The standard coordinates should then be corrected for the effects of radial field distortion:
These effects should be negligible for the JKT f/15, INT f/15 and WHT f/11 foci, but corrections are (in theory) required for INT Prime and JKT f/8. Neither focus currently supports any instruments which require precise location of guide stars, but the formulae are included for completeness:
with b = 
for the JKT (LPTN
64). For the INT at Prime Focus:
with 
, 
,
and 
. This is a
theoretical formula and has not been checked.
Differential refraction between the field centre and the guide probe also affects the conversion, but is dependent on Elevation (and hence on the time of observation). It is not appropriate to include it in the search software.