GRBs appear randomly in the sky. Therefore detecting a large fraction of them requires to design wide-field instruments. ECLAIRs has been designed to ensure both the detection of GRBs with the best possible sensitivity and the distribution of their sky position with the best possible accuracy. Following a detection, the spacecraft will slew in order to repoint the narrow-field instruments in the direction of the burst. Creating wide-field optics focussing hard X-rays and Gamma-rays is currently out of reach. Thus, the instrument ECLAIRs is based on the coded mask technic.
The coded mask technic has been used for decades in Gamma-/X-ray astronomy as it is a robust and efficient way to localize high energy sources over a wide field of view. A coded mask is made of opaque and transparent elements in a given energy range arranged in a particular pattern. The mask is placed above a pixellized detection plane. The light from each source within the camera field of view will then project a unique shadow of the mask on the detection plane, therefore coding the sky light. By measuring this shadow and using deconvolution technics, it is possible to retrieve the position of any source within the camera field of view.
For bright sources, the mask pattern in the projected shadow could be seen easily. However for faint sources, we need to rely on mathematical tools. For the faintest sources, it will not be possible to distinguish the source from the background. It means that the camera reaches its limiting sensitivity.
Another striking aspect of using coded mask technics is the case for the presence of multiple sources within the field of view. Deconvolution technics enable us to decipher the direction of any incident photons forming the shadow on the detection plane. It is then possible for instance to say if a third of them comes from a burst, another third from the background and the last part from another source. Thanks to this, it is possible to build a sky image with all the sources present within the field of view.
In order to reconstruct a sky image using deconvolution algorithm, we need to know what encoding key has been used in the first place to code the signal. The coding of the data relies on multiplexing (i.e. transforming/re-arranging) multiple sources of signal in one signal using a particular encoding key. Then these coded data could be transmitted. Once received, the original signals could be retrieved using demultiplexing technics and the same encoding key. Such technics are often used in telecommunication applications.
The coded mask technics basically work the same way. The signal from the targeted source is mixed through the mask pattern with external signals coming from other sources within the field of view and the background. This creates a shadow on the detection plane that is recorded. Applying deconvolution algorithm to this shadow enables to retrieve the position and intensity of the targeted source.
The design of the mask relies on both the sensitivity and the localization accuracy we want to achieve with the instrument. On one hand small size holes will favor the localization accuracy, with a limit on how small they could be being fixed by the size of the pixels of the detection plane. On the other hans, large size holes will increase the instrument sensitivity i.e. increasing the probability for detecting something in the sky. Therefore, the design of the mask is the result of a complex balance between these two parameters. Indeed, the burst position computed by ECLAIRs will be used by the spacecraft to place the burst within the field of view of the MXT and VT telescopes. The mask design shall also ensure that the burst detection efficiency is optimal to fulfill the mission science goals. The optimization of the mask design has been done by computing thousands of mask patterns and by comparing their performances.
The selected mask (ACS-o40-46x-a) for the ECLAIRs instrument is made of an array of 46×46 transparent and opaque elements. This mask design is the result of many years of hard work. Credits CEA-APC
To mitigate the effects of vignetting, the mask thickness shall be less than 0.6 mm. However manufacturing such a thin mask using only a Tantalum sheet (a metal with a good absorbing power for X-ray photons) would have been challenging. In order to increase the mask rigidity, the Tantalum sheet is sandwitched by two Titanium sheets. A silicone joint is placed between the Ta and Ti sheets to absorb vibrations and deformations induced by temperature changes between the two metals. This assembly has been called « TiTaTi » for the Titanium — Tantalum — Titanium sandwitch.
To mitigate the effects of vignetting, we make sure that the size of the holes in the Titanium sheets are larger than for the Tantalum one.