TAUVEX hardware has been built at El-Op (Israel). The software for the data pipeline and scientific processing and analysis is being developed by the TAUVEX software group at Indian Institute of Astrophysics.
TAUVEX as a payload
The TAUVEX payload consists of two parts - one containing three identical telescopes designed to observe the sky in the ultraviolet (Optical Module or OM; Figure 1) and the other containing the associated electronics (Electronics Module or EM). Figure 1 shows the Tauvex optical module in the clean room at El-Op. The module is wrapped in a dust free cover to prevent contamination.
Both modules will sit on a plate attached to a platform mounted on a motor, similar to that used to drive the solar panels, allowing observation over the entire celestial sphere. The payload will sit on the East face of the GSAT-4 satellite as shown in Figure 2. Instrument consists of three identical telescopes (T1, T2 and T3) designed to observe the sky in the ultraviolet. In Figure 2, the OM is the larger green box on the left side of the satellite. The three detector tubes and the telescopes are within this box. The EM is the smaller box behind the OM. In principle, TAUVEX can look anywhere in the sky but we are limited by scattering from the Sun, both internally and from the external surfaces of the satellite. The challenge is to maximize the observation time while maintaining the quality of the data.
Light from the sky enters through the apertures and is reflected from the primary and secondary and brought to a focus at the detector plane behind the primary mirror. The mirrors are made of Zerodur and are coated with aluminium with a MgF2 overcoating, which protects the aluminium from oxidation (which would destroy the UV sensitivity of the telescope).
There are two baffles, external and internal, on each telescope that reject light from off-axis sources. Despite the baffling, the Sun is so bright in the UV that we will only observe more than 90° from the Sun. The instrument configuration is shown below:
Each detector has a filter wheel with four positions. One of the positions
is occupied by an opaque filter which serves as a shutter to close the detector
when the system is not in use or when strong sunlight may fall on it or for the
purpose of characterizing the dark response of the system. The other 9 positions
altogether are used by filters defining five different UV bands. These bands are
Plotted below (Fig. 4) are the total system responses (q.t.) for each of the TAUVEX filters. Click on the image to download the text files containing the system response. The units are in counts/ (photon cm-2 s-1 Å-1) so the total count rate is given by integrating the spectral energy distribution in units of photon cm-2 s-1 Å-1 with the system response for the given filter. Note that this takes into account all other factors such as the mirror collecting area and response. A zoom into the near zero region of y-axis is shown as inset.
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