Region of Interest Selection-Based Autofocusing for High Magnification Systems

The selection of the region of interest (ROI) in an image for measuring the focus level of high-magnification astronomical observations is arduous. Precise focusing is a principal parameter that controls the quality of images for astronomical observations, leading to proper scientific research. Our proposed scheme detects and evaluates four candidate stars using the full width at half maximum (FWHM) star characteristic. We use the FWHM to assess the detection process, particularly for out-of-focus frames, because no approach for detecting stars at those positions exists. We compare the performance of 18 different focus measure operators in the cases of our proposed method, the entire full-frame image, and the regular sub-frame at the widths of 75, 85, and 95 pixels. The operators were applied to five star clusters acquired using the 74-inch telescope of Kottamia Astronomical Observatory (KAO) under good seeing conditions. The results show that the 18 operators perform better in the case of the proposed sub-frame.