Radek Melich

Design status of ASPIICS, an externally occulted coronagraph for PROBA-3

The “sonic region” of the Sun corona remains extremely difficult to observe with spatial resolution and sensitivity sufficient to understand the fine scale phenomena that govern the quiescent solar corona, as well as phenomena that lead to coronal mass ejections (CMEs), which influence space weather. Improvement on this front requires eclipse-like conditions over long observation times. The space-borne coronagraphs flown so far provided a continuous coverage of the external parts of the corona but their over-occulting system did not permit to analyse the part of the white-light corona where the main coronal mass is concentrated. The proposed PROBA-3 Coronagraph System, also known as ASPIICS (Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun), with its novel design, will be the first space coronagraph to cover the range of radial distances between ~1.08 and 3 solar radii where the magnetic field plays a crucial role in the coronal dynamics, thus providing continuous observational conditions very close to those during a total solar eclipse. PROBA-3 is first a mission devoted to the in-orbit demonstration of precise formation flying techniques and technologies for future European missions, which will fly ASPIICS as primary payload. The instrument is distributed over two satellites flying in formation (approx. 150m apart) to form a giant coronagraph capable of producing a nearly perfect eclipse allowing observing the sun corona closer to the rim than ever before. The coronagraph instrument is developed by a large European consortium including about 20 partners from 7 countries under the auspices of the European Space Agency. This paper is reviewing the recent improvements and design updates of the ASPIICS instrument as it is stepping into the detailed design phase.

A new method of determination of ablation threshold contour in the spot of focused XUV laser beam of nanosecond duration

It is well known that at interaction of femtosecond Extreme Ultraviolet Radiation (XUV) with a surface it is possible – according to local fluency - to distinguish two main regions: the desorption region (when efficiency η of removing particles is <10%), and the ablation region (when efficiency η ~ 100%). In this case, the ablation threshold determination is very simple and relatively accurate. It was e.g. shown that with the help of mapping of morphology of the ablationdug- craters it is possible to determine the fluency distribution in/near the beam focus. However, recently we found that (1) the desorption efficiency η for nanosecond pulses is much higher than that for femtosecond ones and spans from zero at the periphery imprint to ~90% at the ablation threshold; this complicates the ablation threshold determination; (2) the direct nano-structuring of solid surfaces is possible only in the desorption region (e.g. the diffraction pattern generated in windows of in-proximity-standing-grid [K.Kolacek et.al., Laser and Particle Beams 30, 57-63, (2012)] is visible only in these parts of laser-beam-spot, which correspond to the desorption region). This prompted us to use this nano-patterning for determination of ablation threshold contour. The best possibility seems to be covering the laser beam spot by interference pattern. For that, it was necessary to develop a new type of interferometer, which (a) provides as dense interference pattern as possible, (b) uses practically all the energy of laser beam, (c) works with focused beams. Such interferometer has been designed and is described in this contribution.

Design and modelisation of ASPIICS optics

In the framework of development of ASPIICS (Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun), the Centre Spatial de Liege is responsible of the optical design of the coronagraph and the optics will be manufactured by TOPTEC. The particularity of this coronagraph is to have an external occulter located 150 m ahead of the first imaging lens. This external occulter is re-imaged on an internal occulter which function is - as in a classical externally occulted Lyot coronagraph - to block the sun light diffracted by the external occulter and to reduce the straylight on the detector. The selection of this configuration is driven by the requirement to observe the corona as close as possible to the solar limb (i.e. 1 RSun) without imaging the limb itself. A requirement of 1.08 RSun is specified at optical design level to grant 1.2 Rsun at instrument level. The coronograph instrument is designed to have a field of view of 1.6° x 1.6° with a resolution of less than 6 arcsec. Its performances are limited by diffraction in a 530 – 590 nm wavelength range. This paper presents the optical design and demonstrates that by design the requirements are fulfilled within the misalignment, manufacturing and thermo-elastic error contributions.

Recent achievements on ASPIICS, an externally occulted coronagraph for PROBA-3

This paper presents the current status of ASPIICS, a solar coronagraph that is the primary payload of ESA’s formation flying in-orbit demonstration mission PROBA-3. The “sonic region” of the Sun corona remains extremely difficult to observe with spatial resolution and sensitivity sufficient to understand the fine scale phenomena that govern the quiescent solar corona, as well as phenomena that lead to coronal mass ejections (CMEs), which influence space weather. Improvement on this front requires eclipse-like conditions over long observation times. The space-borne coronagraphs flown so far provided a continuous coverage of the external parts of the corona but their over-occulting system did not permit to analyse the part of the white-light corona where the main coronal mass is concentrated. The PROBA-3 Coronagraph System, also known as ASPIICS (Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun) is designed as a classical externally occulted Lyot coronagraph but it takes advantage of the opportunity to place the external occulter on a companion spacecraft, about 150m apart, to perform high resolution imaging of the inner corona of the Sun as close as ~1.1 solar radii. The images will be tiled and compressed on board in an FPGA before being down-linked to ground for scientific analyses. ASPIICS is built by a large European consortium including about 20 partners from 7 countries under the auspices of the European Space Agency. This paper is reviewing the recent development status of the ASPIICS instrument as it is approaching CDR.

Multi-wavelength Solc birefringent filter

There are presented first testing data and a design of a special Solc birefringent chain narrow-bandpass filter that allows an astronomical observation of the Sun in wide number of interesting spectral lines (Hα, CaIIK, Hβ, Hγ, D1, D3, etc.). It is described an idea of tuning of sub-filters that the filter is compound of. A special aberrationless off-axis Maksutov telescope meeting demands for constant solar image scale in wavelength from 380 nm to 760 nm is also presented. This telescope equipped with the proposed filter is designed as a unity providing a top quality imaging.

Development of ASPIICS: a coronagraph based on Proba-3 formation flying mission

This paper presents the recent achievements in the development of ASPIICS (Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun), a solar coronagraph that is the primary payload of ESA’s formation flying in-orbit demonstration mission PROBA-3. The PROBA-3 Coronagraph System is designed as a classical externally occulted Lyot coronagraph but it takes advantage of the opportunity to place the 1.4 meter wide external occulter on a companion spacecraft, about 150m apart, to perform high resolution imaging of the inner corona of the Sun as close as ~1.1 solar radii. Besides providing scientific data, ASPIICS is also equipped with sensors for providing relevant navigation data to the Formation Flying GNC system. This paper is reviewing the recent development status of the ASPIICS instrument as it passed CDR, following detailed design of all the sub-systems and testing of STM and various Breadboard models.

Optical measurements of the mirrors and of the interferential filter of the Metis coronagraph on Solar Orbiter

The paper describes the wavefront error measurements of the concave ellipsoidal mirrors M1 and M2, of the concave spherical mirror M0 and of the flat interferential filter IF of the Metis coronagraph. Metis is an inverted occultation coronagraph on board of the ESA Solar Orbiter mission providing a broad-band imaging of the full corona in linearly polarized visible-light (580 - 640 nm) and a narrow-band imaging of the full corona in the ultraviolet Lyman α (121.6 nm). Metis will observe the solar outer atmosphere from a close distance to the Sun as 0.28 A.U. and from up to 35deg out-of-ecliptic. The measurements of wavefront error of the mirrors and of the interferential filter of Metis have been performed in a ISO5 clean room both at component level and at assembly level minimizing, during the integration, the stress introduced by the mechanical hardware. The wavefront error measurements have been performed with a digital interferometer for mirrors M0, M1 and M2 and with a Shack-Hartmann wavefront sensor for the interferential filter.

Design, manufacturing, performance and application of wide angle aspherical achromatic doublet

The paper describes an achromatic Steinhal type doublet that employs an aspherical surface to allow wide angle imaging. A design criteria, optimization techniques and tolerancing of the doublet are described. Further a manufacturing process of the system and achieved optical performance measurement is discussed. Benefits of the wide angle imaging doublet are recently planned to be used in automotive industry application, namely for optimizing of head-light performance and their final evaluation. The final device is planned to be part of the production line.

Spontaneous and artificial direct nanostructuring of solid surface by extreme ultraviolet laser with nanosecond pulses

Nanostructuring can be either spontaneously appearing (such as laser-induced periodic surface structures, and diffraction patterns – for example, in windows of grid proximity-standing at the ablated target-surface) or artificially created (like – as we hoped – interference patterns) that can be in some extend controlled. Due to that a new interferometer (belonging to wave-front division category) with two aspheric mirrors has been developed. Each of these mirrors reflects approximately one half of incoming laser beam and focuses it into a point image. Both focused beams have to intersect each other, and in the intersection region an interference pattern was expected. However, the first tests showed that some other spontaneously appearing interference pattern with substantially larger fringe-pitch is generated. The origin of this idle interference pattern is discussed.

Design and realization of an aspherical doublet

We have realized an optical design of air space doublet of 100 mm clear aperture and 520 mm focal length that is optimized with respect to a quality of wavefront error better than 0.07 λ RMS for on-axis imaging at wavelengths of 633 nm and 450 nm. To minimize optical aberrations we have designed one of the four surfaces to be an aspherical. Based on a tolerance analyses those take into account planned spherical and aspherical technologies for surfaces realization and measurement equipment we have realized the doublet. In the paper there is described a technique of the optical design, tolerance analysis, technique of objective realization and results of the optical elements realization.