5.1 Restrictions and limitations

The script inherits some limitations from ASTROM, and some from CCDPACK's findoff.

The limitations due to ASTROM are as follows (quoted from SUN/5, sect. 5):

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.

Further, ASTROM requires that at least 10 reference stars are available. Since autoastrom obtains its reference stars from well-stocked catalogues, this is not a problem in practice.

The restrictions arising from findoff are similarly slight, and should not be a problem if the initial astrometry is good enough. It's not at present completely clear exactly what `good enough' means.