The return result from READ_IMAGE.PRO is a 2D array if greyscale and 3D if color (e.g., see https://www.nv5geospatialsoftware.com/docs/READ_IMAGE.html). So yes, if you only want the greyscale image, just use the second two dimensions. To make it a true greyscale image, you can follow the advice in https://stackoverflow.com/a/689547/4005647 to apply proper weights to each of the RGB channels.