D. Walker ([email protected]), S.M. Clark(*) ([email protected]), R.L. Jones(*) ([email protected]) and L.M.D. Cranswick(*) ([email protected]) Lamont Doherty Earth Observatory, Columbia University, Palisades, NY 10964 USA. * CLRC, Daresbury Laboratory, Warrington, WA4 4AD, UK. Deposited at http://www.ccp14.ac.uk/projects/glass-calib/