Charge shuffling is the ability to multiply expose and shift an individual CCD frame backwards and forwards along the chip many times before read out. With the AAO-1 controller and a MIT-LL CCD, a single row of 2048 pixels can be shuffled one row in 50 s whereas to read out the same row takes 40 ms. This technique has been used with TAURUS at the Anglo-Australian Observatory since 1994.
Charge shuffling is essential to effective parallelism testing as it provides a powerful tool for sampling TTF throughput as it is stepped over successive gap spacings, without the need for many discrete CCD images. Similar charge shuffling methods have been applied to other forms of astronomical imaging such as imaging polarimetry ([Clemens & Leach 1987]) and spectroscopy of faint galaxies ([Cuillandre et al. 1994]). Bland-Hawthorn & Jones (1998) describe more sophisticated charge shuffle applications with TTF.
We use both the MIT-LL (rows columns) CCD with 15 m pixels and a Tektronix CCD with 24 m pixels. Both are buried-channel devices with charge-transfer efficiencies (CTEs) in excess of 0.99999. Tests involving repeated shuffling with the Tektronix chip have demonstrated that 100 full-frame shuffles are possible before significant charge-loss occurs ([Bland-Hawthorn & Jones 1998]). Bulk traps are the major cause of charge loss, arising from impurities and defects within the silicon lattice of the CCD ([Janesick & Elliott 1992]). The missing charge leaks out of trailing pixels as a deferred charge or is lost through recombination. The TTF plate spacing can be also be changed and allowed to settle within the time of a single shuffle (50 s). The primary limitation to shuffle speed is the shutter, which keeps the rate at 1 s for one complete shuffle.