First confirm that the ProHDP is correctly setup to drive the display. Display the internal test patterns by pressing "Menu, MISC, TPAT, PATS, Ok, Ok" Use the right arrow button to display the "Overscan" test pattern. The test pattern should be centered on your screen. Use the position controls on the display to center the image.
You can calibrate the position and overscan on each input Select the input. Press "Menu, IN, CONFIG, ADJ, SIZE". Adjust the top left and then the bottom right. Save the new settings when you are finished.
You can do a factory reset to see if the image is centered. Press "Menu 0999". See if the image from the AppleTV is now centered. Do a "Save" if you want to keep the factory default settings. Press "Standby" and then "On" to restore the last saved values.
Best regards,
Randy Freeman