Cross scan error of new prism with 1 arcminutes parallelism is shown below.
A pixel in the image is 3 micron and the distance between lines is 111 microns.
Deviation angle of prism is
Orthogonal wobble is;
A deviation of approx 0.1 mm is observed at distance, d, of 30 mm from prism, which implies planes are not planar within 9.8 arcminutes.
New prisms reduce the cross scan error. The prisms are leveled within 1 micron prior to rotation and sides should be planar at 1 arc minutes (this is guaranteed by supplier)
To ascertain the motor is the cause, one could monitor the height deviation of the prism while rotating using a laser distance sensor.
Another option is to think about the results with the cylindical lenses.
If it is assumed the prism is perfectly planar, the refracted rays remain parallel with the incident rays. The second cylindrical lens maps all parallel rays with the same angle to the same point in the focal plane. Ergo it filters any issues with the motor.
Use of cylindrical lenses proved to be highly beneficial, which implies these problems exist.
If higher accuracy is desired it can be resolved by;
- using only a single facet
- reducing the distance between the prism and the exposure plane
- adding a cylindrical lens pair; this circularizes the laser bundle
and filters out problems caused by rotation.
I will look at the motor in the next post.