Fig. 4. Optical layout of the calibration system in the front end adaptor: 1. tungsten lamp and diffuser, 2. hollow cathode thorium lamp, 3. colour corrector filter. 4. static beam-splitter, 5. relay achromat, 6. mirrors for beam angular splitting, 7. removal mirror and beam splitting (part of the accelerometry experiment), 8. optical fibre of the Fabry-Perot light (part of the accelerometry experiment), 9. circular wedge 10. pupil, 11 achromat (to put the pupil at infinity), 12. calibration mirror, 13. fibre mask, 14. concave mirror with 2 holes, 15. spectrometer linking fibres and aperture converter, 16. aperture converter optics and rhomboid prisms (to bring the beams closer to the focus of the collimator).