2.1

Metis optical design

The 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 (R_s), at minimum perihelion distance (0.28 AU), and from 2.8 R_s to 5.5 R_s, 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.

Figure 1.

Metis layout. The raytraces of the two channels are depicted separately. On the top the UV channel and on the bottom the VL one with a zoom on the VL polarimeter assembly.

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 1

Summary of Metis instrument optical characteristics.

2.2

Estimated UV and Visible-light imaging path optical performance

The 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.

Figure 2.

VL theoretical diffraction RMS spot size.

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.

Figure 3.

Metis UV channel performance. (a) Geometric spot diagrams over the FoV both ordinary and extraordinary rays have been depicted; the square boxes have 60 micron size equivalent to 2x2 pixels of the detector. The Airy disc is also drawn for comparison. (b) Geometrical RMS spot radius versus FoV compared with the diffraction limited RMS spot radius and with the detector equivalent pixel size.

2.3

Metis vignetting function

The 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).

Figure 4.

In (a) opto-mechanical subsystems of Metis. In (b) picture of the flight model of the internal occulter mechanism with the four-spider system painted in black clearly visible [2].

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.

Figure 5.

Dependence of the Metis vignetting function on the radial field of view. (a): Gray scale representation of the bidimensional vignetting function, as obtained by ray-tracing; overlapped dashed circles represent 1° (red), 2° (orange), and 3° (blue) FoVs. (b): plots of the vignetting function along the radial directions shown in (a) as a function of the different colours.

3.1

UV channel performance

For 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).

Figure 6.

(a) UV image of α Leo, the bright spot on the top, acquired with masking and radialization. (b) UV PSF measured in-flight using α Leo.

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).

Figure 7.

UV PSF measured on-ground. In (a) appearance of the acquired pinhole images, in (b) measured radial and tangential angular resolution and FWHM of the PSF.

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.

3.2

VL channel performance

During 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.

Figure 8

Example of a VL image acquired during the IT-6B1. The identified stars are highlighted and named.

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 5^th 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 2

Summary 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.

Figure 9.

Composite VL image. In the insets, zooms of some of the star images.

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).

Figure 10

In (a) PSF FWHM (radial, tangential and mean) measured for δ1 Tau, in (b) for ɛ1 Tau.

Figure 11

VL channel PSF measured on-ground.

3.3

Spatial scale

For 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.