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.

Figure: 1

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.

Figure: 2

Telescope
The OM contains three identical telescopes (T1, T2 and T3), each with a circular field of view of 0.9° in diameter. The telescopes are standard Ritchey-Chrétian with a 20 cm diameter primary mirror as shown in Figure 3. The structure is cut away in one of the telescopes to show the carbon fibre (CFRP) metering structure. CFRP expands very little due to temperature and so the entire telescope will operate over a wide range.

Figure: 3

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:

Detectors
The detectors are identical Wedge and Strip photon counting detectors with a diameter of 40 mm. The photocathodes are solar-blind CsTe. An incident photon will eject an electron from the photocathode (through the photoelectric effect) which is then accelerated through a two-stage MCP to strike the anode. We get the x and y position of each photon from the anode which is then transmitted to the ground along with a time stamp. From this, we reconstruct the position of each photon on the sky and build up an image of the sky.

Filters

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 marked as:
BBF - broad band filter;
SF1 - special filter #1;
SF2 - special filter #2;
SF3 - special filter #3;
NBF - narrow band filter.
The closed filter position is marked as CLS.

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.

Figure: 4

Instrument and Performance Summary