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Generally the positional changes have been less than 0.5 arcsec. However there is a cluster of poor calibrator positions near RA=23 hours, DEC=-40^o. The accuracy and the reference for a calibrator's position is now given on the web page for each calibrator.

The consequences

• Observerations with high astrometric requirements are probably already aware of issues with the accuracy of calibrator positions.
• The change in position can have an effect on observations where multiple epochs are involved. If the nominal position of the calibrator changes between the epochs, then this will lead to a misregistration between the epochs. If the change in nominal calibrator position is "significant" relative to the resolution and the dynamic range requirements, then care is required. One way to avoid these issues, where positional accuracy is of secondary importance, is to continue to use the old calibrator positions. To use the old positions, in SCHED select the old calibrator catalogue as the default one with the command

      defcat old_calibrators
  

  Note the change in the calibrator catalogue does not affect schedule files that have already been developed.

The details

The following calibrators have been added: j0026-3512, j1147-6753, j1354-1041 and j1427-3306. These are new, and are of most relevance to 3mm observers.

The following table gives the list of calibrators whose position have been updated with improved ones, and the size of the change.