SIMBAD references

Gaia Data Release 2 (Gaia DR2) provides high accuracy and precision astrometric parameters (position, parallax, and proper motion) for more than 1 billion sources and is revolutionizing astrometry. For a fast-moving target such as an asteroid, with many stars in the field of view that are brighter than the faint limit magnitude of Gaia (21 Gmag), its measurement accuracy and precision can be greatly improved by taking advantage of Gaia reference stars. However, if we want to study the relative motions of cluster members, we could cross-match them in different epochs based on pixel positions. For both types of targets, the determination of optical field-angle distortion or called geometric distortion (GD) in this paper is important for image calibration especially when there are few reference stars to build a high-order plate model. For the former, the GD solution can be derived based on the astrometric catalogue's position, while for the latter, a reference system called 'master frame' is constructed from these observations in pixel coordinates, and then the GD solution is derived. But, are the two GD solutions in agreement with each other? In this paper, two types of GD solutions, which are derived either from the Gaia DR2 catalogue or from the self-constructed master frame, are applied respectively for the observations taken by 1-m telescope at Yunnan Observatory. It is found that two GD solutions enable the precision to achieve a comparable level (∼10 mas) but their GD patterns are different. Synthetic distorted positions are generated for further investigation into the discrepancy between the two GD solutions. We aim to find the correlation and distinction between the two types of GD solutions and their applicability in high precision astrometry.