The LSST Science Raft Electronics

The Large Synoptic Survey Telescope (LSST) Project is a public-private partnership for a large-aperture, wide-field, ground-based telescope that will survey half the sky every few nights in six optical bands. The 8.4-meter telescope will be located on the El Pen peak of Cerro Pachn, a 2682 metre high mountain in Coquimbo Region, in northern Chile. The 3.2 Giga-pixel camera will take 6.4 GB images every 15 seconds, resulting in 15 TB of new raw image data per night. The LSST camera is made up of 21 modules designated Science Raft Towers. Each raft tower module (RTM) is an autonomous, fully testable and serviceable 144 Mega pixels imager consisting of nine highly segmented CCDs with complete readout electronics chain. To minimize noise and obscuration the RTM is housed in a compact enclosure fully contained within the camera cryostat. The RTM electronics is composed of 3 independent boards called REB (Raft Electronics Boards). Each REB controls and acquires data from 3 4K x 4K fully depleted CCDs with 16 outputs each and send data to a data acquisition system based on ATCA crates through a 3.25 Gbps serial link. The readout time is 2 seconds. The Front End section is based on two ASICs one, the CABAC (Clock & Biases Asic for CCD), that provides CCD clocks and biases and the other, the ASPIC (Analogue Signal Processing Integrated Circuit), for the signal conditioning and amplification. Analog data are digitized using 48, 18 bits low power ADCs. In addition the electronics has many housekeeping features such as temperature voltage and current monitoring. The board also provides heater controls, ASICs configuration and read back and power down modes. A specific section based on 2 fast 12 bits ADCs is dedicated to in-cryostat diagnostics and debugging, and allows the measure of all the essential camera parameters. Each board is controlled by a Xilinx Virtex 5 FPGA. It provides CCD clocks sequences, drives all boards devices and is in charge of the communication with the DAQ system. We will discuss the conceptual design of the science raft electronics and present preliminary tests results.