I hope you can see it in the picture, but my cactus has brown spots on one side on top of the nodes/aireols?? Is this sunburn, and if so, how many hours of direct sun should I limit them to a day? I have other cacti as well. If they show no signs of burn, should I still limit them, or just let them continue to bake? If a better picture is required, just ask and I will try. Thanks.