content long 18-November-2017 16:47:35

Payload Module


The payload consists of a single integrated instrument that comprises three major functions. In the earlier spacecraft designs, the three functions were distributed over three separate instruments. Now the three functions are built into a single instrument by using common telescopes and a shared focal plane:

  1. The Astrometric instrument (ASTRO) is devoted to star angular position measurements, providing the five astrometric parameters:
    • Star position (2 angles)
    • Proper motion (2 time derivatives of position)
    • Parallax (distance)
    ASTRO is functionally equivalent to the main Hipparcos instrument
  2. The Photometric instrument provides continuous star spectra for astrophysis in the band 320-1000 nm and the ASTRO chromaticity calibration
  3. The Radial Velocity Spectrometer (RVS) provides radial velocity and high resolution spectral data in the narrow band 847-874 nm

Each function is achieved within a dedicated area on the focal plane. Afocal elements are located close to the focal plane for the photometric and spectroscopic functions, providing dispersion of the star's spectrum along the scan. This allows both functions to take benefit from the two viewing directions and from the large ASTRO aperture, and to operate in densely populated sky areas. RVS is implemented as a grating plate, combined with four prismatic spherical lenses. This allows the necessary dispersion value to be met while correcting most of the telescope aberrations.

Annotated diagram of the Payload Module


The payload design is characterised by:

  • A dual telescope concept, with a common structure and a common focal plane. Both telescopes are based on a three-mirror anastigmat (TMA) design. Beam combination is achieved in image space with a small beam combiner, rather than in object space as was done in the Hipparcos satellite. This saves the mass of the beam combiner, simplifies the accommodation and eliminates the directional ambiguity of the detected targets
  • The use of silicon-carbide (SiC) ultra-stable material for mirrors and telescope structure provides low mass, isotropy, thermo-elastic stability and dimensional stability in a space environment. This allows to meet the stability requirements for the basic angle between the two telescopes with a passive thermal control instead of an active one
  • A highly robust Basic Angle Measurement system
  • A large common focal plane shared by all instruments

Last Update: 12 October 2013

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