DIY Laser levelling using webcam and laser level

[devmonkey]

So I have 'borrowed' a much higher quality line laser with decent optics, and believe I have verified that line lasers are not straight. Setup was with the sensor on my milling table moving along 10 stations over 500mm against a ruler that was clamped to the table. Note the sensor was moved, the table was stationary to avoid any rocking on the saddle block.

The laser was setup in a fixture about 2m away so that it projected its line across the top of the table hitting the sensor. The fixture allowed the laser to be rotated so I did 4 runs then flipped the laser 180 degrees axially (so the projected line was now inverted wrt the first runs) and did 4 more runs.

I think this suggests that this laser has a parabolic error in its cylindrical lens or the lens alignment, what do you think? Chart shows all 8 runs, you can clearly see two groups of 4 parabolas, one group the inverse of the other, this parabolic error is greater than any error in table straightness.