Focus accuracy is an essential factor that affects the quality of astronomical observations. The accurate measurement of celestial objects' properties depends on focus. Automatic focusing is necessary for celestial imaging systems. This paper presents a modified focus measure operator. It also proposes the use of fuzzy logic to transform images because of its tolerance of imprecise and incomplete data. The focus operators are applied to two sequences of star clusters' observations. The experimental results show that the suggested measure's overall score exceeds those of previous operators.