For precise work there are a number of refinements that should be considered, some of which should be implemented, and some which probably need not be. Things that come to mind include parallax and proper motion of the comparison stars, refraction, aberration, optical distortion, mistakes – which include such things as poor measurements, blends, poor or erroneous catalogue positions or any of a number of mistakes caused by human or instrumental frailty. If you write your own reduction programs, you will know which of these refinements you have included and which you have left out. If you use a “pre-packaged” program, you may not always know whether a given correction has been included.
Let us now look at some of these refinements.