Light distribution and instrument suite proposed for the European Solar Telescope (EST)

In the last months, the EST Project Office at IAC and the review panel of the EST Science Advisory Group chaired by Rolf Schlichenmaier from KIS has worked on the definition of the EST light distribution system and instrument suite, using the guidelines laid down by the Science Requirement Document.

Proposed light distribution system and instrument suite for EST (Sketch by Carlos Quintero)

In December 2019, the second edition of the EST Science Requirement Document (SRD) was published by the EST Science Advisory Group (SAG) chaired by Rolf Schlichenmaier from KIS. In spring of 2020, the EST Science Advisory Group formed a review panel consisting of instrument experts. Based on the scientific requirements and on the 61 science cases presented in the SRD, this SAG review panel together with the EST Project Office at IAC worked out the technical specification for EST as well as the light distribution concept and the instrument suite.

Light distribution. The figure (link to png file) shows the light distribution design based on the previous results. The solar light comes from the left and enters the telescope. It is divided by an optical element, i.e. a dichroic beamsplitter, that sends part of the spectrum to a first arm and the rest to a second arm. In this case, we are not reducing the intensity of the spectrum, just dividing the spectral range. This process is repeated again to create red and near-infrared arms (top dichroic) and visible and blue arms (bottom dichroic). This design allows multi-wavelength observations to be performed. In particular, observers can have access to the Ca II 854 nm and He I 1083 nm lines simultaneously. It also makes it possible to have the most demanded photospheric lines – the Fe I pair at 630 nm – and the requested context chromospheric observations at 390 nm in separate instruments. Therefore, those transitions could be observed strictly simultaneously for the first time ever in any solar telescope.

Instrument suite. After designing how the light is distributed in wavelength, the next step is to define the instruments we will have in each spectral arm. To that end, we checked the OP tables in the SRD again, focusing this time on different aspects. For instance, we checked how the community wants to observe the Ca II 854 nm and He I 1083 nm lines. In both cases, more than 75% of the OPs want to use an Integral Field Unit (IFU) instrument, while the rest of cases requested to observe the spectral lines with Tunable Band Imager (TBI) instruments. Moreover, a similar ratio of instruments was found for the visible photospheric lines at 630 nm. Thus, we decided that we should have one IFU system per arm in the visible, red, and the infrared. On top of that, we thought that we should include a fourth IFU in the blue to perform seamless multi-wavelength observations with state-of-the-art IFU systems (orange boxes in the figure). In fact, EST is going to be the first telescope equipped with one IFU system per spectral region. Currently, microlens arrays and image slicers are the two IFU systems baselined for EST.

More information can be found in the EST newsletter of June 2020.

Text by Carlos Quintero (IAC) and Rolf Schlichenmaier (KIS)

Reference: Science Requirement Document (SRD) for the European Solar Telescope (EST) (2nd edition, December 2019), Schlichenmaier, Bellot Rubio, Collados et al 2019.