GTC/OSIRIS zoomed g′-band image of the PSR J2021+4026 field. The pulsar position is marked by the red circle, whose radius accounts both for the absolute uncertainty on the pulsar Chandra coordinates and the accuracy of our astrometric solution. The intensity levels in the bottom bar are in ADU and have been adjusted for a better clarity. The point-spread function wings of the bright star BD+39 4152 are visible on the left, overflowing from the gap between the two CCDs. The white band on the left corresponds to the CCD gap residual after image dithering.