2.1

COR: a visible and EUV coronagraph

COR is an externally occulted coronagraph designed for performing broad-band polarization imaging of the visible K-corona, narrow-band imaging of the UV corona in the HI Lyman-α (121.6 nm) line, and narrow-band imaging of the EUV corona in the HeII Lyman-α (30.4 nm) line. COR is planned to observe in an annular field of view (FOV) between 1.2 R_⊙ and 3.0 R_⊙ at 0.226 AU perihelion (perihelion will change during the mission).

The coronagraph critical issue of solar disk light rejection is taken care of by means of a novel optical design for a reflecting coronagraph, widely used in traditional externally occulted coronagraphs. This optical design has been thoroughly studied in the design of the SCORE coronagraph developed for the suborbital mission HERSCHEL [1]-[4]. A combination of multilayer coating of the mirrors, optimized to enhance reflectivity in the EUV He line, and of band-pass filters contribute to the capability of imaging the solar corona in three different wavelength bands by means of a single telescope. These mirrors are coated with multilayer optimized for 30.4 nm, but that maintains good reflectivity at 121.6 nm and in the visible.

A schematic of the COR element is shown in Fig. 2, and its optical specifications are summarized in Tab. 1. COR is based on a classical externally occulted design, in which light enters through the external occulter (EO) aperture, that provides the Sun disk occultation, located at the outside panel of the S/C heat shield. The EO is a fixed triple disk system that ensures both thermal protection for the optics and optimal stray-light rejection; it is supported by a suitable truss fixed to the optical bench, protruding from the S/C instrument bay inside the heat shield.

Fig. 2.

Schematic of the COR optical path inside METIS.

Tab. 1.

COR optical specifications.

An annular shaped Sun-disk rejection mirror (M0) back reflects the sun disk light through the front aperture to minimize the thermal load inside the instrument, and leaves the coronal light reaching the telescope through a hole at its center located in the shadow of the external occulter. This hole has also the optical function of COR aperture stop.

Coronal light passing through M0 reaches the COR telescope, which consists of aspherical primary (M1) and secondary (M2) mirrors in off-axis Gregorian mount. The image of EO produced by M1 is blocked by a stop called internal occulter (IO): its function is to block the disk solar light diffracted by EO which could be a strong source of noise for the system. In addition, M1 forms an image of the aperture stop on the plane of M2: this image, potential source of stray light inside the instrument, is collected by a light trap behind M2, acting as the so-called Lyot stop. The expected stray light level on the detector is given in Tab. 1.

The coronal radiation reflected by M2 impinges the filter wheel. The filter wheel mechanism (FM) accommodates two filters: a narrow band multilayer filter (HF) optimized to transmit the HI 121.6 nm line and to reflect the visible light (VL), and an aluminum low pass filter (HeF) to select the HeII 30.4 nm line. With HF inserted, the UV HI 121.6 nm corona and the VL K-corona are imaged simultaneously, respectively on the UV detector (UVD) and on the VL detector (VLD). With HeF inserted, only the EUV HeII 30.4 nm is imaged on the UVD. In the VL channel, before reaching the detector after the filter reflection, the radiation is also processed by an achromatic polarimeter (PL) for measuring the linear polarized brightness.

COR has the capability of being operative also during spacecraft off-pointing by means of a mechanism that inserts an additional external occulter system, called extended external occulter (EEO). The latter is a five disks system large enough to prevent sun disk light from entering the coronagraph aperture stop also when the S/C is off-pointed by 1.25°. In case of off-pointing, COR FOV will not be symmetrical with respect to Sun center.

The VLD considered for COR is a custom developed Active Pixel Sensor (APS). The choice of an APS over a CCD is driven by the harsh radiation environment of Solar Orbiter. Until recently, the readout noise of APS detectors was well above the VLD requirements needed for this application; however, now a readout noise of 10 e^– is in the current technological capability of European companies. Due to the relatively long acquisition frame period, the APS has to be operated at low temperatures (of the order of –40°C) and therefore must be equipped with a thermo-electric cooler device and a suitable radiator.

The COR-UV detector works in photon counting mode and will be an intensified APS (IAPS). A description of the intensified detectors used on METIS is given in Sect. 3.

The optical performance of COR is given in terms of spatial resolution on the focal plane for all three channels in Fig. 3 The effect of diffraction due to the vignetting of the entrance pupil by the external occulter is also evaluated and taken into account.

Fig. 3.

COR spatial resolution as a function of heliocentric height for all three wavelength bands. The curves include the optical aberrations and an estimate of the diffraction.

To estimate COR efficiency, the effective areas in the three COR channels have been calculated, taking into account the vignetting function, the mirror coatings, the filter transmission and detector efficiencies. The COR count rate (see Fig. 4) has then been estimated by means of the coronal radiances values of the K-corona polarized brightness and HI Ly-α obtained at solar minimum with SOHO LASCO and UVCS [5,6], whilst the HeII Ly-α count rates are derived from modeled radiances. For K-corona PB images, the statistics required for achieving 1% accuracy is at least 10⁴ counts; for UV/EUV images the accuracy required is 10%, corresponding to at least 10² counts.

Fig. 4.

Count-rates for observations from 0.22 AU. Top: polarized visible-light K-corona. Bottom: count rates for observations of the HI 122 nm and HeII 30 nm coronal emission.

2.2.

EUS: an EUV disk spectrometer

EUS is the component of the METIS EUV disk spectrometer which includes the telescope and all the related mechanisms. Since EUS instantaneous FOV is rather small, a rotation of the EUS telescope around an axis passing through its vertex is foreseen to scan the sun disk along the East-West direction in order to cover a large part of the sun disk. By means of another mechanism, a mirror can be inserted on the EUS optical path to image selected portions of the solar corona, instead of the disk, on the spectrometer entrance slit. This allows to perform spectroscopic measurement also on the corona, and not only on the Sun disk. In both cases, after the entrance slit, light enters the SOCS element, which includes the dispersive element and the detectors of the METIS spectrometer. The adopted two-element design allows to maintain a high system efficiency since the number of reflections is reduced to the minimum.

A schematic of the EUS element is shown in Fig. 5, and its optical specifications are summarized in Tab. 2. Light from the sun disk passes through the METIS aperture on the S/C sun shield, through a cut-out on M0, and reaches the EUS telescope (ET), an off-axis parabola. Just behind ET, a heat trap (HT) is located. In fact, to reduce the heat load entering the instrument, the mirror is made of high transparent quartz coated with a thin layer of either B4C or SiC [7]: in this way, the mirror mainly reflects the EUV light of interest, but at the same time transmits the majority of visible and IR light arriving from the sun disk. The latter is the radiation bringing the higher thermal load, which is in this way collected by HT and then suitably dissipated. On the plane where ET forms the image of the solar disk, different entrance slits (S) can be placed by a suitable mechanism. From here, the radiation reaches a toroidal grating (TG) with variable-line-spaced grooves (TVLS grating) which gives a stigmatic spectrum of each of the points along the slit height on the focal plane. The TVLS grating design has been found to provide excellent off-axis imaging in configurations with magnification higher than unity, permitting high-quality performance in compact instrument packages [8,9]. Three bi-dimensional micro-channel plate (MCP) photon counting detectors (D) (see Sect. 3) are aligned on the spectral curve to acquire simultaneously the imaging spectra on six spectral intervals in the 1^st and 2^nd diffracted orders. We also expect observing third order lines on detector 3.

Fig. 5.

Schematic of the EUS optical path inside METIS.

Tab. 2.

EUS optical specifications.

Aberrations are very low over all the spectrum, allowing to obtain a spatial resolution limited by the detector within 30 (instantaneous) x 17 (rastered) arcmin² FOV. Outside this FOV aberrations slightly increase, remaining anyway smaller than 3 pixels at the extremes. EUS efficiency has been estimated taking into account the reflectivity of a B₄C coating, the grating diffraction efficiency and the detector efficiency. The reflectivity of ET is assumed to be 15% less than standard B₄C [10,11] reflectivity to account for the loss due to the required “thermal” transmitting capability of this mirror. Moreover, a conservative 50% peak diffraction efficiency of the grating has been considered for safety. The photocathode selected for the photon counting IAPS detectors is KBr [12,13]. However, the visibility of 2^nd and 3^rd order spectral lines is definitely compromised unless the strongest 1st order contribution is reduced at the detector stage. Since the efficiency of the bare MCP is relatively high at short wavelengths, in order to enhance the contrast between 2^nd and 1^st order lines it has been decided to use a detector with a partially KBr coated and partially bare MCP: second order lines will be observed on the bare MCP portions, while first order lines will be observed on the KBr coated portions to reduce blending between orders. Moreover, since the radiance on the HI Ly-α line at 121.6 nm is 2-4 orders of magnitudes higher than that of the other lines on the same spectral channel, the portion of the MCP on which the HI Ly-α line falls will be uncoated, thus reducing the contrast by a factor ≈20. In addition, the HI Ly-α flux will be further reduced by a suitably located 10% transmission mesh, as successfully adopted in SUMER [11]. To summarize the global efficiency performance of EUS, its estimated effective area is schematically shown in Fig. 6.

Fig. 6.

EUS effective areas for the different MCP coating options. The gray areas correspond to the portions of the spectrum covered by the detectors.

2.3.

SOCS: the Solar Orbiter Coronal Spectrometer

SOCS is essentially an independent box which includes the dispersive system and the detectors of the METIS spectrometer. It consists of a fixed array of slits, a TVLS grating and a set of three independent detectors mounted on suitable positions to collect three different interesting portions of the solar spectrum. SOCS allows to perform the spectral analysis of both the Sun disk and corona. A multi-slit system has been implemented in order to allow simultaneous spectroscopic observations of different portions of the corona,. Essentially, in this case, there are four different slits at the SOCS entrance slit plane, which allow to analyze four different portions of the solar corona at different solar heights. Each slit produces its own spectrum, which is then collected by the SOCS detectors.

A schematic of the SOCS element is shown in Fig. 7, and its optical specifications are summarized in Tab. 3. Light from the solar corona passing through the METIS aperture is collected by the insertable SOCS telescope (ST), a low resolution off-axis parabola, and an image is created at the entrance plane of the spectrometer where the field stop with four slits (S) is located. Coronal light passing through the slits is then diffracted by the TVLS grating (TG); the latter creates stigmatic spectral images of the different slits on the same detectors used by EUS. The angular separation among the slits is such to bring the spectral lines of interest on the coated areas of the detector MCPs, to have the highest detection efficiency. The positions of the spectral lines on the three detectors are shown in Fig. 8.

Fig. 7.

Schematic of the SOCS optical path inside METIS.

Tab. 3.

SOCS optical specifications.

Fig. 8.

Multiple spectra on the detectors provided by the multiple slit system. Top: channel 1, 1^st order OVI 103.2-103.7 nm doublet; four slits visible. Mid: channel 2, 1^st order HI Ly-α 121.6 nm; four slits visible. Bottom: channel 3, 5^th order HeII 30.4 nm; two slits visible. The grey portions show the uncoated areas on the detector MCPs.

Since the areas illuminated on TG for on-disk and coronal observations are different, a coating different from B4C, which is optimal for disk spectroscopy but not for the observations in the corona, can be adopted. Hence, for both ST and the portion of TG invested by coronal light, a multilayer coating optimized for the 30.4 nm HeII line and with high reflectivity also at 103.2-103.7 nm and at 121.6 nm [14,15,16] has been selected. As an example, the reflectivity of an optimized Mo-Si multilayer coating is shown in Fig. 9.

Fig. 9.

Reflectivity curve of a Mo-Si multilayer mirror. Parameters: 16.5 nm period, gamma 0.85, 25 periods, SiO₂ cap-layer.

Spatial and spectral aberrations are in this case not negligible, and are the actual limit to the SOCS performance. Spatial aberrations are almost constant within the instantaneous FOV and depend on the slit: they are about 20 arcsec for slit #4, and increase up to 60 arcsec for slit #1. Spectral aberrations are equivalent to about 18, 20 and 5.3 pm for channels 1, 2 and 3, respectively.

The effective area of SOCS has been estimated taking into account the multilayer coating reflectivity, the grating diffraction efficiency, and the vignetting function. The grating peak diffraction efficiency has been assumed to be 50%, that is well within the present capabilities of the grating manufacturers. The multilayer reflectivity has been assumed to be 0.25 @ 30.4 nm and 0.30 @ 103.2 and 121.6 nm, accordingly with present measurements on Mo-Si coatings. The vignetting function depends on both the angular distance from the Sun and the off-axis observation angle, introducing a variation in the effective area of approximately a factor 3. The obtained values of the SOCS effective area range from 2-3 mm² for channel 3, to 2.5-7 mm² for channel 1, to 3-10 mm² for channel 2.