Electronics

The LSST focal plane represents a quantum leap in size and scope over those in use in telescopes today. While both CMOS/PIN diode image sensors and CCDs remain under consideration, we restrict our attention here to the CCD option and for this design option. The readout electronics (see Fig. 3) support 4K x 4K CCD sensors with 32 output ports per device (see Figures 4 and 5). With the exception of the front end module, comprising the front end signal processing electronics, the remaining downstream electronics could also accommodate a CMOS image sensor array.

The combined requirements of large size, high dynamic range, low noise, and rapid readout time, dictates a highly segmented focal plane with approximately 6,400 readout ports. This dictates a high degree of integration for both the front end electronics, those which process the CCD output signals and provide clocks, and the back end, which digitize the data, buffers it, and transmits it off-camera via optical fiber.

A basic choice must be made to distribute functionality either within the camera inner cryostat or outside it. A design with most of the electronics outside cryostat would afford greater accessibility at the expense of a much higher number of cryostat electrical penetrations. An analysis of connector requirements indicates that for a system with two hundred 16 Mpixel CCDs, about 20,000 cryostat penetrations would be required. To avoid this excessive number of penetrations, a much more highly integrated strategy with considerable electronics within the cryostat is preferred.

A second design driver is the low noise requirement, and in particular, low feedthrough of digital activity back to the sensitive analog signals from the CCDs. This dictates front end electronics located very closely to the CCD ports with digital activity somewhat removed. It also dictates use of low-level differential signaling both for analog and digital data transmission.

Finally, the entire LSST focal plane will be synchronous in operation, which means that clocking for all sensors in the array will be synchronous to the level of some tens of nanoseconds. This assures repeatability and robustness against feedthrough and pickup. Timing generation will take place within the timing/control crate under the command of the off-camera control system.

Figure 3. LSST electronics.
Figure 4. LSST sensor segment layout.

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Figure 5. LSST CCD layout.

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