At what focal length is your render camera set to?  Anything set to <50mm will give you a sharper image due to the lens angle capturing more surface area of the dome.  Using telephoto settings, narrows that field significantly, giving you pixelation and what appears to be lower image resolution from the resulting magification.  Hi res images help but the amount of viewing area must be considered if you want everything in the distance relatively sharp.  Problem with wide angles, they do distort the models if they are close to the camera.