The accommodation of 24 normal + 2 fast cameras and associated electronic units has a mass allocation of 533 kg, with the specified camera groups’ orientation and offset on a spacecraft whose volume and mass are constrained by the Soyuz fairing and launch performance of 2165 kg to L2. The maximum allowed mechanical and thermal loads onto the payload units, especially the cameras, as specified in the interface requirements, drive the platform design.
Among others, this affects in particular the mechanical sizing of the optical bench assembly and the camera support structures, which interface with the optical bench to limit e.g. quasi-static, sine and random loads or shocks onto the cameras. The cameras thermal control, even though powered by the platform, is designed by the PMC and ensured by the payload itself. The telescopes’ absolute temperatures are around ‒70 to ‒90 oC in order to obtain nominal working temperatures for the CCDs (‒65 oC) and required PSF focused point. These are not extremely challenging, but once set to the specified level for a given camera, the thermal control must keep this set point within narrow limits on the telescope body (~ 200 mK over 6 hours). The purpose is to guarantee the stability of the telescopes focusing points and to avoid PSF breathing effects, which could affect photometric performances, especially in the frequency range of interest to asteroseismology. In order to do that, the spacecraft must also provide a very stable conductive and radiative environment towards the cameras, including the front end electronics.
The set of 26 cameras leads to an unusual large amount of harness to fulfil all functions, i.e. power, thermal control, data (via space wire), synchronisation. The main issues are the high mass of the harness and its routing between the SVM equipment and the payload units. In addition, the Ancillary Electronics Units (AEU) are providing secondary power to the Front-End Electronics (FEE) to limit high power dissipations close to the cameras. Due to the large distance between the FEEs located on the cameras and the AEUs located on the SVM, the harness losses must be carefully mitigated.