The standard coordinates are then corrected for the effects of radial field distortion:

$\xi \text{'}\; =\; \xi D(\rho )$

$\eta \text{'}\; =\; \eta D(\rho )$

$\rho =\; (\xi 2+\; \eta 2)1/2$

These effects should be negligible for the JKT f/15, INT f/15 and WHT f/11 foci, but corrections are required for WHT and INT Prime and JKT f/8. In particular, WHT Prime requires precise location of guide stars, since the autoguider field is small. The formula:$D(\rho )\; =\; 1\; +\; a\rho +\; b\rho 2+\; c\rho 3+\; d\rho 4$

is appropriate, with coefficients as given in Table 1.

Configuration | $a$ | $b$ | $c$ | $d$ |

WHT Prime | 1.2797 | 27.458 | 24320.0 | 0.0 |

INT Prime | ||||

JKT f/8 | 0.0 | 13.32 | 0.0 | 0.0 |

Mon Mar 1 17:52:09 GMT 1999