The conceptual optical design of the three Tunable Imaging Spectropolarimeter/Fixed-Band Imager instruments foreseen for EST has been completed in 2023.
Figure 1. Conceptual layout of the TIS/FBI instrument.
The Tunable Imaging Spectropolarimeters and Fixed-Band Imagers (TIS/ FBIs) are one of the key instruments foreseen for the European Solar Telescope (EST). The development of the TIS/FBIs is carried out by a consortium formed by the Spanish Space Solar Physics Consortium (S3PC) —led by Instituto de Astrofísica de Andalucía (IAA-CSIC)—, University of Rome Tor Vergata (UNITOV), the National Institute of Astrophysics of Italy (INAF), University of Catania (UNICAT), Stockholm University (SU), Queen’s University Belfast (QUB), Mullard Space Science Laboratory (UCL-MSSL), Istituto di Ricerche Solari (IRSOL), and Leibniz-Institut für Sonnenphysik (KIS).
The TIS/FBIs will be responsible for providing maps of the thermal, dynamic and magnetic properties of the solar photosphere and chromosphere over a circular field of view of 60 arcsec at very high cadence and diffraction-limited spatial resolution. To achieve this goal, they will measure the four Stokes profiles of selected spectral lines by means of a Fabry-Pérot etalon system.
Three TIS/FBIs will be built to cover the following spectral ranges: 390-500 nm, 500-780 nm, and 780-850 nm. Each instrument will be able to observe different spectral lines, typically four to five. This capability grants them exceptional versatility, enabling the simultaneous observation of multiple combinations of three lines. Moreover, the instruments will combine two different operation modes. In one mode, they will work as tunable narrowband spectropolarimeters (TIS mode). In the other mode, they will work as context fixed band imagers (FBI mode).
TIS/FBI concept
The conceptual layout of the instruments is depicted in Figure 1. Light coming from the telescope will first illuminate a filter wheel hosting a set of pre-filters centred about the wavelengths of interest. These filters will let pass a very narrow part of the spectrum (about 0.1 nm) into the instrument. Then, the transmitted light will be divided between the TIS (90- 95%) and the FBI (5-10%).
The TIS will modulate the incoming light polarimetrically with two liquid crystal variable retarders (LCVRs) and will scan the spectral line selected by the pre-filter with a system of two Fabry-Pérot interferometers that will achieve a spectral resolution of 5-10 pm, depending on the wavelength.
Finally, a polarising beam splitter will divide the modulated beam into two orthogonal polarisations that will be recorded by two cameras in order to reduce seeing-induced crosstalk. The FBI is much simpler, as spectral and polarimetric analyses are omitted. In this case a focused channel will be used for image reconstruction through the Multi-Object Multi-Frame Blind Deconvolution (MOMFBD) technique and for alignment of the TIS. A defocused (auxiliary) channel will observe the same scene with a difference of phase (phase diversity) in order to increase the capabilities of the MOMFBD technique.
The TIS arm
The three TIS instruments foreseen for the European Solar Telescope will record the spectrum of the four Stokes parameters of the incident light with a resolving power larger than 50.000, excellent (diffraction-limited) optical quality, and a polarimetric sensitivity as high as 10-3 thanks to the use of dual-beam polarimetry.
Figure 2. Optical design of the TIS/FBI instrument. F3 is the telescope focal plane. FW stands for filter weel, BS for beam splitter, FP1 and FP2 are the two etalons, IM is an intermediate image, and FP represents the final focal plane.
The TIS optical design is depicted in Figure 2. Right after the filter and the beam splitter that divides light between the TIS and the FBI (Figure 1), two group of lenses will produce a slow (f/150) telecentric image of the solar scene. The dual Fabry-Pérot system will be placed close to this intermediate image. Illuminating the etalons with such a large f-number will prevent the harming pupil apodisation effects that appear in this (telecentric) configuration from being too large. Finally, another two groups of lenses will produce a telecentric image with the required plate scale and size on the detector.
One of the major advantages of this design is that no folding mirrors are needed. As a consequence, light absorption in the entire optical system is minimised, which increases the photon flux on the focal plane and, hence, reduces the time needed to achieve the required signal-to-noise ratios. In fact, thanks to the high throughput of both the telescope and the TIS instrument, scanning of a spectral line at 10 wavelength positions will take less than 20 seconds in most cases, according to the photon budget calculations that have been performed by the TIS team.
The main downside of this design is probably that the slow telecentric beam of f/150 requires the etalons (and also the optics) to have diametres of the order of 18 cm, several times larger than those used by similar ground-based instruments. No doubt, manufacturing such large etalons with high optical quality will be one of the biggest challenges of the project.
The FBI arm
FBIs will be employed to make broadband (0.1-0.5 nm) photometric observations of the solar surface with larger signal-to-noise ratio —and potentially faster cadence— than the TISes. These observations will then complement the measurements taken by the TISes, thereby increasing the flexibility and the overall capabilities of the instrument.
One noteworthy potential of the FBIs is their capacity to infer the wavefront degradation introduced by the instrument and the telescope in real time, as well as the residual atmospheric aberrations produced by seeing which are not corrected by the telescope adaptive optics system. The combined effect of these two sources of wavefront degradation is expected to reduce the final image quality. The ability to infer the wavefront error allows for subsequent compensation during image restoration with the MOMFBD technique.
To accurately sense the aberrations introduced by the instrument, the FBI and TIS optical paths must be as similar as possible. Ideally, the FBIs should be an exact replica of the TISes. However, the two largest doublets seen in Figure 2 have a very limited impact on the aberrations, as their focal length is much larger than that of the other lens groups. Aberrations produced by optical elements placed close to an image plane (etalons, filter wheel, LCVRs and beam splitter) will also be negligible, as the footprint of the incident beam on them is very small. Thus, only the first and last groups of TIS lenses must be replicated in the FBI, effectively reducing the size of the FBI arm by a factor of 4.
The current TIS/FBI design results in three 6 m x 1.5 m instruments (Figure 3). These dimensions are compatible with the envelope required to accommodate them into the Coudé room alongside other instruments. In view of the large size of the telescope, these dimensions are actually quite moderate.
Figure 3. Three-dimensional view of the TIS/FBI instrument.