Versatile mosaic CCD controller design

Current astronomical CCDs (Charge-coupled devices) are limited in size. However, there are increasing demands for larger devices for spectroscopic and direct imaging use. A comparison of the areas of a direct Schmidt-survey plate, and the largest CCD likely to be available in the foreseeable future, serves to illustrate limitations of current electronic technology. Similarly, modern spectrographs can serve either high spectral resolution or multi-object capability, but not both. One could conceive of an updated analogue of a classical coudé spectrograph, which would provide both simultaneously, were the enormous detector areas required available. It is with these two long-term applications in view that we consider the prospects for large-scale CCD mosaics. Spectroscopy in particular would benefit from mixed devices in the same mosaic e.g. blue and red-optimised, IR devices etc. We address how optimally to provide the drive waveforms with i) minimum interconnections and complexity, ii) the ability to mix CCDs, iii) the ability to tune the individual CCD waveforms for optimum low-light level performance, and iv) to conceive an architecture which is indefinitely expansible. Our solution is to multiplex the drive waveforms to the CCDs as well as the signals from them. Minimum chip-count in the sequencing electronics is ensured by using high-density PLD technology (Programmable Logic Device), which enables a module of sixteen or more CCDs to be sequenced from one PLD plus memory. We describe a laboratory prototype, and describe how this could be developed into a system with all the drive electronics for large numbers of CCDs immediately behind the focal plane array. We also summarise our software system for efficiently generating and editing the bitmaps which define the CCD waveforms.