The European Solar Telescope (EST) will be Europe’s most ambitious ground-based infrastructure in solar physics. It will have a primary mirror with a diameter of 4.2m, which will make it the largest in Europe, and of identical aperture as the largest solar telescope available worldwide, namely the Daniel K. Inouye Telescope (DKIST) installed at the Haleakala summit, Hawai'i. EST will have two main characteristics devoted to maximising the scientific return. First, the telescope’s optical path will be polarisation-free using pairs of mirrors that compensate for instrumental polarisation. Second, the telescope will be designed together with a complete instrument suite with imaging and spectrograph instruments. EST will also bring many new technologies, such as a multi-conjugate adaptive optics system and integral field spectro-polarimeters. This contribution presents the conceptual design of the infrared (1 to 1.8 microns) integral field spectropolarimeter. The instrument will have an integral field unit composed of a mirror-based image slicer as input to a Czerny-Turner spectrograph. It will have a polarimeter to record the polarisation state of light on a dual-beam configuration to ensure high-precision spectro-polarimetry.
The European Solar Telescope (EST), with its primary mirror of 4.2 diameter, will be the largest solar telescope available in Europe. EST will offer Integral Field Spectropolarimetry (IFS) by incorporating the EST spectropolariMeter Based on slicEr-mirrors for the near-infraRed (EMBER). This instrument is a high-resolution spectropolarimeter that allows for the analysis of a 2D field of view by using a slicer mirror-based Integral Field Unit (IFU) as input to the spectrograph. The slicer mirror, which is placed at the focal plane of the telescope, allows the observation of the integral field of view by slicing the entire field. After that, an optical system reorganizes the field of view and provides the spectrograph with an output slit composed of multiple slitlets. The spectrograph has a Czerny-Turner design and will cover the solar spectrum from 1 to 1.8 μm. Additionally, EMBER will offer spectropolarimetry observations with a dual-beam configuration. In this contribution, we present the conceptual optical design of the spectrograph and the IFU as a solution that meets the scientific requirements.
THEMIS, a 90 cm solar telescope, offers spectropolarimetric observations in the VIS and NIR spectral ranges through a long-slit spectrograph. Now, an image slicer-based Integral Field Unit (IFU) is being developed as an evolution of the long-slit set-up. The IFU will allow observing a 2D Field-of-View (FoV) and several spectral regions simultaneously. Integral Field Spectroscopy mitigates very well the loss of information generated by the atmospheric seeing over a single long-slit and is more suitable for 2D evolutionary studies of solar phenomena. The IFU optical design inherits the concept of the IFU currently installed on the GRIS spectrograph at the GREGOR telescope and has been adapted to THEMIS. Also, we took advantage from the image slicers manufacturing process evolution since the GRIS IFU was created. First, the spatial resolution is increased by reducing the width of the slicer mirrors from 100 to 80 μm. Second, the FoV is increased by changing the slices number from 8 to 16. In terms of optomechanical integration, the slicer mirror unit is placed at the telescope focal plane. The available volume is limited and has constrained the IFU elements position and the mirrors focal length. The design strategy generates 16 different pupil planes that are superimposed and keeps the optical path constant to allow interchangeability between the new IFU and the long-slit mode. The most challenging condition was keeping the pupil plane position unaltered since the input beam is not telecentric. This contribution presents the IFU optical design and describes the challenges faced in design phase.
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