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The ASPIICS instrument on PROBA-3 looks at the solar corona through a refractive telescope, able to select 3 different spectral bands: Fe XIV line @ 530.4nm, He I D3 line @587.7nm, and the white-light spectral band [540;570nm]. The external occulter being located at ~ 150 meters from the instrument entrance, will allow ASPIICS to observe the corona really close to the solar limb, probably closer than any internally or externally occulted coronagraph ever observed.
This paper will present the straylight model and analyses carried out by CSL. A first specificity of the analysis is that the scene on the useful Field of View (FOV) is the solar corona which has a brightness dynamic range as high as 103 between the close corona, close to 1 solar radius (Rsun), and the “distant” corona around 3RSun. The specifications are very stringent for this type of instrument. A consensus was found and will be presented regarding the expected straylight within the FOV. It will also be shown that to achieve realistic estimations it is required to take into account the exact location of the created straylight as well as the entrance field.
The second specificity that had to be analyzed is that the diffraction from the solar disk by the external occulter enters the instrument un-obstructed until the internal occulter, and with a brightness 100 times higher than the close corona (~1RSun) brightness. The simulation of this diffraction as well as its propagation inside the ASPIICS telescope creating additional straylight, had to be carefully established in order to give realistic results of its impact on the performances while being actually possible to compute.
The entire alignment and verification phase has been performed by the Metis team in collaboration with Thales Alenia Space Torino and took place in ALTEC (Turin) at the Optical Payload System Facility using the Space Optics Calibration Chamber infrastructure, a vacuum chamber especially built and tested for the alignment and calibration of the Metis coronagraph, and suitable for tests of future payloads.
The goal of the alignment, integration, verification and calibration processes is to measure the parameters of the telescope, and the characteristics of the two Metis channels: visible and ultraviolet. They work in parallel thanks to the peculiar optical layout. The focusing and alignment performance of the two channels must be well understood, and the results need to be easily compared to the requirements. For this, a dedicated illumination method, with both channels fed by the same source, has been developed; and a procedure to perform a simultaneous through focus analysis has been adopted.
In this paper the final optical performance achieved by Metis is reported and commented.
The stray light calibration was performed in a clean environment in front of the OPSys solar disk divergence simulator (at ALTEC, in Torino, Italy), which is able to emulate different heliocentric distances. Ground calibrations were a unique opportunity to map the Metis stray light level thanks to a pure solar disk simulator without the solar corona. The stray light calibration was limited to the visible light case, being the most stringent. This work is focused on the description of the laboratory facility that was used to perform the stray light calibration and on the calibration results.
Several metrology systems have been implemented in order to keep the formation-flying configuration. Among them, the Shadow Position Sensors (SPSs) assembly. The SPSs are designed to verify the sun-pointing alignment between the Coronagraph pupil entrance centre and the umbra cone generated by the Occulter Disk. The accurate alignment between the spacecrafts is required for observations of the solar corona as much close to the limb as 1.05 RΘ.The metrological system based on the SPSs is composed of two sets of four micro arrays of Silicon Photomultipliers (SiPMs) located on the coronagraph pupil plane and acquiring data related to the intensity of the penumbra illumination level to retrieve the spacecrafts relative position. We developed and tested a dedicated algorithm for retrieving the satellites position with respect to the Sun. Starting from the measurements of the penumbra profile in four different spots and applying a suitable logic, the algorithm evaluates the spacecraft tri-dimensional relative position. In particular, during the observational phase, when the two satellites will be at 150 meters of distance, the algorithm will compute the relative position around the ideal aligned position with an accuracy of 500μm within the lateral plane and 500 mm for the longitudinal measurement. This work describes the formation flying algorithm based on the SPS measurements. In particular, the implementation logic and the formulae are described together with the results of the algorithm testing.
Metis features two channels to image the solar corona in two different spectral bands: in the HI Lyman ∝ at 121.6 nm, and in the polarized visible light band (580 – 640 nm). Metis is a solar coronagraph adopting an “inverted occulted” configuration. The inverted external occulter (IEO) is a circular aperture followed by a spherical mirror which back rejects the disk light. The reflected disk light exits the instrument through the IEO aperture itself, while the passing coronal light is collected by the Metis telescope. Common to both channels, the Gregorian on-axis telescope is centrally occulted and both the primary and the secondary mirror have annular shape.
Classic alignment methods adopted for on-axis telescope cannot be used, since the on-axis field is not available. A novel and ad hoc alignment set-up has been developed for the telescope alignment.
An auxiliary visible optical ground support equipment source has been conceived for the telescope alignment. It is made up by four collimated beams inclined and dimensioned to illuminate different sections of the annular primary mirror without being vignetted by other optical or mechanical elements of the instrument.
ASPIICS is distributed on the two PROBA 3 spacecrafts (S/C) separated by 150 m. The coronagraph optical assembly is hosted by the “coronagraph S/C” protected from direct solar disk light by the occulting disk on the “occulter S/C”.
The most critical issue in the design of a solar coronagraph is the reduction of the stray light due to the diffraction and scattering of the solar disk light by the occulter, the aperture and the optics. In the present article, we deal with two of these issues:
- The analysis of the stray light inside the telescope.
- The optimization of the external occulter edge, in order to eliminate the Poisson spot behind the occulter and to lower the stray light level going through the entrance pupil of the telescope.
This work was performed in the framework of the ESA STARTIGER program which took place at the Laboratoire d’Astrophysique de Marseille (LAM) during a 6-month period from September 2009 to March 2010.
In general, it is a very complicated task to combine the above two stray light issues together in the simulation and design phase as it requires to consider the propagation inside the telescope of the light diffracted by the external occulter. Actually, the present literature only reports diffraction calculations performed for simple occulting systems (i.e., two disks and serrated disk). A more pragmatic approach, also driven by the tight schedule of the STARTIGER program, is to separate the two contributions, and perform two different stray light analyses. This paper is dedicated to the description of both analyses: in particular, the first part is dedicated to the evaluation of the stray light inside the telescope, assuming a simple disk as occulter, and a preliminary baffle design is presented; the second part describes the investigation on the geometry of the external occulter, with a detailed description of the laboratory setup that has been designed and implemented to compare together several types of occulting systems.
This paper provides a description of the overall manufactured system and its performance and shows the additional resources available at the XUVLab laboratory in Florence that make SCOUT exploitable by whatever compact (within 1 m) optical experiment that investigates the UV band of the spectrum.
The solar corona will be observed thanks to the presence on the first satellite, facing the Sun, of an external occulter producing an artificial eclipse of the Sun disk. The second satellite will carry on the coronagraph telescope and the digital camera system in order to perform imaging of the inner part of the corona in visible polarized light, from 1.08 R⦿ up to about 3 R⦿.
One of the main metrological subsystems used to control and to maintain the relative (i.e. between the two satellites) and absolute (i.e. with respect to the Sun) FF attitude is the Shadow Position Sensor (SPS) assembly. It is composed of eight micro arrays of silicon photomultipliers (SiPMs) able to measure with the required sensitivity and dynamic range the penumbral light intensity on the Coronagraph entrance pupil.
In the following of the present paper we describe the overall SPS subsystem and its readout electronics with respect to the capability to satisfy the mission requirements, from the light conversion process on board the silicon-based SPS devices up to the digital signal readout and sampling.
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