The techniques for stray light analysis, optimization and testing are described for two space telescopes that observe the solar corona: the Solar Orbiter Heliospheric Imager (SoloHI) that will fly on the ESA Solar Orbiter (SolO), and the Wide Field Imager for Solar Probe (WISPR) that will fly on the NASA Parker Solar Probe (PSP) mission. Imaging the solar corona is challenging, because the corona is six orders of magnitude dimmer than the Sun surface at the limb, and the coronal brightness continues to decrease to ten orders of magnitude below the Sun limb above 5° elongation from Sun center. The SoloHI and WISPR instruments are located behind their respective spacecraft heat shield. Each spacecraft heat shield does not block the instrument field of view above the solar limb, but will prevent direct sunlight entering the instrument aperture. To satisfy the instrument stray light attenuation required to observe the solar corona, an additional set of instrument baffles were designed and tested for successive diffraction of the heat shield diffracted light before entering the telescope entrance pupil. A semi empirical model of diffraction was used to design the baffles, and tests of the flight models were performed in flight like conditions with the aim of verifying the rejection of the design. Test data showed that the baffle systems behaved as expected. A second source of stray light is due to reflections of the sunlight off of the spacecraft structures and towards the instruments. This is especially the case for SoloHI where one of the spacecraft 8m tall solar arrays is located behind the telescope and reflects sunlight back onto the instrument baffles. The SoloHI baffle design had to be adjusted to mitigate that component, which was achieved by modifying their geometry and their optical coating. Laboratory tests of the flight model were performed. The test data were correlated with the predictions of a ray tracing model, which enabled the fine tuning of the model. Finally, end-to-end ray tracing was used to predict the stray light for the flight conditions.
Solar Probe Plus (SPP) is a NASA mission developed to visit and study the sun closer than ever before. SPP is designed to orbit as close as 7 million km (9.86 solar radii) from Sun center. One of its instruments: WISPR (Wide-Field Imager for Solar Probe Plus) will be the first ‘local’ imager to provide the relation between the large-scale corona and the in-situ measurements.
The SoloHI instrument for the ESA/NASA Solar Orbiter mission will track density fluctuations in the inner
heliosphere, by observing visible sunlight scattered by electrons in the solar wind. Fluctuations are associated with
dynamic events such as coronal mass ejections, but also with the “quiescent” solar wind. SoloHI will provide the
crucial link between the low corona observations from the Solar Orbiter instruments and the in-situ measurements
on Solar Orbiter and the Solar Probe Plus missions. The instrument is a visible-light telescope, based on the
SECCHI/Heliospheric Imager (HI) currently flying on the STEREO mission. In this concept, a series of
baffles reduce the scattered light from the solar disk and reflections from the spacecraft to levels below
the scene brightness, typically by a factor of 1012. The fluctuations are imposed against a much brighter
signal produced by light scattered by dust particles (the zodiacal light/F-corona). Multiple images are
obtained over a period of several minutes and are summed on-board to increase the signal-to-noise ratio
and to reduce the telemetry load. SoloHI is a single telescope with a 40⁰ field of view beginning at 5°
from the Sun center. Through a series of Venus gravity assists, the minimum perihelia for Solar Orbiter will
be reduced to about 60 Rsun (0.28 AU), and the inclination of the orbital plane will be increased to a
maximum of 35° after the 7 year mission. The CMOS/APS detector is a mosaic of four 2048 x 1930
pixel arrays, each 2-side buttable with 11 μm pixels.
The Solar Probe Plus (SPP) mission scheduled for launch in 2018, will orbit between the Sun and Venus with
diminishing perihelia reaching as close as 7 million km (9.86 solar radii) from Sun center. In addition to a suite of in-situ
probes for the magnetic field, plasma, and energetic particles, SPP will be equipped with an imager. The Wide-field
Imager for the Solar PRobe+ (WISPR), with a 95° radial by 58° transverse field of view, will image the fine-scale
coronal structure of the corona, derive the 3D structure of the large-scale corona, and determine whether a dust-free zone
exists near the Sun. Given the tight mass constrains of the mission, WISPR incorporates an efficient design of two widefield
telescopes and their associated focal plane arrays based on novel large-format (2kx2k) APS CMOS detectors into
the smallest heliospheric imaging package to date. The flexible control electronics allow WISPR to collect individual
images at cadences up to 1 second at perihelion or sum several of them to increase the signal-to-noise during the
outbound part of the orbit. The use of two telescopes minimizes the risk of dust damage which may be considerable
close to the Sun. The dependency of the Thomson scattering emission of the corona on the imaging geometry dictates
that WISPR will be very sensitive to the emission from plasma close to the spacecraft in contrast to the situation for
imaging from Earth orbit. WISPR will be the first ‘local’ imager providing a crucial link between the large scale corona
and the in-situ measurements.
The space-born coronagraph is an instrument used to observe the solar corona, the outer atmosphere of the Sun, typically over a range of altitudes from close to the limb of the solar disk to tens of solar radii. The brightness of the solar disk is many orders of magnitude greater than that of the corona. A coronagraph is designed to reject the light from the solar disk such that the corona is observable. An externally-occulted coronagraph is basically a telescope that forms an image of the corona, with the addition of an external occulter before and an internal occulter after the objective elements and stops, positioned and sized to reject light from the solar disk. The main source of stray light is diffraction of solar light around the edge of the external occulter, which is then scattered into the image plane by the optical elements. The occulters and stops are designed to reduce the intensity of diffracted and scattered light in the coronagraph as much as possible.
We have developed a numerical model of the diffraction by an external occulter system and validated the model experimentally. We used the model to optimize the external occulter design for the SECCHI COR2 instrument, which is part of the NASA STEREO mission. We also used the model for the GOES-R SCOR concept design to predict the sensitivity of the instrument to misalignment and off-pointing from the Sun. In this paper, we will present the results of this experimental and numerical study of the performance of the external occulters on these instruments.
In order to obtain an image of the solar corona, coronagraph optical design needs to be optimized with respect to stray light reduction. Despite the accurate optical design, some stray light is present on the focal plane in addition to the coronal signal. The stray light level has to be estimated in order to test the quality of the optical design. The stray light is given by scattering off the surfaces of the optical elements and by diffraction from the instrument apertures. In order to estimate the stray light level on the focal plane, a diffraction calculation is necessary. In this paper we describe the diffraction calculation for a coronagraph with an innovative stray light reduction design. For the same optical configuration we used two different algorithms, based on different approaches to Fresnel diffraction computation. By using the Fresnel-Kirchhoff scalar theory we developed an algorithm, and we used it to write codes in IDL (Interactive Data Language, by Research System Inc.), and C programming languages. By using the GLAD (General Laser Analysis and Design, by AOR) software, which diffraction algorithm is based on the principles of Fourier optics, we wrote a further code. In this paper we compare the results of the different codes and we discuss their efficiencies.
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