1.1

Current technology

The French state of the art of high-resolution Earth observation technologies can be found in the Spot and Pléiades satellites [3], which deliver sub-meter ground images while boarding focal plane arrays a few decimeters wide. This is achieved by using folding mirrors inside the focal plane - while this size is acceptable, next generations of high-resolution imagers cannot rely on this specific technology to continue increasing their resolution.

Figure 1.

The Pléiades satellite and its focal plane array. The use of multiple folding mirrors to reduce the size of the image greatly complexifies the focal plane and its integration.

1.2

Proposed solution

The solution brought by LAM is based on its expertise on Integral Field Units, with image slicers, for spectro-imaging in Astrophysics [4]. Now studied in a mission-like scenario with Thales Alenia Space, it intends to optically reduce this dimension by segmenting the linear FOV into smaller sub-fields that are stacked on a CMOS TDI sensor; the array is this way more compact. This is done at a small expense in terms of the optical system’s complexity and of surface shapes, as the use of freeform optics become mandatory.

Figure 2.

Segmentation solution described in this paper. The values (not scaled) are extrapolated from a hypothetic high-resolution Earth-observation mission.

2.1

Focal plane array

TDI versions of CMOS detectors are currently developed at various places in the world [5] [6] [7]. The detector we choose for our study is a multi-spectral CMOS TDI sensor that we virtually divided in 7 segments (called modules), each one being sub-divided in 5 spectral bands, allowing for multi-spectral and panchromatic imaging. The modules, corresponding to sub-fields, are isolated from each other by a 50-pixels blank band.

Figure 3.

Schematics of the detector used in our design.

2.2

Mission specifications

Our case study is a mission configuration that can be either a hypothetic high-resolution Earth-observation mission or a future high-resolution planetary mission in the solar system. The high-level specifications for this hypothetical mission are presented in Table 1.

Table 1.

General specs of the hypothetical mission.

The specs of the ground-images are derived from the Pléiades satellites, and slightly updated to simulate some of the needs of next generations of high-resolution spatial telescopes.

The point is not to demonstrate that we can do much better - because we can - but to prove that it is possible to segment the FOV into smaller sub-fields and still reach the diffraction limit across the whole image. This implies the following characteristics for the telescope:

Table 2.

Telescope characteristics of the hypothetical mission.

3.1

Optical layout and mirrors’ characteristics

In this first study the whole optical system is divided in two parts : {Korsh telescope + slicing unit}. This is due to an AIT (Assembly, Integration & Tests) trade-off : this way, the two parts can be integrated and aligned independently. A second version is currently under study, with the slicing unit and Korsh telescope integrated, allowing optimization of a simpler global system with better performances - but at the expense of a new philosophy for the AIT phase.

As the slicing unit is becoming a part of the whole system, its characteristics impact the one’s of the Korsh telescope. Following Jahn’s parametric study and in order to facilitate the design, the unit’s magnification has been chosen to be 1.

Korsh telescope:

The Korsh is first optimized independently to the diffraction limit across the whole field. Bearing in mind the alignment process, the distances and radii of curvature are constrained to reduce overall sensitivities. The bulk of the system is known to be high, but as the Korsh is not part of the study, the sensitivity overrules the size.

Figure 4.

The Korsh telescope is first optimized, then frozen.

Slicing unit:

Once this is done, the slicing unit is constructed step by step:

• ▪ General positioning of the two sets of mirrors, all centered and spherical, while paying attention to the bulk of the system: the unit shall fit in the Korsh’s footprint.

• ▪ First round of optimization using only conics, to set up the system and have an approximate location of the sub-images (modules) on the detector plane.

• ▪ Freeze of the whole system and modification of the unit’s surfaces type to Zernike Polynomials. Gradual incrementation of the number of Zernike, one module at a time to greatly reduce the number of variables (which improves the convergence of the optimization) : when a module is optimized, it is frozen and the next one is considered.

Figure 5.

The whole system in its final configuration. The slicing unit is entirely fitting in the Korsh’s footprint

Special attention is given to the second mirror set: they shall not overlap, otherwise parts of the beams would be obstructed while having a small overall lateral size.

3.2

Optical performances

The seven linear sub-FOVs are now stacked on the different modules defined on the image sensor, allowing for a drastic reduction in the linear size of the array.

For the slicing unit, Fringe Zernike up to the 5^th order on the first mirrors set and 4^th order on the second mirrors set have been used. This is due to the proximity of the first set to the Korsh’s image: this set geometrically contributes less to the overall aberrations correction.

The image quality for the seven linear sub-FOV are listed below:

Figure 6.

MTF charts of the 7 linear sub-FOV up to the Nyquist frequency.

As expected, the image quality deteriorates on the sub-FOV at the edge of the field. Residual aberrations remain: mainly coma and astigmatism, which will be corrected more precisely in future designs. Looking at the spot-diagrams gives another insight about the image quality, as shown in figure 7 :

Figure 7.

Spot-Diagrams of the 7 linear sub-FOVs on top of the Airy Disks.

The consistency in image quality that we reached across the seven sub-FOVs is a critical parameter in high-resolution observations : the ground images shall deliver the same standards of information whether it’s at the center or at the edge of the field.