If you've ever had a raccoon enter your home through the cat door, it's not an experience you want to repeat. But it doesn't mean you have to get rid of the cat door. You can keep raccoons from going through cat doors, although it might involve installing a different type of pet exit and entrance. If that's not an option, scheduling changes and lighting devices can deter the masked creatures.
The best option for keeping unwanted critters from going through the cat door is installing an electronic door. They aren't cheap, but they do the job. Your cat wears a collar with an electronic disc on it that signals the door to open. If your cat loses his collar, or the disc falls off, he can't open the door. However, unless your raccoon intruder mugs your cat and puts on the collar and disc -- they are smart critters -- he's not getting into your house. (ref 1)
Since raccoons are nocturnal, one way to keep them from coming in through the cat door is by closing the door each night and opening it again in the morning. Of course, you must make sure that all of your cats are in the house, present and accounted for, when you shut or block the cat door. The best way to ensure that kitties show up in the evening is by making that their primary feeding time. If your cat doesn't show up to eat, you've got a bigger problem than possible raccoon entry. (ref 2)
Remove Food Sources
There's one primary reason that a raccoon enters your house via the cat door. He figures -- probably correctly -- that the inside of your home offers fine dining opportunities. Stop raccoons from becoming interested in what's on the other side of the cat door by keeping any sort of food away from either side of the pet entrance. That means not leaving pet food outdoors, and feeding your pets inside as far away from the pet door as possible. Pick up any uneaten pet food inside your house once your animals finish and don't leave human food out in the open. (ref 3)
Other Raccoon Deterrents
Try installing motion detectors above your pet door, so that a light goes on when a raccoon -- or other animal -- is in the vicinity. If it doesn't affect your sleep, leave a radio turned on low near the pet door at night, tuned to a talk station. The sound of human voices could make a raccoon think twice about entering, but shouldn't bother your cats. You also can place a raccoon repellent outside of the door every evening if the cats are inside. Liberally sprinkling cayenne pepper should do the trick. Another option is installing a motion-activated sprinkler designed to repel wildlife. Just remember to disconnect it in the morning before letting the cats out. (ref 3)