With imaging instruments, it is not feasible to transmit the entire image to the ground due to the limited communications bandwidth. Therefore some on-board data processing must be undertaken. For the photometry, simple data compression schemes can be used to significantly reduce the amount of data, as we are looking for only small changes on a stable background. In the case of the spectroscopic instruments, we will isolate the regions of interest, and transmit only those portions of the spectra. The nature of the data reduction in both cases lends itself to parallel or pipelined architectures. The continuing development of very fast processors (for example transputers and dedicated signal processing devices) means that the computational load is not excessive, and a large amount of preprocessing can be undertaken onboard. The processing required is very modest compared with some other space science experiments (e.g. Scherrer et al. 1990).