Spacecraft development

2002 Payload Optimisation Reduce mass and size for being compatible with Soyouz launcher Total launch mass ~ 2270 kg (A5) : mass objective < 1500 kg ( - 34%) Reduce the PLM mass ~ 900 kg: Astro focal plane assemblies ~ 168 kg (with video electronics) mass decrease obtained by reducing the number of CCDs Optics with their mounts ~ 270 kg : mass decrease is obtained by reducing the reflector sizes Structure ~ 290 kg structure mass decrease is a consequence of size and supported mass reduction Mass reduction objective is in conflict with accuracy requirement. Design improvement is mandatory for Soyouz compatibility. Reduce overall complexity and cost– Try to harmonise CCD definitions– Minimise hardware tests– Suppression of mechanisms whenever possible2002 Design Features Astrometric Instrument– Superimpose the two focal planes, by making use of a focal plane combinerlocated close to the exit pupil– Increase the overall optical path length for practically suppressing opticaldistortion– Enlarge CCDs along scan (~ 60 mm x 45 mm)– Passive cooling of the FPA detectors down to 160 K – 170 K (radiation) Spectrometric Instrument– Along scan dispersion for RVS– Suppression of RVS mechanism– RVS spectral resolution improved by a factor 2– Medium Band Photometry improved