A.

Optical design

Six months will be the maximum period in which global stereo mapping, first scientific goal of STC, will be completed. To reach this objective the optical design (proprieties are shown in Table 1) was conceived to reach a coverage of 40×19 km² for the panchromatic filter and 40×3 km² for the color ones (periherm configuration on equator).

Table 1

Optical and on-ground propriety of STC instrument.

Two independent elements compose the STC optical solution [4]: two folding mirrors per each channel, and a common telescope unit. The couple of folding mirrors redirects the ±20° wide beams along direction much closer to the system optical axis (±3.75°). The optical path is in common for both the channel and the rays are focused on a 10 μm pixel size SiPIN CMOS detector, useful in terms of radiation hardness and for the capability of snapshot image acquisition, allowing very short exposure times (< 1 ms).

Result of this innovative design is a light instrument which cuts down on energy and permits stereo reconstruction with a narrow focal length of 95 mm to reach the 50 m/pixel scale factor at periherm.

B.

Stereo Validation Set-up (SVS)

Considering the brand-new design adopted a pre-flight verification was needed. Base of the pipeline for the generation of the DTM of Mercury surface is the knowledge of the intrinsic and extrinsic parameters of the optical system. With this aim a stereo validation setup was realized for an indoor reproduction of the flight observing condition [5] [6]. See Fig. 2 for a scheme of the stereo validation set-up concept.

Fig. 2

Setup for STC stereo validation concept: a. STC observing Mercury surface at periherm (flight conditions). b. In laboratory simulation of STC observing with “Forward channel” (F) and “Backward channel”

Two principal problems were engaged: the need of acquire satellite images in a limited indoor space and the necessity of supply two different perspectives of a surface to use both the optical channels.

First hindrance was solved by the use of an auxiliary optical system to observe with the STC instrument not a planetary surface from an altitude greater than 400 km but a selected small stone target at about 1 meter distance: a lens (collimator) that collimates the light rays coming from the target (essentially placing it at infinity) has been foreseen. Optical analysis by means of ray-tracing software led to the choice of an achromatic doublet with focal length of 1000 mm with a diameter of 80 mm.

Neglecting for the sake of simplicity the Mercury curvature, multi-perspective acquisition was supplied by the use of two high precision rotational stages (see Fig. 3), which permit the acquisition of images from two different points of view. These stages allow to rotate the instrument and the target without the need of additional space not available in the close clean room environment. The first rotation stage, where the STC instrument (in a thermo-vacuum chamber) is positioned, has the function of aligning the optical axis of the active STC channel with the collimated light coming from the collimator lens; the second rotation stage has the function of rotating a target positioned on the collimator lens focusing plane, reproducing the angle between a pair of homologous rays.

Fig. 3

In (a) Setup for the stereo validation: mechanical layout. From left side: on the optical bench the collimator, on the rotation stage the light source and the rock sample. In (b) example of target (stone) acquisition with lamp in extreme orientation position and rotational stage on optical path of high channel in panchromatic (-21°).

With the aim of reproduce different light conditions, a narrow-spot halogen lamp coupled to a collimator has been assembled on a curve rail system. Considering that the target illumination condition has to be maintained in the two acquisitions with STC channels, the rail was mounted on the target rotational stage in order to be moved together with the target.

C.

Scientific requirements scaling

The ratio between the observable feature size (horizontal and vertical accuracy) in the laboratory images and in the in-flight ones are proportional to the ratio between the collimator focal length and the distance of the SC from the Mercury surface during the acquisition. At periherm the observing distance is about 400 km: considering a collimator focal length of 1 m the scale ratio will be 1:400000.

So, considering that the area of the Mercury surface that will be imaged by STC is approximately 40x20 km², the corresponding target area that should be imaged with this setup in the laboratory is of approximately 9x4 cm², referring to the panchromatic FoV.

Rescaling parameters of Table 1 to obtain the spatial resolution corresponding to that of STC (50 m) with a 1 m collimator focal length, the target surface should be sampled with a step of 105 μm and the elevation should be determined with an accuracy of 190 μm.

D.

Rock target definition

Scaling factor obtained permitted to choose a correct dimension for the target. To reproduce the same albedo condition of the ones attended from the Mercury surface basalt and anorthosite were chosen as composition material of the target stones.

These rock samples were acquired by scanning by a CAM2® FaroArm Platinum, a portable Coordinate Measurement Machine (CMM) with a Z axis accuracy of 0.02 mm which supplied a DTM with point spacing of 0.05 mm, i.e. about half of the image resolution [5][6].

This permits to have a reference DTM with an accuracy better than the one required by scientific requirements. DTM obtained by laser scanner is than use to compare stereo reconstruction and to define the effective validity of the instrument approach.

A.

Zhang camera calibration

A sequence of about 50 image pairs of the chessboard target has been used to compute the relative orientation (extrinsic parameters) of the camera pair for STC FM. Since the relative geometry between the cameras is nominally unchanged, such parameters can be used in the derivation of the DTM, when the stone samples are substituted for the calibration target; however, a not trivial co-registration of the reconstructed DTM with the laser measured one is necessary to compare the two surfaces.

Results of the Zhang calibration impose the rotation between the two optical axes of the channels angle are equal to 42.28° (with a relative error around 2%) and stereo base line equal to 1.23 m. These data correspond to a distance pupil-target equal to 1.59 m.

In the case of intrinsic parameters while principal point and skew factors are determined, both the channels present a focal length around 164 mm, but the error band is greater and reach the 8%. It has to be considered that these parameters identify just a starting point for bundle adjustment refinement.

B.

Bundle adjustment refinements

In the second part of the pipeline, the orientation is computed by means of a bundle block adjustment of a series of automatically identified tie points and some Ground Control Points (GCP) extracted from the reference DTM. A stereo pair of the rock target in the region of the tie-points is shown in Fig. 5.

Fig. 5

The stereo pair of one of the rock samples with markers.

Tie points are automatically extracted and matched by structure from motion algorithms [8], [9]. GCP’s are object points of known coordinates, independently determined with an accuracy better than that obtainable by photogrammetric techniques; using them in a bundle adjustment allows to fix the reference system to the object and to control residual errors in intrinsic parameters.

Tie points have two important scope: on one side they are well-recognizable both on images, DTM results and on points cloud results of the laser scanner acquisition; on the other side they permit a fast and precise comparison between stereo results and laser scanner acquisition without resorting to closest point algorithms.