The main achievements reached by the project are:
- Detectors: The overall objective of this WP has been the development of new prototype sensors and the evaluation of an existing camera technology. The diverse requirements of solar observations cannot be fulfilled by a single camera type and a specialisation is needed. To properly address these various applications and the corresponding camera requirements, this WP was subdivided into 3 sub-WPs, each one addressing a specific camera type. (i) The development has been addressed of a prototype for a large-format (4k x 4k) fast-readout low-noise back-side illuminated camera. The characteristics and optical performance of the prototype camera was evaluated, revealing that the vast majority of the specifications are met. (ii) A second objective has been the feasibility study for the development of a new, high speed polarimetric camera based on the detector / amplifier structure DEPFET. Such a device will allow for much higher modulation frequencies than conventional polarimeters based on standard imaging detectors. A first Quadropix prototype matrix was operated and characterised. The basic functionality of the Quadropix was verified, and all key performance indicators were within the expected parameter range. A science ready camera system might be available within the next 6 years. (iii) For the purpose of potential application in EST, the study and testing has been done of the sCMOS technology currently under development at ESO for the wavefront sensing system of the night-time ELT. The consequences on the polarimetric modulation derived from the rolling shutter and of linearity deviations were studied and quantified with laboratory tests and measurements. The effect of adaptive optics residual aberrations on polarimetric measurements has also been analysed.
- Fabry-Perot Interferometer: A customised Capacitance-Stabilised digital control system (ADS100) for the control of an etalon laboratory prototype (GREST FPI 50) has been designed, fabricated and tested. A significant objective has been to assess the noise level of the controller and the overall spectral performance of the etalon. A fully customised piezo driver electronics was designed and prototyped. The embedded piezo driver was designed to be easily adjusted to different piezo power and voltage requirements, thus making the new controller potentially applicable to a large set of available etalons. The integration of the ADS100 controller with the GREST FPI 50 was successfully achieved.
- 2D spectropolarimetry: High-spatial resolution observations with a spectrograph demand configurations of these instruments using Integral Field Units (IFU), in contrast to standard long-slit configurations used in solar physics. Two possibilities to reach that goal have been studied in GREST: one based on image slicers and the other on the use of microlenses. (i) An optical and mechanical design of a multi-slit image slicer has been produced that meets the requirements of available space at GREGOR (as a test-bed telescope for the development of this technology), and compatibility with the present single-slit IFU developed in SOLARNET and polarimeter. The adopted solution takes care of the restrictions imposed by the IFU material (Zerodur) and by the manufacturing process. The multi-slit optical layout has been analysed, tuned and validated from the mechanical engineering perspective. (ii) Within the framework of the SOLARNET project, a 2D spectrometer (MiHI) was developed that was based on a microlens array. The instrument is capable of true imaging spectroscopy in the sense that the spatial and spectral information of the observed solar scene is sampled at one instant. Within GREST the existing concept has been modified and extended for a microlens-fed spectrograph with polarimetric capabilities. The main outcome has been a flexible and technically easy concept where the full spatio-spectral image formed after the spectrograph is split-up into six beams, each representing one polarimetric modulation state. These beams can be imaged onto individual image detectors. The opto-mechanical design of the polarimetry package has been produced in GREST as an add-on to the existing SOLARNET MiHI prototype.
- Large modulators: Two rounds of large-format liquid-crystal variable retarders have been fabricated based on different concepts: Anti-Parallel Aligned Nematic, Parallel Aligned Nematic (Pi Cell) and ferroelectric cells. The units have been tested at the lab to study their homogeneity, response to applied voltage, response time, optical quality and dependence of all these parameters with size. The final conclusions are: (a) Repeatability problems due to charging and discharging times contribute to the retardance repeatability much less than the electric noise caused by ripple effects. (b) The retardance homogeneity of FLC cells is considerably better than the one obtained with NLC cells but, it should be remembered that the area of the clear aperture of the big FLCs used corresponds to a 40% of the clear aperture that has been achieved with NLCs. (c) Neither NLC nor FLC cells meet the requirement of a wavefront error (WFE) < 0.095 λ. (d) For most cells, the WFE has an important power contribution which can be attributed to aberrations present in the substrates. For high quality optical substrates, the WFE and retardance homogeneity requirements might be met for both NLC and FLC. (e) Both NLC and FLC cells meet the minimum response time (RT) requirement of RT<100ms. Nevertheless, FLC are near the goal of RT<100μs while NLC are near RT<1ms.
- Adaptive optics: A specific simulation software to model the behaviour of tilted deformable mirrors (DMs) in different conditions has been developed to study the performance of the MCAO system of EST. Using the new software, the analysis and the characterisation of the spatial response of tilted DMs have been performed under different scenarios. The simulations have been complemented with laboratory measurements, which show that the DMs have a polarisation footprint that is particularly evident only in the pupil plane. However, these effects are rather localised and the integrated polarimetric effect of the DM membrane is negligible. Finally, since the EST MCAO system will make use of DMs on a divergent beam, the loop convergence and stability has been confirmed for a wide range of f-numbers including the ones foreseen by the EST optical scheme.
