How to remove oven light cover in Thermador oven?

I have a Thermador "professional" oven and no instruction book or visible model number. The interior light in the oven no longer works and I assume that the bulb needs to be replaced. I can't get the thick glass bulb cover off. I can turn it about 10 degrees, that's all. I've tried pushing, pulling, turning it harder and cleaning around it with oven cleaner and vinegar. Nothing seems to work. Is there some trick to this?