A.

METIS optical design

The METIS instrument is conceived to image the solar corona from a near-Sun orbit in three different spectral bands: the EUV narrow band HeII Lyman-α at 30.4 nm, the UV narrow band HI Lyman-α at 121.6 nm, and the polarized broad-band visible light (590 – 650 nm). It also incorporates the capability of multi-slit spectroscopy of the corona in the UV/EUV range at different heliocentric heights.

The annular Field of View (FoV) covered by METIS telescope ranges between 1.4 and 3.0 solar radii, when the spacecraft is at the perihelion, at 0.28 AU; the attained scale factor is 20 arcsec per pixel.

METIS is an externally occulted coronagraph which adopts an “inverted occulted” configuration [4][5]. The Inverted External Occulter (IEO) is a small circular aperture on the spacecraft Sun facing thermal shield. The disk-light passing through the IEO is rejected back by a spherical heat-rejection mirror (M0). The coronal light, on the other hand, is collected by an on-axis Gregorian telescope. To allocate the spectroscopic part, one portion of the secondary mirror, M2, is covered by a grating (i.e. approximately 1/8 of the solar corona will not be imaged). Fig. 1 shows a schematic layout of the METIS “inverted occulted” coronagraph.

Fig. 1.

METIS telescope layout.

The suppression of the light diffracted off the edges of the IEO and M0 is achieved, respectively, with an Internal Occulter (IO) and a Lyot Stop (LS) [6][7].

METIS consists of a single optical head which incorporates 3 different sub-channels: UV and EUV imaging sub-channel, in which the UV and EUV light paths have in common the detector and all of the optical elements but a filter; the polarimetric Visible Light (VL) sub-channel which, after the telescope optics, has a dedicated relay optics and a polarizing unit; and the spectroscopic sub-channel, which shares the filters and the detector with the UV-EUV imaging one, but includes a grating instead of the secondary mirror.

B.

UV/EUV and Visible-light imaging paths

The coronal light gathered by the METIS Gregorian telescope is divided in the different UV, EUV and VL paths through the filters mounted on a filter wheel mechanism. The filter wheel is inclined at 12° with respect to the telescope optical axis and it is inserted in the converging beam exiting the M2 mirror. The filter wheel hosts:

The primary (M1) and secondary (M2) telescope mirrors are coated with multilayer (ML) coatings which are optimized to enhance the reflectivity for the narrow bandpass at 30.4 nm [8]; those ML coatings have good reflectivity also in the UV and visible-light bands [9].

Inside the polarimetric path [10], see Fig. 2, a broad band filter selects the VL bandpass (590-650 nm). The VL polarimeter sub-channel includes a polarization modulation package (PMP) with a liquid crystal variable retarder (LCVR) [11] together with a fixed quarter-wave retarder and a linear polarizer in “Senarmont” configuration. The PMP is placed inbetween a relay optics system that collimates, through the PMP, the linearly polarized VL from the K-corona and refocuses it on the VL detector.

Fig. 2.

METIS VL polarimetric path layout.

C.

Spectroscopic path

The METIS spectroscopic path is shown in Fig. 3. It is obtained through the insertion of two optical elements: a multi-slit block in the focal plane of M1 and a grating in front of M2 [12].

Fig. 3.

METIS spectroscopic path layout.

The slit block is composed by three slits positioned at 1.5°, 1.8°, 2.1° FoV; the grating is a spherical varied line-spaced (SVLS) diffraction grating which diffracts the HI 121.6 nm line at 1^st order and the HeII 30. 4 nm line at the 4^th order.

Concerning the optical performance at least 50% of the energy is enclosed within 2 pixels for each of the slit in the direction of the spectral dispersion. With regards to the spatial resolution, it is limited for each of the slit respectively to 2, 3, 4 pixels.

A summary of the METIS instrument characteristics and performance are presented in TABLE I.

TABLE I.

SUMMARY OF METIS INSTRUMENT CHARACTERISTICS

The estimated optical performance of the EUV, UV and VL path, including the diffraction effect, is shown in Fig. 4.

Fig. 4.

METIS telescope performance.

A.

Telescope Error Budget

The telescope tolerance analysis has been done at the focus of the UV/EUV configuration and at the intermediate focus of the VL configuration, i.e. the telescope focus.

In TABLE II the ‘manufacturing and alignment’ tolerances are reported; the manufacturing tolerance includes also the effect due to the element mountings (for example the deformation induced on the mirror surface due to its mounting).

TABLE II.

TELESCOPE MANUFACTURING AND ALIGNMENT TOLERANCE

The long term and short term tolerances are reported respectively in TABLE III and TABLE IV.

TABLE III.

TELESCOPE LONG TERM STABILITY TOLERANCE

TABLE IV.

TELESCOPE SHORT TERM STABILITY TOLERANCE

For the alignment and integration phase, the position of the detector has been considered as a compensator, while for the VL path, the position of the whole polarimetric module has been considered as a compensator.

As for the filter, the tolerance analysis considers both the tolerance on the positioning of the filter inside the filter wheel frame (F) and the tolerance on the position of the whole filter wheel frame (FWF).

B.

Visible Light path

VL path tolerances analysis is divided in two parts: element analysis (i.e. decenter and tilt of the lenses/doublets) and surfaces analysis (i.e. decenter and tilt of each lens/doublet surface).

The results of the analysis are summarized in TABLE V; where the tolerances have been divided considering the optical elements composing the relay collimating optics and the PMP: the first collimating doublet (Din), the PMP itself, the out doublet (D_out) and the focusing lens (FL).

TABLE V.

VL PATH TOLERANCE

C.

Spectroscopic path

The characteristic and performance of the spectroscopic path are different from those of the imaging paths, so a different acceptable degradation has been considered.

In particular, the tolerances corresponding to a maximum acceptable degradation of 50% (encircled energy) along the spectral direction, and to a decrease of 20% of the flux passing through the first inner slit have been calculated. Also, it has been assumed a possible error of 1% in the grating radius of curvature.

The spectrum is shifted by 0.25 mm, equivalent to 10 px, if the grating is rotated by 1 arcmin in x or y and by 10 arcmin in z (for the axes orientation see Fig. 5).

Fig. 5.

Detail of the spectroscopic path with the adopted reference system.

The tolerance analysis for the spectrometer path includes also the position tolerance analysis of the slit block.

The spectroscopic path tolerances are summarized in the following table (TABLE VI).

TABLE VI.

Spectroscopic path tolerance

D.

IEO and IO tolerance analysis

The IO and the LS block the diffracted light produced respectively by IEO and M0 edges. Since the IEO and M0 edges are conjugated, via the M1 mirror, with IO and LS, their blocking properties are directly related to the M1 tolerance.

For the LS an appropriate oversizing of the element is sufficient to deal with the problem, considering that the position of this element is not particular affected by M0 shift and M1 rotation. For IO instead, which is more critical, an appropriate mechanism is foreseen to reposition it during the flight.

IO and IEO tolerances for the integration phase on ground and for the stability long term and short term in-flight are summarized in TABLE VII.

TABLE VII.

IEO AND IO TOLERANCES (‘MANUFACTURING AND ALIGNMENT’, LONG AND SHORT TERM STABILITY).