The pointing accuracy of the INT is sufficiently high that acquisition near the centre of the slit or prime-focus CCD (within 5 arcsec, say) is usually possible. Positions should therefore be measured to better than 2 arcsec. Blind offsets on the INT are reliable to less than 1 arcsec using the AUTOFFSET command, which uses the Autoguider probe motions to determine the offset.
Two coordinate systems are supported : mean pre-IAU76 and geocentric apparent of the date and time of observation; post-IAU76 (eg J2000) coordinates are accepted and should be used. Neglect of the difference between pre- and post-IAU76 systems gives rise to a maximum error of < 1 arcsec in practical cases, which is small compared with the pointing error. Optional parameters are proper motions (epoch assumed to be the same as equinox) and differential tracking rates in RA and Dec.