To best describe the oval circle issue with a misaligned laser, I’ve attached a diagram to show the issue (in principle).
In this diagram you will see the correct function of the galvos with respect to a properly aligned laser emitter (A & B).
However, there are some geometrical challenges if your laser emitter is not straight. In this example I show a 2˚ divergence in the emitter path in both X & Y axis. This results in hitting the first galvo at a slightly different angle than expected. However, the real issue has little to do with angles, but more to do with divergence length. When the beam is offset in the X and Y axis (as mine was) the beam will strike the first galvo mirror at different distances pending the angle of the first galvo (C & D). So when the beam strikes the first mirror (at +10˚), the distance the beam travels from the emitter to the mirror is longer than when the first mirror is at -10˚.
As a result, the divergence of the beam will offset more at +10˚ than it will at -10˚. Further, this effect is amplified along the second axis when the beam hits the second galvo mirror. The divergence of the beam comes into play using the extra distance between the two mirrors, allowing more divergence pending the angle of the second mirror (yellow dotted line) because the beam travels further to the second mirror pending the angle of the first mirror — amplifying the divergence effect on the second mirror. But before we forget, the beam divergence is only part of the issue shown here. The offset path (C & D) represents an inherent shift in dimension (grey arrows). Which could amplify or reduce that axis pending the direction of the divergence.
So while you can turn the galvos mechanically and centre a square box with the vat surface, the arrangement will always be inherently distorted (sinusoidal parallelogram), and I fear a substantial amount of complex software will be needed in order to correct a simple error in the laser emitter assembly. I feel it would be far more productive to have a beam alignment feature in the mechanical design of the Moai. All you would need is a piece of card-stock with a target on it it — placed inside the opening of the framework on one side. Hit that beam in the middle of the target and then push the first galvo up into place: problem solved.
I would even design the part for you so it could be easily machined. Best of all, it would allow for a factory alignment step before packaging it for shipping — less risk for the user experience.