[1] UK Astronomy Technology Centre, Royal Observatory, Edinburgh, EH9 3HJ, United Kingdom
[2] Cardiff University, Cardiff, CF24 3YB, United Kingdom
[3] Natl. Inst. of Std. and Technology, 325 Broadway, Boulder, CO 80305, United States
[4] Scottish Microelectronics Centre, University of Edinburgh, Edinburgh, EH9 3JF, United Kingdom