Fig. 6

Data-reduction procedure. a The wide-band image generated by taking the median of the three dark-subtracted wide-band images. b The v-band image generated by taking the minimum of two dark-subtracted v-band images as a reference frame for the stray light. c The stray-light-subtracted image \(W(x,y)-V(x,y)\). d Same as c. e A normalized flat-field image (Suzuki et al. 2018). f The flat-field-corrected image SKY(x, y), which is the final reduced image of the sky used for scientific analysis