A STUDY OF THE ACCURACY OF NARROW-FIELD ASTROMETRY USING STAR TRAILS TAKEN WITH THE CFHT

We extend the study of Han (1989, AJ, 94, 213) to angles as small as 7 arcsec. To investigate the relation between the relative-image motion and the separation of two stars in astrometric observations, we analy zed star-trail plates taken with the CFHT by Christian and Racine (198 5, PASP, 97, 1215). The result shows that on Mauna Kea the rms relativ e-image motion for 1 s of integration time is given by sigma=0.054 arc sec (theta/10 arcmin)(0.35), where theta is the separation between two stars. The standard theory on atmospheric turbulence predicts that si gma is proportional to the one-third power of theta. Our study shows t hat it is valid for the range of 7 arcsec<theta<1100 arcsec. Han, usin g data taken with MAP [Multichannel Astrometric Photometer, Gatewood ( 1987, AJ, 94, 213)] of Allegheny Observatory of the University of Pitt sburgh, showed that in Pittsburgh sigma=0.143 arcsec (theta/10 arcmin) (0.34). This implies that the accuracy of very-narrow-held astrometric observations at an excellent site may be more than 7 times higher tha n those of the currently most-precise astrometric programs.