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The all-sky pointing accuracy of the 4m and 2.1m telescopes are typically 10" (RMS), with offsetting capabilities of <0.3" over 10'. Thus, you will save a tremendous amount of time if you come prepared with accurate coordinates. Doing astrometry used to be an arcane art, but modern tools make this assessible even to the likes of us, thanks in large part to the incredible investment made by the folks at the Space Telescope Science Institute. We refer you to their instructions found on the Web at http://www-gsss.stsci.edu/support/phase2.html for complete instructions.