The Subaru telescope provides a feature of auto guiding the telescope using the slit viewer (SV). The SV guide uses the
target star as a guide star. There are advantages to guide the target star directly. However, the guide accuracy was not
good. The guide star is located on the slit and some lights is vignetted by the slit. But the SV guide simply took the
centroid of the light to measure the position of the star without taking the vignetting of the slit into account. In 2006, we
improved the SV to detect center of gravity with the vignetting of the slit taken into account. It assumes a Gaussian
distribution of light except for the slit. By this improvement, the guiding accuracy of the telescope improved from 0.37
arcsec to less than 0.2 arcsec. The effect of the improvement was also confirmed with actual observations.