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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.
Next: Reduction in Observed Place
Up: ASTROM Basic astrometry program
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ASTROM Basic astrometry program
Starlink User Note 5
P. T. Wallace, Norman Gray
2nd May 2003
E-mail:ussc@star.rl.ac.uk
Copyright © 2003 Council for the Central Laboratory of the Research Councils