In early 1988 the Israel Space Agency (ISA) solicited academic and commercial research and development groups in Israel to propose a scientific payload for a National Scientific Satellite (NSS) of the OFEQ-class. Among the many proposals submitted, that of Tel Aviv University, to orbit a cluster of small, wide-field telescopes to image astronomical objects in the ultraviolet (UV), was finally selected with the highest priority. This payload is referred to below as TAUVEX, the Tel Aviv University UV Experiment.
Observations in the UV region longward of Lyman alpha, up to the atmospheric transmission limit of ~ 3000A° , take advantage of the reduced sky background. This is because of a fortuitous combination of zodiacal light decreasing shortward of ~ 3000A° , and other backgrounds remaining low up to near the geocoronal Lyman alpha. In this spectral region it is therefore possible to observe faint astronomical sources with a high signal-to-noise ratio even with a modest telescope. This was first explained by O'Connell (1987 AJ. 94 876) and measured by various space experiments before and after this publication.The sources best studied with a small aperture telescope are QSOs and AGNs, that radiate significantly in the UV. Other sources of UV photons are hot stars of various types, the most interesting being white dwarfs and mixed-type binaries. Young, massive stars, that emit copious amounts of UV radiation and ionize the interstellar medium, are important in the context of star formation and evolution of galaxies; here the advantage of a wide-field imager is obvious. This has been demonstrated amply by the UIT instrument flown on the Space Shuttle. The obvious advantages of TAUVEX are reduced sky background, the longer observing time per target, and the long duration mission.
The option to orbit TAUVEX on a NSS was abandoned by ISA due to various constraints and an alternative platform was sought. In June 1991 it was proposed that TAUVEX will be launched and operated from the Spectrum Rontgen-Gamma (SRG) spacecraft, as part of the SODART (Soviet-Danish Rontgen Telescope) experiment. The SRG satellite will be launched in late 1997 by the Russian Space Agency into a high elliptical orbit and including TAUVEX requires a 0.5% increase in its mass and 2% in its power consumption.
SODART consists of two X-ray imaging telescopes, each with four focal-plane instruments, to perform observations in the 0.2-20 keV band. SODART is built by Denmark, Russia, Finland Germany and USA. TAUVEX will provide SODART with aspect reconstruction and will assist SRG in pointing and position keeping. In September 1991 the TAUVEX experiment was officially invited to join other instruments aboard the SRG spacecraft. ISA agreed in November 1991 to provide SRG with the TAUVEX instrument. The official confirmation from the Russian side was received in December 1991.
The TAUVEX imagers will operate on the SRG platform alongside numerous X-ray and Gamma-ray experiments. This will be the first scientific mission providing simultaneous UV-X-Gamma observations of celestial objects. The instruments on SRG include, apart from SODART, the JET-X instrument (UK, Russia and Italy: 0.2-10 keV imager with 40' FOV and 10-30" resolution), MART (Italy and Russia: Coded aperture 4-100 keV imager with 1° FOV and 6' resolution), EUVITA (Switzerland, Russia and Canada: 100-400A°, imagers with 1° FOV and 10" resolution, MOXE (USA: all sky X-ray burst detector in the 3-12 keV band), SPIN (Russia: all sky Gamma-ray burst detector in the 10keV-10MeV band, with 0°.5 optical localization), and DIOGENE (France and Russia: Gamma-ray burst detector).
TAUVEX will be bore-sighted with SODART, JET-X, MART and EUVITA, and will obtain simultaneous imaging photometry of objects in the UV with three independent telescopes. A combination of various filters will accomodate wide, intermediate and narrow spectral bands. These have been selected to take maximal scientific advantage of the stability of SRG, the image quality of the optics (90% of the photons in ~ 6"), and long staring times at each SRG pointing. During a single pointing it will be possible to change filters, thus more than three UV bands can be used on one observation.
The present design of TAUVEX includes three co-aligned 20 cm diameter telescopes in a linear array on the same mounting surface. Each telescope images a field of 0°.9 diameter onto photon-counting position-sensitive detectors with wedge-and-strip anodes. Such detectors are space-qualified and have flown in a number of Space Astronomy missions. The payload was designed and is assembled by El-Op Electro-Optics Industries, Ltd., of Rehovot, the top electro-optical manufacturer of Israel, under close supervision of Tel Aviv University astronomers.
The individual photon events are localized on the focal plane with 10 bit accuracy. An internal algorithm "tracks" a bright star (m(UV)<8) in the field of view and provides pointing change corrections every 2 sec or longer. These are used by TAUVEX to register all the incoming photon events in their proper image buffer memory location. The image buffer is a doubly-redundant three-frame array, with 2x3x2MByte capacity. At the end of an observation the image buffer data is transferred to the long-term storage. This consists of a doubly-redundant two-disk modules. Each disk module is a pressurized, dry-nitrogen filled container with two 84MByte 2.5 inch laptop hard disks. The disk modules have been qualified by El-Op long before the qualification of TAUVEX itself. Similar disks operate on the GRANAT space observatory.
The development of TAUVEX follows a number of stages, where the predicted behavior is verified by extensive tests. El-Op already produced a number of models of the experiment that were delivered to the Russian constructors of the spacecraft. The delivered models include a size mockup, a mass and center of gravity model for satellite vibration tests, a thermal simulation model (TM) and an engineering model (EM). The TM, in particular, is identical to the flight model except for its lack of electronics and working detectors. All construction details and surface finishes were included, the telescopes have actual aluminized mirrors, etc. The EM corresponds to the flight model (FM) in all its electrical behavior, mainly in interfacing with the carrier platform. The performance of the EM has been tested in a stand-alone mode and together with other SRG instruments in spring 1995. Additional tests, to include the entire SRG scientific complement, were conducted in spring and autumn 1996. The third complex test is scheduled for March 1997
The thermal model was tested at an ESA (European Space Agency) facility in Germany in late-January 1993 prior to its shippment to Russia. The test was a full space simulation, including Solar radiation, and the measured behavior verified the theoretical model developed at El-Op. The engineering model (EM) of TAUVEX, which has fully operational electronics, has been completed and fully tested at the El-Op facility. A combined interface test with Russian and Hungarian equipment intended to be installed on-board SRG took place successfully at Tel Aviv University in December 1993. After being updated at El-Op, the EM was shipped to the Russian Space Research Institute in Moscow in March 1994, for integration in a complete engineering model of SRG. Since than, it has been tested at least three times by itself and with other SRG moduls.
El-Op constructed a qualification model (QM), that was extensively tested for vibration and shock to the requirements of SRG. The qualification tests include full functional tests of the imaging sections, as well as thermal vacuum performance verification. These tests of the telescope assembly were conducted in March-April 1996. The electric box will be vibreted after the CT3 session in March 1997. The thermal vacuum test of the QM started on summer 1997. The QM will also be submitted to a calibration procedure similar to that of the flight model.
The flight model (FM) of TAUVEX is almost completely assembled. Small modifications, that may result from the testing of the QM, may still be incorporated at this stage. The burn-in and calibration of the FM are planned for summer 1998. The flight model will be ready for integration immediately after this, from early 1999.
The Proton launcher intended to orbit the SRG mission is in fabrication and the launch date has tentatively been set to the end of 1999. The guaranteed lifetime of SRG is three years, but the instruments are constructed for a five-year operational lifetime with a duty fraction of 75% or higher.
In parallel with the hardware development, the Tel Aviv team is studying the physics of UV space sources. A predictor model was developed that calculates the expected number of UV sources to any observation direction. The model tested well against the few existing data bases of UV sources (Brosch 1991). We also are attempting to predict the UV properties of normal sources from their known optical properties. This will allow us to detect extraordinary sources, through a comparison of their predicted and measured UV properties. Finally, we are creating at Tel Aviv University a large and unique data base of UV astronomy, by combining a number of existing data sets obtained by various space missions.
The combination of long observing periods of SRG (three days out of every four), a high orbit with low radiation and solar scattered background, and long staring sessions to every target (typically 4 hours, up to 72 hours), implies that TAUVEX will be able to detect and measure star-like objects of 20.5 mag with a S/N of 10. This corresponds to V~ 22.5 mag QSOs, given typical UV-V colors of QSOs and at least 10 such objects are expected in every TAUVEX field-of-view. During the guaranteed life of SRG (three years) at least 50,000 QSOs will be observed, if the targets will be different and at high galactic latitude. This is an order of magnitude more QSOs than cataloged now.
Diffuse objects, such as nearby large galaxies, will be measured to a surface brightness of about 20 UV mag/square arcsec for each individual pixel. A survey of Local Group galaxies and nearby clusters of galaxies, that cannot be conducted with the Hubble Space Telescope because of its narrow field-of-view, will be a high priority item in our target list. TAUVEX will detect hundreds of faint galaxies in each high latitude field. It is even possible that most faint, high latitude UV sources are galaxies. The data collection of TAUVEX will represent the deepest UV-magnitude-limited survey of a large fraction of the sky.
An additional major contribution of our experiment to astrophysics is the opportunity it presents, together with the other scientific instruments on board SRG, to study time-dependent phenomena in all energy ranges from MeV in the Gamma-ray band to a few eV in the UV. The combination of many telescopes observing the same celestial source in a number of spectral bands is unique and offers unparalleled opportunities of scientific research. For the first time it will be possible to study the physics of accretion disks around black holes and neutron stars, from the hard X-rays to near the optical region. Other subjects of study include the inner regions of QSOs and AGNs, where the physics of the accretion phenomenon, probably powering all such sources, are best studied with simultaneous multi-wavelength observations.
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