Aims. This paper describes the Polarimetric and Helioseismic Imager on the Solar Orbiter mission (SO/PHI), the first magnetograph and helioseismology instrument to observe the Sun from outside the Sun-Earth line. It is the key instrument meant to address the top-level science question: How does the solar dynamo work and drive connections between the Sun and the heliosphere? SO/PHI will also play an important role in answering the other top-level science questions of Solar Orbiter, as well as hosting the potential of a rich return in further science. Methods. SO/PHI measures the Zeeman effect and the Doppler shift in the Fe i 617.3 nm spectral line. To this end, the instrument carries out narrow-band imaging spectro-polarimetry using a tunable LiNbO 3 Fabry-Perot etalon, while the polarisation modulation is done with liquid crystal variable retarders (LCVRs). The line and the nearby continuum are sampled at six wavelength points and the data are recorded by a 2k × 2k CMOS detector. To save valuable telemetry, the raw data are reduced on board, including being inverted under the assumption of a Milne-Eddington atmosphere, although simpler reduction methods are also available on board. SO/PHI is composed of two telescopes; one, the Full Disc Telescope (FDT), covers the full solar disc at all phases of the orbit, while the other, the High Resolution Telescope (HRT), can resolve structures as small as 200 km on the Sun at closest perihelion. The high heat load generated through proximity to the Sun is greatly reduced by the multilayer-coated entrance windows to the two telescopes that allow less than 4% of the total sunlight to enter the instrument, most of it in a narrow wavelength band around the chosen spectral line. Results. SO/PHI was designed and built by a consortium having partners in Germany, Spain, and France. The flight model was delivered to Airbus Defence and Space, Stevenage, and successfully integrated into the Solar Orbiter spacecraft. A number of innovations were introduced compared with earlier space-based spectropolarimeters, thus allowing SO/PHI to fit into the tight mass, volume, power and telemetry budgets provided by the Solar Orbiter spacecraft and to meet the (e.g. thermal) challenges posed by the mission's highly elliptical orbit.

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The MTS, MIRI Telescope Simulator, is developed by INTA as the Spanish contribution of MIRI (Mid InfraRed Instrument) on board JWST (James Web Space Telescope).The MTS is considered as optical equipment which is part of Optical Ground Support Equipment for the AIV/Calibration phase of the instrument at Rutherford Appleton Laboratory, UK.It is an optical simulator of the JWST Telescope, which will provide a diffractionlimited test beam, including the obscuration and mask pattern, in all the MIRI FOV and in all defocusing range. The MTS will have to stand an environment similar to the flight conditions (35K) but using a smaller set-up, typically at lab scales.The MTS will be used to verify MIRI instrument-level tests, based on checking the implementation/realisation of the interfaces and performances, as well as the instrument properties not subject to interface control such as overall transmission of various modes of operation.This paper includes a functional description and a summary of the development status.

The Polarimetric Helioseismic Imager for Solar Orbiter (SO/PHI) is an instrument on board in the Solar Orbiter mission. The Full Disk Telescope (FDT) will have the capability of providing images of the solar disk in all orbital faces with an image quality diffraction-limited. The Heat Rejection Entrance Window (HREW) is the first optical element of the instrument. Its function is to protect the instrument by filtering most of the Solar Spectrum radiation. The HREW consists of two parallel-plane plates made from Suprasil and each surface has a coating with a different function: an UV shield coating, a low pass band filter coating, a high pass band filter coating and an IR shield coating, respectively. The temperature gradient on the HREW during the mission produces a distortion of the transmitted wave-front due to the dependence of the refractive index with the temperature (thermo-optic effect) mainly. The purpose of this work is to determine the capability of the PHI/FDT refocusing system to compensate this distortion. A thermal gradient profile has been considered for each surface of the plates and a thermal-elastic analysis has been done by Finite Element Analysis to determine the deformation of the optical elements. The Optical Path Difference (OPD) between the incident and transmitted wavefronts has been calculated as a function of the ray tracing and the thermo-optic effect on the optical properties of Suprasil (at the work wavelength of PHI) by means of mathematical algorithms based on the 3D Snell Law. The resultant wavefronts have been introduced in the optical design of the FDT to evaluate the performance degradation of the image at the scientific focal plane and to estimate the capability of the PHI refocusing system for maintaining the image quality diffraction-limited. The analysis has been carried out considering two different situations: thermal gradients due to on axis attitude of the instrument and thermal gradients due to 1º off pointing attitude. The effect over the boresight at the instrument focal plane has also been analyzed. The results show that the effect of the FDT HREW thermal gradients on the FDT performance can be optically corrected. The influence of the thermal gradients on the system is also presented.

MTS (MIRI telescope simulator) is the Spanish contribution to the JWST Project. MTS is a part of the Optical Ground Support Equipment (OGSE) for the Assembly Integration and Verification (AIV) and Calibration phase of the MIRI instrument at the RAL (Rutherford Appleton Laboratory) facilities. Briefly, MTS main objetive consists on delivering a diffraction-limited test beam, including the obscuration and mask pattern, to the MIRI instrument that reproduces the output beam of the JWST in environmental conditions similar to those corresponding to the flight.In this work, the current status of the project is reported on. Mainly, after a description of the whole instrument and the optomechanical performances required, the paper will be focused on the current status of the purchase and characterization of certain critical elements belonging to the different subsystems.The first step has been the verification of the thermoelastic behaviour of its structure, employing a mass prototype. Both extensometer measurements and optical measurements with alignment mirror cubes have been carried out during a thermal vacuum test of this MTS prototype. The correlation of the measurements, optically and mechanically, will provide a better knowledge of the structure behavior and will be used to define the integration process.