Aiming the digital heliograph was an interesting part of my project. The sighting system included the 1/4" hole in the mirror silver (or aluminum), a sight arm (which can be pointed left or right), sight post and sight card. The mirror yoke can be manually pivoted left or right, and the mirror can be manually tilted up or down (by first loosening the wing nut at the rear of the solenoid bracket).
When I designed and built the digital heliograph, I made the height of the center of the sight card the same height as the center of the un-silvered spot in the center of the mirror.
But before aiming, the sight must be calibrated. I used the side of a neighbor's house, about 50 yards away. I could position the heliograph mirror to reflect a spot of sunlight on the house (while the mirror was in the solenoid-energized position). I then moved the sight arm to that direction and turned on the Arduino (which had a text message stored in it). I adjusted the sight card left or right in the slot of the sight post until the small shadow spot (from the un-silvered spot on the back of the mirror) was visible bouncing up and down on the sight card as the mirror was tilted by the solenoid. The goal was to look through the un-silvered spot, through the narrow vertical slot in the sight card, and to the distant target. At that point, the sight was aligned in the direction of the reflected, pulsed sunlight. I could now aim the heliograph wherever I wanted to.
I intended to use threaded bolts and nuts, or plastic gears, to make precision aiming easier, but I never had time to design or build that. I also thought that a digital heliograph should have a automatic solar tracker, but there was no time for that. The tracking system should be improved.