<![CDATA[Photometry is one of the astronomical measurement methods that provides astronomers with information about the brightness of astronomical objects at various wavelengths. Over time, various photometric systems have been developed. Therefore, transformations are necessary to convert results from one photometric system to another, with the aim of facilitating scientific communication among researchers working with various photometric systems. The CAT-ITB, or City Astronomical Telescope ITB, is an 11-inch optical telescope located at CAS ITB, equipped with detectors, one of which is the ZWO ASI 183MC that uses the RGB Bayer filter. Hence, transformation equations from the RGB Bayer filter system to the Johnson-Cousins BVR filter system must be established. By utilizing transformation equations that have historically been widely used by previous researchers, this study focuses on deriving the coefficients needed to solve those equations. Observational data of open clusters M67, RU 135, IC 4665, and M11 were obtained using CAT-ITB, which were used to optimize the transformation equations. The coefficients derived from M67 photometric data yielded the best results. The total RMS values for the magnitude transformations of clusters M67, RU 135, IC 4665, and M11 in each of the BJohnson, VJohnson, and RCousins conversions are 0.296, 0.152, and 0.161 respectively. All transformations produced small residuals and colour-magnitude diagrams that closely resemble those from catalog data. This study contributes a validated set of transformation coefficients and zero-point constants specific to the CAT-ITB RGB Bayer detector system, offering a practical reference for converting RGB-based photometric observations into the standard Johnson-Cousins system for future small-telescope and citizen-science photometric research.]]>
