6 Limitations

ASTROM aims to deliver results better than 1 arcsec from typical Schmidt plate measurements, and better than 0.1 arcsec from carefully measured JKT and AAT plates etc. Astrometric specialists will, nonetheless, be aware of a number of shortcomings, including the following:

• The fit is limited to a 6-coefficient linear model plus cubic distortion and plate tilt. Colour effects – arising for example from chromatic aberrations in the camera optics – are not allowed for, no magnitude or image shape terms are included in the model, and the refraction cannot be adjusted automatically.
• The zonal distortions of the reference catalogues are neglected.
• There is no provision for the simultaneous fitting of more than one plate. This prevents an extended area being modelled via overlapping plates, and the determination of proper motion and parallax from plates taken at different epochs.
• Only rudimentary error information is produced.

Despite these limitations, which stem mainly from the need for simplicity of use, the accuracy of the result tends in practice to be dominated by the quality of the input data rather than by ASTROM itself.