Hey pointthinker, check out Calibr8 app for Apple TV (supports 4th gen and later that have App Store)!
Because Calibr8 app runs natively on the Apple TV, you get the most accurate patterns to calibrate your TV with. There's a huge library of dynamic test patterns which you won't find in other calibration apps. The Calibrate! app only contains a handful of static test patterns. This means certain features such as DCR and Dynamic Backlight on many TVs can't be tested.
Calibr8 includes all the popular test patterns which would be of great interest to the enthusiasts (and the pros) on this forum!
Search "Calibr8" on App Store, or Google "Calibr8" for more information.