Pleiades High Resolution instrument

Optical Architecture

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

Mechanical Architecture

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.

Mechanisms

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.

Detection

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-vibrations levels 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 from light spreading along the columns.

The multi-spectral detection is realized with 5 detectors of 1500 pixels each, 13 µm large. Each detector consists in a four lines assembly, enabling four colors imaging (blue, green, red, near infrared). Interferometric filters directly sticked down on the detector glass window provide coloring of 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 30000 columns in the PAN channel and 7500 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 channels registration possible by a simple ground processing (re-sampling).

PAN and Multispectral bands linking and separation principle

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 realizes the functions of detection and conversion of video signals into digital data. The Printed Circuit Boards are placed just behind the focal plane.

Focal plane assembly

The detection unit has its own thermal control to ensure maximum performance. It is constituted mainly by a dedicated radiator, and by 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