Line the terrain edge up with your camera view so you can't see the square edge drop off. As a texture ash works well but I use a large lunar photo I got from a NASA website. Apply the photo texture to the terrain as parametric. Apply the same texture to the ground plane as parametric scaled. Change the scale value to about -7 XYZ. This should keep you from having infinitely repeating tiles that are obvious to see in your background. You can repeat more tiles add mountains whatever to hide edges. I used the Starfield Sky from the presets because it has no fog or haze.