SPOT series camera improvement for the HRG: very high resolution instrument of SPOT 5

Derived from the SPOT series, the new SPOT-5 instrument, called HRG, is now in development phase for a launch in 2001. The HRG instrument will perform Earth mapping still with 60km swath, but with considerably improved resolutions: 5m and 3.5m in the panchromatic band, and 10m multispectral, to be compared with 10m, respectively 20m for the SPOT-1 to SPOT-4 camera series. This improvement is obtained by the implementation of a new focal pane concept and also by a telescope MTF significant enhancement. This paper describes how the optical quality of the SPOT-5 camera has been improved for meeting the new resolution requirements, while still using the same type of telescope as in SPOT-1 to SPOT- 4. First the technique of computer assisted polishing of the mirrors has been adopted to reach a telescope image quality compatible with the demanding resolution. Moreover, for reducing mirror optical surface distortion, specific isostatic fixation devices have been developed. The telescope structure has been also optimized in terms of thermo-elastic and hydro-elastic behavior to ensure stable performances.Lastly, significant progresses have been made in telescope alignment and testing.