R~40,000-100,000 echelle spectra providing an ability to study the dynamics of hot plasma and separate multiple stellar and interstellar absorption components.

However, there are a few emerging problems with this provision. First, since HST has UV, optical and infra-red capabilities, the available observing time is divided across these bands. In addition, the total observing time is oversubscribed by a factor ~10. Hence demand for access to UV observations far outstrips what is available from HST. Secondly, the STIS instrument is already operating on the redundant electronic systems and the instrument will not be serviced. Therefore, it is necessary to plan for a replacement of this high resolution UV capability.

The World Space Observatory - Ultraviolet (WSO-UV) is the proposed solution to the problem of future access to high resolution far-UV spectroscopy. The planned instrument sensitivity will exceed that of HST/STIS by a factor 5-10, but all the observing time will be available for UV astronomy. The present mission design consists of a 1.7-meter telescope. The focal plane instruments consist of UV spectrometers covering the spectral band from Lyman alpha to the atmospheric cut-off with R~55,000 and offering long-slit capability over the same band with R~1,000.

In addition, it is expected a number of UV and optical imagers view adjacent fields to that sampled by the spectrometers. The imaging performance compares well with that of Advanced Camera for Surveys (ACS) of HST while the spectral capabilities are comparable to the HST/STIS echelle modes. However, with a smaller number of instruments in the focal plane, compared to HST, the required number of optical elements in each subsystem is reduced. Hence, the WSO delivers considerably enhanced effective area. This throughput is similar to that offered by the Cosmic Origins Spectrograph (COS) at the longer wavelengths, but COS has an inferior spectral resolution (R~24,000 max).

WSO-UV is ideally placed to provide follow-up studies of the large number of UV sources expected from the GALEX sky survey. Thus the main operation modes of observatory will be the following,

  • Spectroscopic observations in UV area with high spectral resolution (resolution capability up to 60 000);
  • Spectroscopy with long (high) slot allows getting spectrums with spectral resolution of 2500 and objects images simultaneously;
  • Imagery of space objects with high resolution (up to 0.1 angle seconds in UV) and (in addition) visible bands of spectrum.

 

Key facts

 A telescope 1.7m diameter-class (the T170M) and a unified Service Module NAVIGATOR (intended for fulfilment of projects Electro, Spectrum-R, 
Spectrum-UV, that is WSO-UV, Spectrum-RG).


The current baseline regarding orbit and operations is:


  • Geosynchronous orbit. 

  • SOC operations shared between two sites (Madrid and Moscow).

Mission

The WSO-UV telescope will be launched by a Proton medium-class rocket. All launch facilities will be provided by Russia. The observatory is expected to operate during 5+5 years.

The orbit of WSO-UV has been optimized for:

  • minimizing the geocoronal contribution to the UV background (especially Lyman-alpha and OI emission);
  • allowing efficient monitoring on time scales from hours to days of astrophysical sources (transiting planets, active stars, quasars...);
  • optimizing the coverage from the tracking stations in Spain and Russia.

The foreseen WSO-UV orbit is geosynchronous, with an inclination of 51.8 degrees and ascending node over the meridian of the Canary Islands (Spain) to guarantee visibility for, at least, 20 hours/day. Additional criteria taken into account for the orbit selection are: avoidance time of the Earth radiation belts, continuous visibility zones (when two or three ground stations are working), minimum stay in the Earth shadow, stability of the orbit and the characteristics of the available technical equipment of the space and ground segments for radio communication. The projection of the WSO-UV orbit on the Earth's surface is plotted below.

The telescope is planned to be launched by a Proton medium-class launcher equipped with a Fregat accelerating module. WSO-UV will be launched from the Russian facilities in Baikonur.

WSO-UV uses the NAVIGATOR platform that is designed as a unified unit for several missions. This platform has flown the Russian missions SPECTRUM-R and ELECTRO. The main characteristics of the platform are: