The optical solution chosen for the telescope is a Korsch type combination. The imaging geometry optimization induces a primary mirror size of 650 mm diameter.
Primary mirror during tests at INTESPACE
The architecture chosen is organized around a central plane structure supporting the primary mirror, the tertiary mirror, the plane mirror, and a central cylinder that supports the secondary mirror. The solution is based on carbon material for the structure and on Zerodur material for the mirrors.
The instrument includes a focus function based upon a specific finely regulated thermal control of secondary mirror structural support.
The instrument includes also an internal shutter to protect it from the sun radiation in non-operational phases such as launch, attitude acquisition, or safe modes. This solution avoids an external shutter that is generally heavy and complex.
Thinned TDI detectors are used for panchromatic detection, with typically 15 integration lines. They can be used thanks to an optimized guidance strategy of the satellite line of sight including micro-vibration level minimization, specific geometrical accommodation of detector lines in the focal plane to minimize optical distortion effects. Five detectors of 6000 pixels each are used; each pixel having a size of 13 µm. An anti-blooming structure prevents light from spreading along the columns.
The multi-spectral detection is made with 5 detectors of 1500 pixels each, 13 µm large. Each detector consists in a four assembly lines, enabling four colour imaging (blue, green, red, near infrared). Interferometric filters directly stuck down on the detector glass window provide the colour for these four channels.
The focal plane is constituted by two symmetrical arrangements of those detectors. To acquire images over a field of view of 20 km, each line of sight is composed by juxtaposing of 5 linear arrays, generating images of 30,000 columns in the PAN channel and 7,500 columns in the XS channel. Among 5 linear arrays of each retina, 2 operate by reflection and 3 by transmission across a beam splitter mirror device (Divoli) which allows all the points in the field to be acquired almost simultaneously. This great proximity in time makes the reconstruction of the continuous image line insensitive to the parallax effects introduced by the relief, and by temporal attitude variations between the 2 acquisitions of the same point on the ground (by 2 adjacent linear arrays).
Thanks to a separation mirror, the XS and PAN viewing planes are separated by 1.5 mrad in the field only, which makes PAN and Multispectral channel registration possible by a simple ground processing (re-sampling).
These mirrors and the detectors are mounted on a SiC beam and the resulting focal plane assembly is supported on the telescope structure by isostatic invar bars.
The focal plane is electrically coupled with the detection electronics to form a highly integrated detection unit. The overall unit detects and converts the video signals into digital data. The Printed Circuit Boards are placed just behind the focal plane.
The detection unit has its own thermal control to ensure maximum performance. It is constituted mainly of a dedicated radiator and of two heat pipes. The radiator is part of the instrument structure, and a hole is reserved on the coldest satellite panel for it.
Integrated Detection Unit