LSST’s Focal Plane Components Take Shape

When the survey begins near the end of the decade, an array of 201 specially-designed CCDs and electronics buried within LSST’s massive camera will begin converting the light captured by the telescope into digital data at a rate of about 11 trillion bits per hour. Accomplishing this task requires an entirely new generation of CCD image sensor, and a team of engineers within the Camera collaboration has been working for the past several years with experienced vendors to demonstrate the needed new technologies. To meet the exacting requirements of LSST’s science program, the CCDs have to be made from specially-grown silicon crystals and then mounted in precision packages that will form the “paving stones” of the mosaic focal plane.

The key characteristics needed by LSST’s sensors are high sensitivity across the ultraviolet-to-near-infrared wavelength range, fast readout, and excellent spatial resolution. The thickness of the silicon layer required to achieve good near infrared response while preserving fine spatial resolution is about five times greater than current CCDs use, and its electrical resistivity must be over 100 times higher. Furthermore, the optical system which gives LSST its wide field of view has a very shallow depth of focus meaning the CCDs’ imaging surfaces must be exceedingly flat, no more than 5 microns deviation from a perfect planarity. Finally, the CCD packages must be able to be butted closely together on all four sides to minimize dead area as much as possible. The vendors partnering with LSST have recently demonstrated successful prototypes satisfying these characteristics.

Imaging properties are determined by the CCD’s silicon technology, but reading out the entire 3 gigapixel array in 2 seconds requires a highly-parallel electronics system to transform the microvolt-level signals generated by the CCDs into digital data. Unlike other astronomical cameras, LSST’s focal plane will be constructed from an array of 21 identical modules known as “raft towers”, each containing a 3 x 3 mosaic of CCDs with their associated processing electronics in an enclosure roughly the size of a shoebox. Processing the raft’s 144 independent video streams in such a small volume is possible only by using custom-designed integrated circuits. Engineers in LSST’s collaborating universities have designed and successfully tested readout ICs that will be used in the raft towers.

Over the next year the team at Brookhaven Lab and collaborating institutions will assemble sensors, electronics, and mechanics together and start building up the test capability to verify the performance of the modules which will become the heart of the LSST camera.

Article written by Paul O’Connor, Brookhaven National Laboratory