The design of the optical system of EST is constrained mainly by the science requirements. However, there are also some fundamental requirements that drive the design:
- EST must have an entrance-pupil equivalent to a circular aperture of diameter 4 metres. With this, it will provide a significantly improved angular resolution over what is achievable with the current 1-metre class telescopes. The increase in resolution will allow reaching the small spatial scales required to study the solar magnetic phenomena.
- EST must cancel the instrumental polarization introduced by the telescope. This entails the polarization of the incoming light not being modified, independently of the pointing of the telescope to any direction on the sky. This property must hold for all wavelengths.
- EST must provide excellent image quality, limited by diffraction over a circular FoV of 1 arcmin in diameter through a wavelength range from 0.39 mm to 2.3 mm. The system must be seeing-limited in an unvignetted FoV of 2x2 arcmin.
- EST must have high-order AO and MCAO systems integrated into the main telescope light path to provide the highest possible spatial resolution.
- EST must be optimized to give a high throughput, being composed of a minimum number of optical surfaces.
- The Science instruments of EST have to operate simultaneously to maximize operational efficiency.
General description of the optical system
EST will have an on-axis Gregorian configuration in its main telescope to achieve, after two additional main optical subsystems, a diffraction-limited Coudé Focus in a spectral range from 0.39 µm to 2.3 µm that also shows a good polarimetric performance. It will supply a main station with three types of instruments (broad-band imagers, narrow-band tunable filter spectropolarimeters and grating spectropolarimeters) each one comprising different channels to observe different wavelengths simultaneously.
The telescope includes a multi-conjugate adaptive optics system that is integrated into the telescope optical path. This provides simultaneously a corrected image at the Coudé Focus for the three types of instruments mounted on the instrument platform.
EST can be divided into three main subsystems (see Figure), where each one has a relative movement with respect to the others:
- The first is the main telescope (M1 and M2) defined by an on-axis Gregorian configuration.
- The second is the main axes subsystem (M3 to M8) and integrates those mirrors that define the elevation and azimuth axes. This subsystem houses an on-axis magnification stage (M5) to produce the pupil used by the AO system.
- The third is the transfer optics subsystem (M9 to M14) whose mirrors transfer the light from the main axes subsystem to the Science Coudé Focus. In addition, this assembly integrates the MCAO mirrors inside its light path and also works as the field de-rotator of the telescope. This subsystem houses two off-axis magnification stages (M9 and M13) to get an adequate f-ratio at the MCAO post-focus DMs and the Science Coudé Focus.
EST will have active optics in open and closed loop fed, respectively, by the telescope behaviour model and by the measurements of two wavefront sensors. The latter, which also closes the AO loop, are located before and after the transfer optics subsystem reporting the Science Coudé Focus performance and the correction capabilities of the active and adaptive elements.
The instruments, placed at the Science Coudé Focus and consisting of different channels, will be enclosed in an instrumentation laboratory with a controlled environment. A light distribution unit consisting of dichroic and intensity beam–splitters will be placed at the Science Coudé Focus feeding different instrument channels and accomplishing different ways of light distribution using a flexible number of simultaneous instruments/channels.