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Photometric standards, and their WHT names, are listed in the appendices to the WHT Users Manual. The positions of these stars are stored online by the WHT Telescope Control System, and can be selected using the WHT names.
The original source for the Landolt standards is:
'UBVRI photometric standard stars around the celestial equator', A.U.Landolt, 1983, Astron.J., 88, 439.
Photometric sequences for CCD observations are given in:
C.A. Christian et al, 1985, PASP, 97, 363
R.E. Schild, 1983, PASP, 95, 1021
R.S. Stobie et al, 1985, A&A Suppl., 60, 503.
Note that many of the Landolt standards are too bright for a 4--m telescope with a reasonable integration time. Integration times should be kept above about 5 seconds in order to avoid errors due to shutter timing (See section 21). In order to do this without saturating the detector, it may be necessary to defocus the telescope.