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The coordinates used to design a mask should be of the highest precision. Once a mask is made there is no way to adjust the position of the slits. Even errors of 0.5 arcsec can make a significant difference in S/N, considering that most masks have a slit width of 1.5 arcsec and are made for the study of faint objects. Therefore it is recommended that the RELATIVE positional accuracy of the coordinates is kept to a few tenths of an arcsec. The accuracy of the LEXT software is limited by its predominantly REAL*4 implementation, giving positions accurate to about 0.01 arcsec.
It is not that important to beat down the zero-point errors to the FK4 or FK5 system. The accuracy of the field centre position is also not that important, at least during the design of the mask. It can be tweaked during the design, and any offsets resulting from the manufacture can be measured and at the telescope, and the field centre adjusted. However, it is very important that the fiducial stars' positions are measured at the same time as the program objects. These are used to position the telescope. If they are not precisely on the same coordinate system as the program objects it is quite likely that objects will not fall on the slits.