Questions: How much of the fiducial frame needs to be left uncovered for the automatic image alignment to work in Space Ranger?
Answer: The performance of the alignment degrades when fewer than about 270 of the 555 fiducial spots are detectable. Try to have at least 3 sides with at least 50% of the sides uncovered. The automatic alignment does not distinguish between the spots that make up the corner shapes and the spots that make the sides of the fiducial frame. The corner shapes are used as landmarks in the manual alignment wizard in the Loupe Browser.