Here’s a simple fix that you can apply to fix Marco lensmeter targets that separate. Remove the target cell from the instrument. Remove the Risley prism assembly also. Turn the instrument so it is pointing straight up. Put the target back in the instrument backward with the loose part of the target facing up at you. Use the diopter power wheel to bring the target into focus. By turning the target by the axis wheel and by turning the loose part of the target with your fingers you can reset the target.

The 90 degree difference between the target halfs can be established by using the axis scale in the eyepiece. It is not necessary to center the target at this time, just establish the 90 degree setting of the target. Glue the target together; I use four small drops of watch crystal cement around the circumference of the target. I do not remove the glue between the target halfs, as doing so may damage the target. Check the alignment before the glue sets and adjust if necessary. After the glue is dry check the alignment again: if it is correct run a bead of glue all around the circumference of the target and let that dry. Turn the target over again and put it back in the original way. Recenter as necessary. Make any other calibrating adjustments necessary, reinstall the Risley prism and the job is done.