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1.INTRODUCTIONSolar Orbiter (SO) is a mission dedicated to study solar and heliospheric physics[1]. It was selected as the first medium-class mission of ESA’s Cosmic Vision 2015-2025 Programme. With a combination of in-situ and remote sensing instruments, SO is conceived for the circumsolar region exploration with the purpose of giving an answer to the scientific questions on how the heliosphere is generated and controlled by the Sun. Launched in February 2020, the mission will provide close-up, high-latitude observations of the Sun. SO will have a highly elliptic orbit (0.9 AU at aphelion and 0.28 AU at perihelion). Thanks to its mission profile, for the first time the poles of the Sun and the circumsolar region will be seen and studied from a privileged point of view near the Sun. Soon after launch SO underwent the Near Earth Commissioning Phase (NECP), the spacecraft and the payload were functionally checked and calibrated. Also the Metis coronagraph [2], which is one of the remote sensing instruments mounted on-board the SO spacecraft, was switched on, functionally tested and characterized [3]. In particular, the two mechanisms of the instrument, the one-shot sealing cap and the internal occulter mechanism (IOM), were activated. Metis has been conceived to acquire both visible and UV images of the solar corona. It has two channels: one to study the UV HI Lyman α at 121.6 nm, and the other for the polarized visible light band (580 – 640 nm). Metis adopts a configuration called ‘inverted externally occulted’ that is able to prevent the solar disk light to enter the telescope; this light is back reflected outside by a mirror (M0) [4]. 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 mirrors have annular shape. The on-axis field is not available since it is anyway completely blocked by the M0 mirror; for the alignment and test on-ground an ad hoc alignment set-up has been developed for the telescope alignment [5]. In this paper, after a description of the Metis optical design, the preliminary measured telescope performance in-flight will be presented and compared to the one obtained during the on-ground calibration campaign [6]. 2.METIS OPTICAL DESIGN AND PERFORMANCE2.1Metis optical designThe Metis instrument is conceived to image the solar corona from a near-Sun orbit in two different spectral bands: the UV narrow band HI Lyman-α at 121.6 nm, and the polarized broad-band visible light (580 – 640 nm). The instrument is designed to image the structure and dynamics of the full corona in the range from 1.6 to 3.1 solar radii (Rs), at minimum perihelion distance (0.28 AU), and from 2.8 Rs to 5.5 Rs, at 0.5 AU. The annular Field of View (FoV) covered by the instrument is between 1.6° and 2.9°, and the attained spatial resolution is 20”. The “inverted externally occulted” configuration (see Figure 1) [7][8] allows to reduce the thermal loads inside the instrument. The inverted external occulter (IEO), a small aperture on the spacecraft thermal shield, also acts as the entrance pupil of the instrument. The solar disk light passing through the IEO is back-reflected by a spherical heat-rejection mirror (M0) and focused on the IEO itself. The coronal light, on the other hand, is collected by an on-axis Gregorian telescope. The primary mirror (M1) focuses the gathered coronal light in its focal plane, where a field stop (FS) is placed and limits the outer FOV of the coronagraph. Then the secondary mirror (M2) focalizes the light again; part of the light passes through the interference filter (IF) and then is collected by UV detector; the other part is reflected by the IF and is directed to the polarimeter assembly. Stray light minimization is a mandatory goal in any coronagraph and the worst offenders are the surfaces hit by the direct solar disk light, IEO and M0. The internal occulter (IO) and the Lyot stop (LS) are, respectively, conjugated with the IEO and M0 via the primary mirror M1. IO blocks the solar disk light diffracted by the IEO edge, while LS blocks the light diffracted by the M0 edge [9][10]. Metis consists of a single optical head that incorporates 2 different channels: a UV imaging channel and a polarimeter visible light (VL) channel. The telescope optics (M1 and M2) and the stop system (IEO, M0, FS, IO and LS) are common to both channels, while the VL channel has a dedicated relay optics and a polarizing group. Each channel has its own detector respectively called UVDA and VLDA. The Metis instrument optical specifications are summarized in Table 1. Table 1Summary of Metis instrument optical characteristics.
2.2Estimated UV and Visible-light imaging path optical performanceThe coronal light collected by the Metis Gregorian on-axis telescope is divided in two different paths: The UV light detector assembly (UVDA) consists of three main elements: (a) an intensifier with KBr photocathode in optical contact (via a phosphor screen) with (b) a fiber optics 2:1 de-magnifying coupler connected to (c) a CMOS APS STAR1000 (1k x 1k format, 15 μm square pixel). So the equivalent pixel size at the focal plane for the UV channel is 30 μm. The visible light detector assembly (VLDA) consists of a visible light camera equipped with a CMOS active pixel sensor (APS), 2k x 2k, 10 μm pixel [2]. The two different UV and VL paths are separated through a UV interference filter (IF), inserted in the converging beam exiting M2, and inclined at 12° with respect to the optical axis of the telescope. The UV narrow bandpass interference filter acts as VL-UV dichroic beam splitter by selecting the 121.6 nm UV band in transmission and reflecting the VL. The mirrors are coated with Al+MgF2 with high reflectivity in the VL band and optimized to have an enhanced reflectivity at 121.6 nm [11] [12]. The VL path aberrations are rather limited and the theoretical performance is diffraction limited. See Figure 2 for the expected VL channel performance in terms of RMS spot radius, both the tangential (tg) and radial (r) RMS have been calculated. Tangential and radial refer to the solar corona. The UV path performance is not radially symmetric, the IF is a thick 6 mm tilted plate placed in a converging beam, thus some aberrations (mainly astigmatism and coma) are introduced by the filter [13]. To mitigate these aberrations, the IF surfaces are not parallel but tilted by about 0.15°. The IF is made of a birefringent material, another birefringent window is mounted in front of the detector. The theoretical optical performance of the UV channel, including an estimation of the diffraction effect, is shown in Figure 3. In Figure 3(a) the spot diagrams over the FoV are shown; the different colors refer to ordinary and extraordinary rays for both the IF and the detector window. In Figure 3(b) the geometrical RMS spot radius calculated at the UV paraxial focus for both the tangential and radial direction is compared with the diffraction RMS and with the equivalent pixel size of the UV detector. The UV pixel equivalent size considered is 15 μm, i.e. half of the UV detector equivalent pixel size. 2.3Metis vignetting functionThe light passing through the Metis telescope body is highly vignetted by M0 and then vignetted further by the IO and LS. Moreover, on the diagonals of the image, the two four-spider systems supporting respectively the M0/Lyot Stop assembly and the Internal Occulter/Field Stop assembly are vignetting the beam even more. In Figure 4(a), the whole opto-mechanical layout of Metis is presented and in the bottom right part of the picture the two Internal Occulter/Field Stop and M0/Lyot Stop assemblies with the supporting spiders are shown. The internal occulter mechanism, with the four-spider system clearly visible, is shown in Figure 4 (b). Over the nominal FoV of the telescope the fraction of unvignetted rays is always less than 50%. See Figure 5 for the Metis theoretical vignetting function given both in terms of 2D image in (a) and as radial profiles in (b). The further vignetting due to the spiders is clearly visible by comparing the profiles at 0° and 90° with the 45° and 135° ones. 3.IN-FLIGHT OPTICAL PERFORMANCEOn the 9th of February 2020, the Solar Orbiter spacecraft has been launched by an Atlas V rocket from Cape Canaveral. Soon after launch, the switch on and commissioning of the payload instruments started. Star observations with Metis are very important to characterize and calibrate the instrument, but at present only some specific sequences have been run specifically for stellar calibration. Anyway in the VL channel stars are commonly seen in the FoV since some are bright enough to stay above the coronal continuum. Some specific UV stars, useful to characterize the UV channel, have also been observed. 3.1UV channel performanceFor the UV channel α and ρ Leonis stars have been observed transiting throughout the Metis FoV from east to west approximately at the center of the frame, this direction corresponds to the Y direction of the detector. Images have been acquired using masking and radialization, an example of the images is reported in Figure 6(a). A gaussian fit has been used to estimate the FWHM of the images of the stars and the radial and tangential FWHM have been derived (see Figure 6(b)). Also a comparison with the data obtained on-ground has been done (see Figure 7a and b). The preliminary estimated UV PSF in-flight is consistent with the one measured on-ground, even if the one measured onground has been measured along the X axis of the detector, while the one in-flight is measured along the Y axis. In fact, the spots are elongated in different directions, i.e. in flight they are along the X detector direction and on-ground along the Y (see Figure 6(b) and Figure 7(a)), nevertheless the PSF radial and tangential FWHM values can be directly compared. 3.2VL channel performanceDuring the IT-6B1 activity [3], performed in mid-May 2020, some of the brightest stars of the Taurus constellation have been observed. Unfortunately, this sequence had been foreseen with the aim of doing the IO coarse alignment, and so not all of the acquired images are useful and moreover, due to the IO different position from one image to the other, the vignetting function was slightly varying over the different acquired images. Nevertheless, some of the stars have been recognized and their PSFs have been measured. In Figure 8 an example of a VL acquired image is given. Superimposed on the image the detected stars are clearly marked with a blue star symbol and the name and the visual magnitude (mV) of each star are written in green color. This image has been rotated in order to be quasi east-west (EW) oriented along the horizontal direction and south-north (SN) in the vertical direction. The red circles correspond with the inner and outer dimensions of the FoV of Metis, the inner is determined by the vignetting function while the outer by the field stop size. The red cross, almost at the center of the image, corresponds to the boresight. In Table 2 a summary of the brightest identified stars is given. The spectral types were obtained from the Simbad database [14]; in parenthesis spectral type from the Bright Star Catalog 5th Revised Edition [15]. The R magnitude is also provided when available. The R band is centered at 658 nm, and thus it is closer than the V band to the Metis VL channel wavelength band (580-640 nm). Table 2Summary of brightest stars detected by Metis during the IT-6B1 activity performed 15 May 2020
At least 5 of the acquired images have been judged worth detecting the stars and measuring the FWHM of their PSFs. A composite image has been created, after bias and dark removal, summing some of the different consecutive images acquired. The SO spacecraft is pointing the Sun, so the stars appear to move from one frame to the other approximately along the columns of the detector which are approximately oriented EW with respect to the Sun. See Figure 9 for the composite image and the appearance of the stars moving on the detector. On the whole image some zoomed insets are depicted to highlight the regions where the stars are visible. The 5 considered images were acquired respectively 30, 100, 10 and 70 minutes apart one from the other and the stars appear to move from left to right in the frame, i.e. EW. The VL channel images analyzed so far are of stars not completely well exposed and not uniformly distributed on the detector. There are hints, though to be assessed, that on the edges of the FoV the PSF is larger than expected, particularly in the region where the image vignetting is highly determined by the spiders obscuration as discussed in par. 2.3. The quite well-exposed stars analyzed, epsilon and delta Tau, suggest that the mean FWHMs, on the left side of the detector, are of the order of 1.5-2 px (see Figure 10(a) and (b)), which are in quite good agreement with the data acquired on-ground (see Figure 11). 3.3Spatial scaleFor the VL channel, having about 10 identified stars in the FoV, an initial estimation of the plate scale and pointing of the spacecraft have been done. Firstly, the positions of the identified stars on the sensor have been determined using the peak pixel, i.e. the pixel with the highest DN value, of each stellar PSF. Then a set of linear transformations has been defined mapping sensor coordinates to celestial coordinates, using the convention detailed in [16], and consisting of a rotation, a translation and a scaling, followed by a gnomonic projection. The corresponding five parameters (the angle of rotation, a pair of translation coordinates and a pair of scaling factors) were then optimized by minimizing the distance on the sensor between the expected position with the measured position of each visible star on the sensor, using a least squares fitting algorithm. The resulting scaling factors along the X and Y direction of the detector were 10.139 and 10.127”/px respectively, with a standard deviation of 0.005”/px, which are in agreement with the measurements done on-ground. The measured on-ground focal length for the VL channel after alignment is about 203.5 mm, that is slightly larger than the theoretical 200 mm one. The measured on-ground focal length corresponds to a pixel scale of about 10.13”/px. 4.CONCLUSIONSSome results on the in-flight optical performance of the Metis coronagraph have been presented. As a preliminary achievement, the PSF FWHMs of the UV channel measured in-flight are perfectly matching those measured on-ground. As for the VL channel, the present available images are not entirely useful to correctly determine the PSF behaviour all over the FoV. Future specific acquisitions will be envisaged to better characterize the system performance; in particular to better sampling the FoV with well exposed stars, to correlate the optical characteristics of one channel with the other one and to verify the relative alignment. For the VL channel, also a preliminary assessment of the plate scale has been done. The results obtained are in agreement with the measurements done during the on-ground calibration campaign. ACKNOWLEDGMENTSThe Metis program is supported by the Italian Space Agency (ASI) under the contracts to the co-financing National Institute of Astrophysics (INAF): Accordi ASI-INAF N. I-043-10-0 and Addendum N. I-013-12-0/1, Accordo ASI-INAF N. 2018-30-HH.0 and under the contracts to the industrial partners: ASI-TASI N. I-037-11-0 and ASI-ATI N. 2013-057-I.0. The Metis team thanks Barbara Negri, Enrico Flamini, Marco Castronuovo of the Italian Space Agency and Roberto Della Ceca, Giuseppe Malaguti of the Istituto Nazionale di Astrofisica for their continuous support during the development of the coronagraph. A special thanks to Filippo Marliani of the European Space Agency for his dedication to the program and his high standard of excellence. The testing of Metis was performed by an industrial consortium constituted by OHB Italia S.p.A. (acting as Prime Contractor towards ASI), Thales Alenia Space Italia S.p.A. 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