The lenses in the cell are misaligned far enough that light in the center of the aperture barely makes it through the lens.
The PSM video screen displays a partial set of rings produced by a combination of the PSM and Axicon grating. The orientation and curvature of the rings indicates how the lens cell must move to center it with respect to the Axicon grating axis. Note that the PSM objective focus is at an arbitrary height about the lenses, not at a back focus, or center of curvature of one of the lens elements
The first video picture (left) shows the partial ring pattern with the lens misaligned as in the picture above. As the lens is moved by the screws it is clear the centering gets better (middle). The next picture (right) shows the center of the Axicon grating pattern now in the field of view of the video screen. The magenta crosshair is barely visible in the center of all three pictures and is the reference for centering.
The bright spot in the center of the Axicon grating pattern is much more intense than any of the rings. This intensity forces the Auto Gain function to reduce the gain and shutter exposure time so there are no saturated pixels in the display so the final centered pattern looks like the picture on the left side above where only the center spot and first few rings are visible centered on the crosshair. The picture on the right is a blow up of the one on the left to make the crosshair and scale bar easier to see.
The full screen shot shows that the central spot is centered to less than 1 μm. Also, the Auto Gain was turned off so that more detail in the rings is visible.
The centering was accomplished as fast as the adjustment screws could be turned. This contrasts with the conventional situation where the centering objective must first adjusted to focus at a back focus or center of curvature so there is sufficient focused intensity to view on the video screen. Then the lens cell is moved around to find the focused spot that is lying outside the field of view of the microscope. This is often the most difficult part of the alignment, finding the focused spot when there is no signal on the detector until you are within the field of view of the objective, typically within 0.5 mm when using a 10x objective.
With the Axicon grating the alignment is much faster than conventional methods and less tedious because there is a useful centering indicator even though the lens system is vastly decentered. The lack of tedium makes the work of centering pleasurable rather than a chore.