1.1

SIMBIOSYS

The MPO module is carrying instruments which are devoted to the close range study of Mercury surface, to the investigation of the planet gravity field, of the inner magnetosphere and of the exosphere. Imaging and spectral analysis are performed in the IR, visible and UV range. These optical observations are complemented by those of gamma-ray, X-ray and neutron spectrometers, which yield additional data about the elemental composition of the surface, and by those of a laser altimeter, BELA [5], dedicated to high accuracy measurements of the surface figure, morphology and topography.

The imaging and spectroscopic capability of the MPO modulus will be exploited by the Spectrometers and Imagers for MPO BepiColombo Integrated Observatory SYStem (SIMBIOSYS), that is an integrated system for imaging and spectroscopic investigation of the Mercury surface. A highly integrated concept is adopted to maximize the scientific return while minimizing resources requirements, primarily mass and power [6].

SIMBIOSYS incorporates capabilities to perform 50-110 m spatial resolution global mapping in both stereo mode and color filters, high spatial resolution imaging (5 m/px scale factor at periherm) in panchromatic and broad-band filters, and imaging spectroscopy in the spectral range 400-2000 nm. This global performance is reached using three channels: STC [7], the STereo imaging Channel; HRIC, the High Resolution Imaging Channel [8]; and VIHI, the Visible and near-Infrared Hyperspectral Imager [9].

2.1

STC scientific requirement

The main scientific requirement for the STC is to provide the global panchromatic coverage of the surface and secondly to study selected areas in predefine spectral bands [10]. Both imaging tasks should be accomplished with a mean scale of 110 m/px (minimum of 50 m/pixel at the equator). The spectral bands have been chosen with the aim of studying surface composition and defining the main geological units. The STC stereo channel will allow to analyze the morphology of the tectonic features, the impact craters and the volcanic edifices. The STC will also provide the context for the HRIC investigation.

A stereo imaging system has to satisfy a series of scientific requirements linked not only to common optical performance definition, but also to the additional constraints imposed by photogrammetry.

The overall camera requirements are reported in Tab. 1. The scale factor and the swath at periherm have been defined to be able to map the whole Mercury surface during the mission lifetime. For reaching the required 80 m vertical accuracy in the DTM, a standard satellite stereo imaging solution [11,12] has been chosen, with two panchromatic sub-channels looking at ±20° from nadir. The required optical performance of STC is expressed in terms of the Ensquared Energy (EE) inside one pixel, or alternatively in terms of the Modulation Transfer Function (MTF) at the detector Nyquist frequency.

Tab. 1

STC scientific requirements.

2.2

STC optical design

The design of the camera has been driven not only by the just described scientific requirements but also by the aim of saving mass and power. In this attempt, an optical solution in which the detector and most of the optical elements are shared by the two channels has been studied. The design has been kept as short as possible, compatibly with the need of limiting the cross-talk between the two channel and to cope with stray-light problems due to the common optics choice. The desired 50 m/px scale factor at periherm is reached with a 90 mm effective focal length, considering the 10 microns pixel size of the Si_PIN hybrid detector chosen. This kind of detector is particularly useful both in terms of radiation hardness, given the hostile Mercury environment, and for the capability of snapshot image acquisition, which is less demanding in terms of S/C pointing and stability.

The adopted telescope optical solution (see Fig. 2) is a novel design, in which a couple of rhomboid prisms redirects the ±20° wide open beams along directions much closer to the system optical axis. After the two prisms, a modified Schmidt telescope follows, in which a correcting doublet positioned at about half distance between the spherical mirror M1 and its center of curvature replaces the classical Schmidt correcting plate. In this way, the telescope length is reduced by about a factor of two with respect to the classical solution. Finally, a field correcting system has been included just in front of the detector.

Fig. 2

STC overall optical design layout. The inset shows a different projection of the focal plane region.

The nominal FoV of each sub-channel is 5.3° × 4.5° subdivided in 3 narrow strips, one for each filter, covering three quasi-contiguous areas on Mercury surface (see Fig. 1); the scientific useful FoV is actually slightly smaller, 5.3° × 3.2°, imposed by the filter strip solution choice. At periherm, each panchromatic strip corresponds to an area of about 40 × 19 km², and each colored strip to an area of about 40 × 3 km².

The optical characteristics of the camera are summarized in Tab. 2.

Tab. 2

STC optical characteristics.

Simulation and optimization of the camera design have been done by means of Zemax ray-tracing software.

The design has been chosen to satisfy the desired optical performance for all the filters in the whole FoV of each filter. Being the wavelength range rather extended, the on-axis and lateral chromatic aberrations tend to be the worst offenders in the optimization of the system. Given the high radiation dose to which the optical elements will be exposed to, the choice of the glasses has to be restricted to rad-hard ones, and to simplify the procurement all the lenses are foreseen in fused silica.

The off-axis value of the telescope has been driven by the free back focal length, that has to be sufficient to easily integrate the Focal Plane Assembly, maintaining the required optical performance.

The correcting doublet has an essentially null optical power, and its function is to correct the residual aberrations of the primary mirror. Being the doublet optical power near to zero, the residual chromatic aberration in terms of primary and secondary colors is negligible over the whole 410-930 nm spectral range.

The Aperture Stop (AS) position, placed in the front focal plane of the spherical M1 mirror, just after the correcting doublet, has been chosen to allow a good aberration balancing over all the FoV and to guarantee the telecentricity of the design. In fact, being the filter window placed in the converging light beam near the detector, to avoid wavelength shifts due to non orthogonal incidence of the beam on the filter itself, the camera design has been kept as telecentric as possible.

To cope with the field dependent aberrations (i.e field curvature, lateral color,..) a two-lens field corrector has been designed to be placed in front of the detector (see Fig. 2). In addition the solution chosen for the filter manufacturing, i.e. a single optical element composed of stripe-butted filters, allows a better control of the chromatic aberration in the single FoV of each filter.

The rhomboid prism couple, as already mentioned, redirects the ±20° inclination with respect to nadir of the incoming beam to a much smaller ±3.75° one, allowing a much simpler system optical design. These prisms can be made of either BK7G18 or Fused Silica radhard glasses; the reflection of the beam inside the prism takes advantage of the total internal reflection at the glass-air interface.

2.3

Stray-light control

From the point of view of stray-light control the system is very challenging being most of the optical elements common to both sub-cannels. The useful optical beam of the two sub channel has been kept as far apart as possible within the allocated volume, including the choice of using an inclination as high as possible on the beam exiting the prism and directed toward the M1 mirror. In this way, the internal edges of each sub-channel radiation beam are well separated, about 4 mm on the detector active surface.

To prevent cross-talk between the two paths of the two sub-channels, a suitable internal and external baffling system, including baffles at the entrance and exit of the prism and in the middle of all the optical elements, has been designed. A good means to control the stray-light is given by the position chosen for the AS, just after the first correcting doublet: the majority of the stray-light coming from the preceding optical elements, is blocked at this point.

Simulations done with Opticad software show that no direct cross-talk between the two subchannels is present. Moreover, it has been verified that the vignetting function is sufficiently steep to maintain totally decoupled the two optical paths even if the closest distance between the respective images on the common detector is of only 4 mm.

Also the in-field stray-light has to be taken into account and assessed. The main contribution to the infield stray-light is given by the ghost images coming from the filter and the detector surface beam reflections. The relatively high inclination (~23°) of the beam with respect to the filter and detector normals keeps the ghosts far from the primary useful image, but, owing to the filter “spatial” extension small portions of the ghosts fall anyway over detector useful areas. A correct choice of filter position and size in the filter strip stack helps to reduce the phenomenon.

A preliminary analysis of the ghost image intensities has been done on the basis of the expected reflective properties of filters and detector, and of the spectral distribution of the incoming flux. The results show that the stray-light level coming from the ghost images can be controlled if the panchromatic filter is positioned at the center of the filter stack and if an appropriate filter masking is foreseen. One, or two, masks on the filter strip assembly can stop the ghost beam coming from each filter strip and prevent it to be reflected on the adjacent one. In that case the ratio between the ghost image intensity and the useful image one can be less than 10^–3; otherwise, if the mask is not realized, in the worst case ghost intensity ratios as high as 10^–2 can arise.

A preliminary stray-light analysis also for light sources outside the STC FoV has been performed, based on the expected properties of the coating for the internal and external baffle vanes and the optical surface scattering and reflectivity properties.

The primary source of stray-light outside FoV is the Mercury surface itself: in the worst case, at periherm, the surface of Mercury covers a FoV of about 120° in diameter. This stray-light contribution is mainly blocked by the suitably sized external baffle and the AS; its level is reduced to a value smaller than the noise appreciable by the detector.

In fact, the results of the simulation of the system show that a point source outside the FoV, for directions which are inclined more than 30° with respect to the sub-channel boresight direction, gives a relative stray-light signal smaller than 6 10^–9. For sources in the range between the edges of the FoV and the 30° inclination, the relative spurious signal is lower than 5 10^–5.

Integrating over all the surface of Mercury, the global stray-light value on one pixel, compare to the useful signal measured for one pixel extended element source on the Mercury surface, is about 10^-4. This value is low if compared to the noise expected from the other sources (i.e. electronics noise, photonic noise,..).

2.4

STC performance

The performance of the camera has been calculated for all the filters in the whole FoV; spot diagrams and EE have been considered.

The spot diagrams and the relative EE for the panchromatic filter are shown respectively in Fig. 3 and Fig. 4. It can be seen that the spots are well within the overlaid box having the 10 µm square pixel size of the foreseen detector; a small lateral color residual is present in one corner of the field, but it has been verified to be less than 1/3^rd of a pixel, that is completely tolerable.

Fig. 3

Spot diagram for the panchromatic filter. Spot at the center, corners and edges of the FoV are shown for minimum (600 nm), center (700 nm) and maximum (800 nm) wavelength in filter band.

Fig. 4.

EE for the panchromatic filter. Different lines correspond to different points in the FoV (center, edges, corners). The values are a mean over the wavelength band of the filter.

The mean diffraction EE has been calculated over all the FoV of each filter and over the wavelength band of the filter itself. The EE including diffraction effects is of the order of 80% all over the FoV of each filter (5.3° × 2.4° panchromatic; 5.3° × 0.4° color filter); it is a few percent smaller only for the filter centered at 420 nm, where some chromatic focal shift is present.

Also the MTF of the optical system has been derived for all the filters in the whole FoV. The mean MTF, at the Nyquist frequency of 50 cycle/mm, is of the order of 60-70%. As an example, the mean MTF for the panchromatic filter is shown in Fig. 5.

Fig. 5

MTF for the panchromatic filter. Different lines correspond to different points in the FoV (center, edges, corners). The values are a mean over the wavelength band of the filter.

Considering that a reasonable value for the detector MTF is 50-60%, the global MTF of the system, including detector sampling, is of the order of 30 %.

Finally, a preliminary analysis of the camera distortion has been done. Thanks to the adopted Schmidt design and to the position chosen for the AS, the distortion effect is rather limited, being less than 0.3% for all the filter. As an example, the distortion effect for the panchromatic filter is shown in Fig. 6: to clearly see the deformation pattern, the differences between predicted (underlying grid) and real (‘×’) positions of the chief rays have been multiplied by a factor of twenty; for the considered panchromatic filter FoV, which corresponds to about 8 × 4 mm² on the detector, the distortion is about –0.2%.

Fig. 6

Grid distortion for the panchromatic filter. Note that real chief ray displacements from theoretical values are multiplied by a factor of twenty to make the distortion effects clearly visible.

2.5

Exposure time

The choice of the exposure time for the images will be derived not only by the intensity of the scene the system will be looking at, but also by the need to prevent the smearing of the image on the pixel due to S/C motion during the acquisition time. The acquisition philosophy is push-frame, or matrix scanner, imaging, in which the composite surface image projected on the detector will be firstly acquired, then buffered and read while the S/C moves; only when the image on the detector has been shifted along track by an amount corresponding to the FoV of each filter, another composite image will be acquired.

Since the S/C velocity at periherm is about 2.2 km/s, this implies that the exposure time has to be less than 5 ms to assure that the image is not smeared, (during 5 ms the surface, imaged on the detector, shifts of about one fifth of a pixel); the repetition time is of the order of 2-5 seconds depending on the position of the S/C during the orbit.

To compute the expected flux and the relative convenient exposure time, a radiometric model which considers the expected properties of the camera (optics and filter transmission and reflectivity properties, detector quantum efficiency) and of the Mercury surface has been implemented. A mean planetary albedo of 0.12 [13] has been considered. For the color filters, an exposure time of 2 ms yields a mean expected flux per pixel of about 4 10⁴ photons, with a signal to noise ratio of 200. For the panchromatic filter, the expected mean flux per pixel, for an exposure time of 0.4 ms, is 9 10⁴ photons with a signal to noise ratio of about 300.

2.6

Tolerance analysis

In the analysis of an optical design, the tolerance budgeting is of primary importance. The tolerancing of a stereo camera is a challenging task: in fact not only the desired performance has to be reached and maintained separately for each channel, but also the combination of the two channels and their mutual orientations have to be kept as fixed as possible during all the mission lifetime; in addition, all the optical parameters relative to the DTM reconstruction accuracy have to be fully taken into account. Having in mind these considerations, a preliminary assessment of both manufacturing, alignment and stability tolerance has been undertaken.

The philosophy in tolerancing the system follows directly from the clear subdivision of the system in two part: the fore-optics unit, i.e. the two rhomboid prisms, and the modified Schmidt telescope unit.

Being the prisms positioned into the collimated incoming beam, in principle they are not introducing aberrations or focal variations; they can only be responsible for image shifts. Actually, a rhomboid prism positioned in the object space is insensitive to tilt with respect to three reference axes, and consequently it does not introduce co-registration errors between sub-channels.

For what concerns the prism manufacturing tolerances, it can be mentioned that by means of a suitable manufacturing process (for example the two prisms can be obtained starting from a double prism and cutting it at the middle), the two prisms will have the same angles and consequently possible impacts on sub-channels co-registration can be avoided. However, planarity errors between entrance and exit surfaces have to be controlled to avoid possible increase of aberrations.

Regarding the telescope unit tolerances, the adopted solution with common focusing optics is very performing to avoid, or to limit, co-registration errors and focus differences between the two channels. Possible asymmetries in the actual system can only arise by asymmetries in the realization/mounting of the optical elements. Since optical shop machines usually work spherical surface in symmetric way, the primary reasons for possible asymmetries in the system will be a different realization of the two prisms and/or different relative mounting of the lenses in the correcting doublet or field corrector.

Moreover, any perturbation caused by temperature change or accidental optical element movements have the same repercussions on the two images, in terms of amplitude and direction of the error vector.

The preliminary tolerance analysis results show that the achievable standard manufacturing tolerances of the optical shops for lenses and mirrors, (0.1-0.2% on curvature radius, 1 arcmin on surface parallelism, 10^–3 on refractive index,..), can be easily compensated. The required compensating motions of the various elements are of the order of ±0.2 mm for decentering and ±0.2° for tilt. The most critical element is the primary spherical mirror, i.e. the only element with optical power in the design, and thus determining the focal length of the full instrument. The primary mirror curvature radius value and its variation clearly propagates into focal length variation of the system.

For the stability tolerances, being the optical power of the correcting and field doublet zero, the thermal stability changes can be neglected if the primary mirror and the bench have the same thermal expansion coefficient.